Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 558–567
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FTIR spectra and density functional theory P.E.D. assignments of oxiranes in Ar matrix at 12 K L. Gontrani a,⇑, S. Nunziante Cesaro b, S. Stranges c,d, L. Bencivenni b, A. Pierett e,1 a
CNR-ISM, Istituto di Struttura della Materia, Via del Fosso del Cavaliere 100, 00133 Roma, Italy Dipartimento di Chimica, Università di Roma La Sapienza, P.le A. Moro 5, 00185 Roma, Italy c Dipartimento di Chimica e Tecnologie del Farmaco, Università Sapienza, P.le A. Moro 5, 00185 Roma, Italy d IOM-CNR, TASC Laboratory, Basovizza, Trieste, Italy e CASPUR, Via dei Tizii 6, 00185 Roma, Italy b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Trifluoroepoxypropane is studied in
argon matrix for the first time. Some vibrational modes were
reassigned. DFT calculations employed the
6-311++G(3df,3pd) basis set. Potential energy distribution (PED) is
performed with GAMESS and VIBCA.
a r t i c l e
i n f o
Article history: Received 25 July 2013 Received in revised form 3 December 2013 Accepted 4 December 2013 Available online 10 December 2013 Keywords: FTIR spectra of oxiranes in argon matrix DFT calculations Vibrational assignment P.E.D. analysis
a b s t r a c t The FTIR spectra of a series of oxiranes were studied in Ar matrix at 12 K. The interpretation of the spectra was accomplished on the basis of density functional theory calculations employing the 6-311++G(3df,3pd) basis set with the B3LYP functional. Potential energy distribution was carried out for each molecule employing the B3LYP/6-311++G(3df,3pd) force field and a non-redundant definition of internal coordinates. The study of the FTIR spectra led to the reassignment of some vibrational modes of the molecules. The FTIR spectrum of trifluoroepoxypropane measured in Ar matrix and its assignment is reported for the first time. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Oxiranes, thiiranes and their derivatives present a variety of industrial applications since they are quite stable but their ring can be easily opened by acids or bases in catalytic reactions or by electrophiles and nucleophiles in stoichiometric reactions in order to synthesize various complex organic molecules and macro⇑ Corresponding author. Tel.: +39 3387615798. 1
E-mail address: [email protected] (L. Gontrani). Present address: CINECA, via dei Tizii 6, 00185 Roma, Italy.
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.12.005
molecules [1–4]. The ring can be also opened on solids and therefore employed for synthesis of epitaxial layers through Metal Organic Chemical Vapour Deposition (MOCVD) [5,6]. The increasing interest for practical applications of oxirane, thiiranes and derivates has been paralleled by an increasing attention to their fundamental properties. Among chiral molecules, methyloxirane is one of the smallest and is considered a good prototype for theoretical and experimental studies focused on chiral effects and chiroptical activities, due to the presence of a stereogenic center, that is bonded to the methyl group [7–13].
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The first determination of the equilibrium configuration of the molecule is due to Swalen and Herschbach [14] who, from microwave spectroscopy measurements, evaluated the height of the torsional barrier of the methyl group (2.71 kcal mol1). Afterwards, far infrared spectral data of methyoxirane in its gaseous and solid states provided an average torsional barrier of 2.573 ± 0.023 kcal mol1 and 3.05 kcal mol1, respectively [15]. Vibrational spectroscopy studies were also performed in all the aggregation states of the molecule [16,17], in CS2 and CCl4 solutions [18] or in N2 [17,18] and Ar matrix [18]. Lowe et al., however, did not report the spectrum of methyloxirane isolated in Ar matrix because of its complexity with respect to the N2 matrix [17,18]. Particular attention was always devoted to the high frequency region (3250–3100 cm1) where the carbon–hydrogen stretching modes are observed together with a number of combination bands of the methyl group torsion and asymmetric stretching vibrations whose presence makes their attribution difficult. The first assignment [13] was revised by Winter and Hummel [19] studying gaseous methyloxirane using high spectral resolution. Their assignment was confirmed in a recent paper where the low resolution infrared spectrum of room temperature and jet cooled methyloxirane was compared to results of high resolution spectroscopy [20]. Methylthiirane shares with methyloxirane the same conformational rigidity of the three-membered ring characterizing the molecule. In the past years, particular attention was devoted to the study of the methyl torsional vibrations [15,21–24]. The available spectroscopic data allowed the evaluation of the torsional barrier of the methyl group (3.26 kcal mol1), in agreement with previous microwave data providing 3.24 kcal mol1 [25]. Only later, the complete spectrum of liquid and Ar matrix-isolated methylthiirane was experimentally and theoretically studied [26,27]. More recently, gas phase electron diffraction data and theoretical calculations were used to determine the structure of methylthiirane in order to explain the ease of ring opening during polymerization [6]. Epichlorohydrin, at present largely employed for the preparation of epoxy resins [28] has been object of structural investigations mainly dealing with conformational equilibria in different physical states of the molecule. This conformationally flexible molecule was extensively studied in all its physical states using infrared spectroscopy [29–36], NIR-VCD [37], nuclear magnetic resonance (NMR) [38], microwave spectroscopy [39], electron diffraction [40], dipole moment measurement [41], angle resolved photoelectron spectroscopy [42], and electron velocity map imaging [43]; the optical rotatory dispersion was determined by a theoretical study [44]. All the quoted investigations were concerning with the determination of the stability of the conformers of the molecule, namely Gauche-2 (G2), Gauche-1 (G1) and Cis (Cis), their relative abundance in gaseous, liquid and solid states and just for their solutions, the influence of solvent polarity on the conformational population. The G2 conformer is the predominant gas-phase conformer, while the G1 and Cis rotamers are less abundant. Studies of the infrared spectra based their conclusion on the comparison of calculated and experimental relative intensities of multiplets (doublets and triplets) observed in the spectral ranges 980–930, 850–830, 795–760, and 530–400 cm1. As a further support, the torsional modes of both G-type conformers were observed and assigned in the far infrared spectrum of gaseous epichlorohydrin. Infrared and Raman studies performed on solid and liquid epichlorohydrin, confirmed the presence of G1 in the former physical state. Furthermore, the observation of doublets and triplets in the spectrum of the molecule in its liquid phase, suggested the simultaneous presence of G1 and G2 rotamers together with a minor amount of Cis form [32,36]. These results were afterwards confirmed by spectroscopic studies in liquid state at variable temperature or in liquid
559
xenon solutions [33]. Analogous conclusions were achieved studying the molecule in different solvents [35]. The populations of G2 and G1 conformers are strongly dependent on the polarity of the solvent, which, in contrast, does not affect that of the Cis conformer. Quite recently, infrared and Raman spectroscopy have been utilized to determine the fundamental vibrations of solid epichlorohydrin whose assignment is supported by theoretical calculations [44]. Studies on chiroptical properties and electronic structure of the rotamers of epichlorohydrin have recently been carried out in the valence ionization [42,43] and near infrared regimes [44]. Surprisingly, no fundamental data are found in literature concerning the structural and vibrational behaviour of 1,1,1-trifluoroepoxypropane despite its applicative importance in the synthesis of chain-end functionalized polymers [45]. For this reason, the vibrational absorptions of 1,1,1-trifluoroepoxypropane isolated in Ar matrix were measured in the 4000–400 cm1 spectral range. In addition, the complete vibrational spectra of methyloxirane, methyltiirane and epichlorohydrin in their pseudo-gaseous phase have been obtained and the data compared with previous ones when available. The experimental data reported in this work were interpreted by a theoretical treatment based on density functional theory for geometry optimizations and harmonic frequency calculations carried out for all the molecular species using extended basis sets. Materials and methods The experimental apparatus consists of a IFS 113v Fourier Transform Interferometer (Bruker) and a Cryostat (Displex CSA-202, Air Products and Chemicals) equipped with a gold plated copper coldfinger, free to rotate in a high vacuum shroud. The vacuum chamber is connected under rotary vacuum (103 torr) to the interferometer. Infrared spectra (range 4000–50 cm1) were recorded in reflection cumulating at least 200 scans at a resolution of 1 cm1 or better. High purity samples of methyloxirane, epichlohydrin, methylthiirane and 1,1,1-trifluoroepoxypropane were supplied by Sigma–Aldrich. The samples were vaporized in the temperature range 273–363 K from a suitable device and vapours were deposited onto a reflecting copper surface kept at 12 K. High purity argon (Linde) was employed as isolating gas. Annealing cycles, performed at 35 K, did not produce appreciable spectral changes ruling out presence of matrix aggregates. Computational section Calculations were run at CASPUR on an HP cluster, based on AMD Opteron CPUs. The lowest energy equilibrium structures and harmonic frequencies of vibration of all the molecules were calculated at the B3LYP and MP2 levels using the 6-311++G**, 6-311G++(3df,3pd) and aug-cc-PVDZ basis sets employing the GAUSSIAN-03 program package [46]. Calculated infrared spectra depend on the theoretical level as well as on the basis set because both influence vibrational frequencies and band intensities of molecular normal modes. The most satisfactory and homogeneous results were obtained for all the molecules at the B3LYP/6-311G++(3df,3pd) level providing the best agreement between calculated vibrational wavenumbers and relative infrared band intensities. Vibrational assignments of the computed harmonic frequencies were accomplished on the ground of potential energy distribution analysis carried out for each molecule at the B3LYP/6-311++G(3df,3pd) level. Harmonic frequencies of vibration were scaled adopting the scale factors suggested in literature [47,48].
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Fig. 1. Oxiranes: atom numbering.
Fig. 2. (red) FT-IR spectrum of methyloxirane (Ar matrix, 12 K); (blue) DFT spectrum. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 Internal coordinate definition.a S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S22 S23 S24 a
Stretching Stretching Stretching Stretching Stretching Stretching Stretching Stretching Stretching Stretching Bending Bending Bending Bending Bending Bending Bending Torsion Torsion Torsion Torsion Torsion Torsion out-Plane
C1H8 C3H5 C3H6 C1H9 C1H10 C2H7 C2C3 C1C2 C2O4 C3O4 H8C1H10 H5C3O4 H5C3H6 H9C1H8 H10C1H9 H7C2O4 C1C2O4 H8C1C2C3 H5C3O4C2 H6C3O4C2 H9C1C2C3 H10C1C2C3 H7C2O4C3 C1C3O4C2
(C1F8)
(C1Cl8)
(C1F9) (C1F10)
(C1Cl9) (C1Cl10)
(C2S4) (C3S4) F8C1F10 H5C3S4 F9C1F8 F10C1F9 H7C2S4 C1C2S4 F8C1C2C3 H5C3S4C2 H6C3S4C2 F9C1C2C3 F10C1C2C3 H7C2S4C3 H7C2S4C3
Cl8C1H10
Cl9C1H8 Cl10C1H9
Cl8C1C2C3
Cl9C1C2C3 Cl10C1C2C3
Atom numbering is given in Fig. 1.
P.E.D. analysis was calculated using VIBCA program [49] implementing the force constant matrix and coordinate transformations described in Califano’s book [50]. Force constants and coordinate sets were obtained from GAMESS software [51] after re-optimizing the molecular geometry and re-calculating the vibrational frequencies of each species. P.E.D. contributions below 10% are not reported in the paper. The molecular model of the molecules, including atom numbering, is shown in Fig. 1 and the set of not redundant internal coordinates is given in Table 1. Results and discussion
Table 2 FT-IR spectra of methyloxirane: experimental (Ar matrix, 12 K), B3LYP/6311++G(3df,3pd) vibrational frequencies (cm1) and vibrational assignment based on P.E.D. analysis.a Calc.
Scaled
Exp.
Description of the normal mode
3168 3113 3092 3087 3082 3029
3064 3011 2991 2986 2981 2930
S3 S1 S6 S5 S2 S4
1529
1479
1498 1484
1449 1435
1438 1408 1294 1188 1168 1158 1130
1391 1362 1252 1149 1130 1120 1093
1043 974 908
1009 976 910
844 773 411 368 211
846 774 412 369 211
3055.4 3001.6 2972.1 2935.5 2907.3 2874.4 2805.0 1500.5 1475.7 1459.1 1447.0 1440 1410.2 1370.5 1269.7 1174.7 1147.3 1137.7 1107.0 1056.4 1046.9 1027.0 953.4 898.0 879.8 833.3 745.8 411.6 375.5 213.1
52% 52% 68% 51% 50% 41%
S2 S6 S1 S4 S3 S5
47% 29% S5 10% 24% 48% 47% 35%; S1 23%
S13 69% S11 41%; S15 33%; S22 13% S14 46%; S11 20%; S15 10%; S18 10% S16 S14 S16 S23 S19 S20 S12
20%; 32%; 39%; 28%; 26%; 34%; 47%;
S23 17%; S15 16%; S8 12% S1519%; S11 18% S9 15%; S7 13%; S10 10% S20 13%; S16 11%; S12 10% S12 14%; S21 10%; S24 10% S19 27% S18 17%
S12 22%; S18 21%; S23 12%; S16 10% S10 39%; S8 27%; S21 10% S19 25%; S20 21% S7 56%, S9 19% S9 36%; S10 21%; S8 17%; S21 10% S24 41%; S17 35%; S22 10% S17 46%; S24 39% S21 38%; S22 35%; S18 21%
a Atom numbering is shown in Fig. 1 and internal coordinates definition is given in Table 1.
Methyloxirane The infrared spectra of methyloxirane in gaseous, liquid, solution and solid state have been extensively studied in the previous years [14–19]. The spectra of the single molecule in N2 [17,18] and argon low temperature matrices [18] were investigated. Lowe, however, did not report in his paper the spectrum of methyloxirane in argon matrix because of its complexity with respect to the nitrogen environment, attributed to site heterogeneity [18]. In
the present paper the vibrational spectrum of methyloxirane isolated in argon matrix measured in the range 4000–50 cm1 is reported and discussed for the first time. Surprisingly enough, frequency values and relative intensity of the spectrum of solid methyloxirane studied by Kirchner [16] presents a very good matching with the results of the spectrum in argon matrix. The infrared spectrum of methyloxirane isolated in argon matrix is shown in Fig. 2, where it is compared with the DFT spectrum
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calculated at the B3LYP/6-311++G(3df,3pd) level, and the experimental frequencies are listed in Table 2 along with the theoretical results (calculated frequencies and corresponding P.E.D. analysis). It is worth mentioning that comparison among spectra obtained from different experimental conditions shows a very good agreement in spite of the physical state of the molecule. The spectra of methyloxirane in all its physical states, in solution as well as in pseudo gaseous phase are indeed quite similar, as only minor frequency shifts and relative band intensity changes are observed, thus excluding occurrence of whatsoever intermolecular interaction. The assignment of some of the fundamental bands needs sometimes to be reconsidered in the light of more sophisticated calculations, in particular for the modes falling in the high frequency range where the CH stretching modes are expected. The present calculations do not fully agree with those reported by Lowe et al. [18] carried out at the HF/4–31G level. The disagreement mainly regards the assignment of some of the CH stretchings of the molecule, as one may realize from comparing our revised assignment with the previous one [18]. This fact is evidently due to the HF-SCF approximation and to the use of rather small basis sets. Furthermore, the not entirely matching between our conclusions and the earlier ones is also due to scaling procedure adopted by Lowe et al. [18]. The combined effect of both these facts might be responsible of the failure of the HF approximation when vibrational coupling is considerable as for this molecule. According to the revised assignment, the peaks at 3055.4 and 2907.3 cm1 are attributed to the antisymmetric and symmetric stretchings of the CH2 group, respectively, the band 3001.6 cm1 to the antisymmetric stretching of the CH3 group, strongly coupled with the CH bond stretching. Among the remaining bands, that measured at 2972.1 cm1 is attributed to the CH bond stretching, coupled with the CH3 group stretchings, and those at 2935.5 and 2874.4 cm1 are related to the antisymmetric and symmetric stretching vibrations of the methyl group, respectively. Therefore, for this molecule the carbon–hydrogen stretching frequencies are distributed as follows mas CH2 > mas CH3 > m CH > mas CH3 > ms CH2 > ms CH3. The matrix spectrum under consideration shows a number of weak and very weak intensity bands quite likely attributable to combinations or overtones, having no importance for the purpose of this work. On the other hand, no conclusive assignment can be reached for these absorptions because several assignments might be proposed for them. An improvement of the description of the molecular vibrations is reached for several bending and ring stretching modes (see Table 2) often showing vibrational coupling among them. For instance, the band measured at 1500.5 cm1 is described as the bending of the CH2 group of the molecule having small vibrational coupling ( m CH (3048.5 cm1) > ms CH2 (2994.8 cm1) instead of mas CH2 > m CH3 > m CH as established for methyloxirane. The CH3 group stretching frequencies of methyltiirane associated with the internal coordinates S1, S5 and S4 are observed in the matrix spectrum at 2988.1, 2971.3 and 2932.7 cm1. The two antisymmetric strechings of the methyl are predicted to occur at higher wavenumbers than the corresponding symmetric mode and show high vibrational coupling among them. In summary, the revised assignment for these vibrations reproduces that reported by Polavarapu et al. [26] based on HF/6–31G calculations, as well as that proposed by Alper and Lowe [52] and Polavarapu et al. [26]. Surprisingly enough, our B3LYP/6-311++G(3df,3pd) P.E.D. analysis does not confirm the conclusions reported for the carbon–hydrogen stretchings in a later investigation performed at the MP2 level [53]. As far as the remaining fundamentals are concerned, there are some differences between our P.E.D. analysis and the interpretation proposed by other authors [26,53]. The different conclusions reached in the previous studies evidently depend on the computational level, which, as assessed in advance in this work, is particularly sensitive to the combination of the method and of the basis set, resulting in not harmless consequences when some vibrational modes are closely lying. Actually, this fact occurs for the CH2 and CH3 bendings, calculated, and as expected observed, between 1500 and 1420 cm1. The present calculations show, in conformity with the results accomplished for methyloxirane and with a few exception, strong vibrational coupling among most of the modes. Thus, the carbon–sulphur stretchings are meaningfully coupled with the ring deformations. In particular, the C2S4 (S9) and C3S4 (S10) stretchings calculated at 639 and 602 cm1 and corresponding to the bands measured at 634.5 and 608.7 cm1, are strongly coupled with one another. For this molecule, the C1C2 stretching calculated at 881 cm1 and measured at 872.7 cm1 is largely mixed to the C2C3
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563
Table 4 FT-IR spectra of 1,1,1-trifluoroepoxypropane: experimental (Ar matrix, 12 K). B3LYP/ 6-311++G(3df,3pd) vibrational frequencies (cm1) and vibrational assignment based on P.E.D. analysis.a Calc.
Scaled
Exp.
Description of the normal mode
3205
3100
S2 56% S3 43%
3142
3039
3109
3007
1528
1478
1449
1402
1298
1256
1276 1171 1167 1157 1147
1234 1133 1129 1119 1109
1094 1011
1058 978
928 882
930 884
788
789
647 582 523
648 583 524
3093.0 3089.1 3087.8 3053.4 3023.6 2956.3 1490.2 1486.2 1452.5 1440.2 1433.2 1410.4 1402.0 1321.4 1294.6 1277.3 1261.0 1178.0 1173.2 1166.1 1141.6 1079.4 1074.2 1000.8 971.6 965.2 936.6 918.2 856.0 846.2 799.4 788.5 772.9 758.6 752 650.6 589.2 530.4
436 382 238 218 75
437 383 238 218 75
440.7 385.7 221.2
S6 98% S3 57% S2 42% S13 73%
S16 19% S8 16% S13 16% S23 16% S9 10%
S16 35% S1 17% S8 11% S23 10% S8 13%; S10 12%; S20 12%; S1 10% S19 50%; S20 16% S12 38%; S23 27%; S16 17% S5 45%; S20 11% S4 53% S12 47%; S16 12%; S23 10%; S20 10% S20 25%; S19 23%; S1 15%; S23 11%
S7 48%; S9 16%; S23 12% S10 60%; S9 23%
S1 23%; S8 15%; S5 14%
S15 S21 S11 S15 S14 S15 S24 S17 S18
23%; 31%; 32%; 11%; 45%; 43%; 47%; 46%; 96%
S4 14%; S17 10% S5 15%; S11 11% S14 16%; S22 15% S8 10% S17 16%; S11 22% S8 16%; S24 12% S22 35% S21 31%
a Atom numbering is shown in Fig. 1 and internal coordinates definition is given in Table 1.
carbon–carbon stretching (S7) and to the molecular torsion described by the internal coordinate S20. The matrix spectrum of the molecule shows a large number of weak and very weak absorptions for which the same conclusions already reported for methyloxirane are likely to hold. 1,1,1-Trifluoroepoxypropane In this paper we discuss the Ar matrix spectrum of trifluoroepoxypropane for the first time since no report of the vibrational spectrum of this molecule was found both in the mid- and farinfrared regions. Absorption bands of trifluoroepoxypropane isolated in Ar matrix at 12 K are reported in Table 4 together with the theoretical frequencies of vibration calculated at the B3LYP/ 6-311++G(3df,3pd) level and the corresponding P.E.D. analysis. It might be interesting to mention that the theoretical vibrational spectrum of this molecule is more affected than any other molecule of the series by the combination of computational method and basis set. This observation comes out when the DFT and
Fig. 4. (red) FT-IR spectrum of trifluorooxirane (Ar matrix, 12 K); (blue) DFT spectrum. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
MP2 theoretical frequencies and band intensities calculated from the different levels of theory are compared. The effect is particularly appreciable in the region 1270–1130 cm1 of the spectrum. Improvement between theory and experiment was at any rate achieved from B3LYP/6-311++G(3df,3pd) calculations, that reproduce nicely the measured FT-IR spectrum. The Ar matrix spectrum and the calculated one are shown in Fig. 4. P.E.D. analysis performed at the B3LYP/6-311++G(3df,3pd) level shows that the higher wavenumber bands measured in the carbon–hydrogen stretching region are due to the antisymmetric CH2 (3093.0 cm1), CH (3087.8 cm1) and symmetric CH2 (3023.6 cm1) stretching modes respectively, following thus the same order established for methyloxirane and methyltiirane. The CF stretching vibrations are strongly coupled among them as well as with other vibrations (see Table 4). The antisymmetric CF3 stretchings (1166.1 and 1141.6 cm1) occur at much higher wavenumbers than the corresponding symmetric stretching (788.5 cm1); the corresponding theoretical values for these stretching frequencies are 1157 cm1, 1147 cm1 and 788 cm1. The higher frequency antisymmetric mode is coupled with the torsional mode described by the internal coordinate S20 (H6C3O4C2 torsion) and the symmetric stretching, associated with the S1 and S5 internal coordinates, shows large coupling with the C1C2 stretching (S8) (see Table 4). All the frequencies ranging between 647 and 382 cm1 are related to S11, S14 and S15 bending coordinates of the CF3 group. In particular, the largest contributions of these coordinates are found in the frequencies computed at 523 cm1 (S11, 32%; S14, 16% and S15, 11%), 436 cm1 (S14, 45% and S11, 22%) and 382 cm1 (S15, 43%) and for this reason the CF3 bendings should be consequently attributed as follows: 530.4, 440.7 and 385.7 cm1. Also the bands calculated at 647 cm1 (exp. 650.6 cm1) and 582 cm1 (exp. 589.2 cm1) have a moderately large CF3 bending character, that is S15 (23%) for 647 cm1 and S11 (11%) for 582 cm1. The 647 cm1 frequency would thus be associated to a mode where the S15 internal coordinate predominates over S4 (CF stretching) and S17 (C1C2O4 bending) and the frequency 582 cm1 would be related to torsional mode of the CF bond with respect to the C1C2C3 axis. Concerning some frequencies lying between 1500 and 1200 cm1, the band at 1490.2 cm1 is without any doubt assigned
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Fig. 5. (a) G2, (b) G1 and (c) Cis rotamers of epichlorohydrin and (d) TS1 and (e) TS2 transition states.
to the CH2 bending involving H5 and H6 (S13 = 73%) and the band at 1433.2 cm1 to a complex mode of the molecule wherein the role of the S16 internal coordinate (OCH bending involving H7) is comparable to that of S8 (C1C2 stretching), S13 and S23 (see Table 4). The highest amount of the S16 coordinate is found in correspondence of the frequency 1294.6 cm1: this OCH bending mode is appreciably coupled with the C1C2 stretching as well as with the CH2 group bending. On these premises, the carbon–hydrogen bending mode of the CH2 group (calculated 1449 cm1, exp. 1433.2 cm1) is predicted at higher wavenumbers than the CH bending vibration (1298 cm1, exp. 1294.6 cm1). The frequency measured at 1261.0 cm1 would be related to the C1C2 stretching, coupled with the CF, CO stretchings and the torsional mode H6C3O4C2 (S20 internal coordinate). Among the bands just below 1200 cm1, those at 1178.0, 1173.2, 1079.4 and 1000.8 cm1 should be assigned to bending and torsional modes involving the CH2 and CH groups of the molecule (see details in Table 4). The further modes to be discussed are the CO and C2C3 stretchings, being the former mode attributed at 856.0 cm1 and the latter at 918.2 cm1 (coupled with the C2O4 stretching and the H7C2O4C3 torsion). The low frequencies calculated at 238, 218 (exp. 221.2 cm1) and 75 cm1 correspond to complex bending modes involving the carbon–fluorine bonds and that at 221.2 cm1 (calculated at 218 cm1) to an oxygen–carbon–carbon bending, strongly interacting with the FCC bending vibration. Epichlorohydrin Among oxirane derivatives, epichlorohydrin is one of the most extensively studied molecule in all its physical states
Fig. 6. (red) FT-IR spectrum of epichlorohydrin (Ar matrix, 12 K); (blue) DFT spectrum (the overall spectrum is the weighted sum of the theoretical spectra of the three rotamers). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
employing a number of techniques aiming to verify the presence of its conformers and their relative abundance. Only the most polar conformer G1 is present in solid phase [32,36] while all of them are simultaneously present in gas, liquid phases or in solutions. The G2 conformer should predominate over the G1 and Cis rotamers in gas phase and the amount of the Cis form is indeed small [33,43]. The same trend was confirmed diluting the molecule in liquid Xe at variable temperature [33]. Our MP4/6-311++G(3df,3pd), G2MP2 and G3MP2 calculations provide some further information about the Gauche-2, (G2) Gauche-1 (G1) and Cis conformers (see Fig. 5) suggesting that the three rotamers are not separated by high energy differences (see Table 5). These calculations agree that the G2 conformer is the lowest energy form and that the less stable G1 and Cis rotamers are 2.5 kJ mol1 and 5.4 kJ mol1 respectively higher in energy than G2. Moreover, the transition state TS1 between G2 and G1 and the corresponding one TS2, between G1 and Cis (see Fig. 5) are estimated relatively low from MP4/6-311++G(3df,3pd) single-point energy calculations. The conversion of the less stable G1 conformer into the most stable one G2 would require 8 kJ mol1, whereas the conversion of the Cis into the G1 form would require a higher amount, that is 15 kJ mol1. Gas-phase population at 298.15 K was estimated using the G2MP2 and
Table 5 Stability study of the stable conformers of epichlorohydrin at the MP4/6-311++G(3df,3pd) level for total electronic energy and B3LYP/&-311++G(3df,3pd) for zero-point energy and thermal corrections. DEe and DEo energy differences (kJ mol1) are the electronic energy differences and electronic plus zero-point energy differences. G2
G1
DE e 0 2.8 DE 0 0 2.7 DEe(G2 ? TS1) 17.7 14.1 DEe(G1 ? TS1) 11.3 DEe(G1 ? TS2) DE0(G2 ? TS1) (cis ? TS2) 14.6 10.5DE0(G1 ? TS1) 7.9 DE0(G1 ? TS2) G2MP2 and G3MP2 energy differences, calculated with respect to the G2 conformer and gas-phase conformer population at 298.15 K
Cis 5.5 5.4 20.4 DEe(cis ? TS2) 17.3 DE0
Energy differences (kJ mol1) G2MP2 (0 K) G2MP2 G3MP2(0 K) G3MP2
0 0 0 0
2.4 2.4 2.5 2.2
5.6 5.5 4.9 4.7
Gas-phase conformer population at 298.15 K G2MP2 G3MP2 Average
70% 68% 69%
24% 24.5% 24.25%
6% 7.5% 6.75%
565
L. Gontrani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 558–567 Table 6 FT-IR spectra of epichlorohydrin: experimental (Ar matrix, 12 K), B3LYP/6311++G(3df,3pd) vibrational frequencies (cm1) and vibrational assignment based on P.E.D. analysis.a Calc.
Scaled
Exp.
Description of the normal mode
3197 3185 3177 3154 3150 3129 3124 3123 3102 3094 3089 3091 3084 3084 3071
Cis G2 G1 G2 G1 Cis G2 G1 Cis G2 G1 G2 G1 Cis Cis
3092 3081 3073 3051 3047 3027 3022 3021 3001 2993 2988 2990 2983 2983 2971
3071 3059.2 3032 3032 3032 3012.5 3012.5 3012.5
S2 S3 S3 S1 S1 S4 S6 S6 S3 S2 S2 S5 S4 S6 S6
1528 1526 1529
G1 Cis G2
1478 1476 1479
1490 1470 1472 1439 1430 1432
G2 G1 Cis G2 G1 Cis
1441 1422 1424 1392 1383 1385
1322 1304 1298 1294 1285 1277
Cis G2 Cis G1 G1 G2
1261 1256 1252 1243 1235
1307.7 1307.7 1277.5 1277.5 1269.1
1233 1228
Cis G1
1193 1188
1211 1176 1174 1168 1162 1157 1158 1113 1107 1096
G2 G2 Cis G1 G1 Cis G2 G2 G1 Cis
1171 1138 1136 1130 1124 1119 1120 1077 1071 1060
1256.6 1256.6 1249 1193 1147.4 1147.4 1147.4 1134.8 1134.8 1134.8 1107.1 1107.1 1107.1
1095 1072
G1 G2
1037 1037
1096.7 1027.1
1041 988 983 948 931 919 890
Cis Cis G2 G1 G1 Cis G2
990 985 950 933 921 892
970.4 966.4 952.7 934.8 914 894.3
863
G2
865
856.3
863 851 816 795 791 739
G1 Cis G2 Cis G1 G2
865 853 818 797 793 740
726
G1
727
846.2 840.1 833.1–830 797.7 797.7 784 771.1 754.1
2990.7 2972.6 2972.6 2972.6 2972.6 2936.3 2906.4 2894 1511.9 1511.9 1511.9 1486.2 1476.6 1458.5 1452.4 1452.4 1447.6 1410.0 1410.0 1403
64%; 54%; 53%; 55%; 59%; 51%; 90% 90% 63%; 55%; 52%; 59%; 62%; 50%; 47%;
S3 S2 S2 S5 S4 S5
35% 44% 44% 36% 32% 49%
S2 S3 S3 S1 S1 S5 S4
35% 45% 45% 40% 35% 25%; S4 22% 27%; S5 26%
S13 78% S13 70% S13 75%
S11 S14 S15 S23 S16 S16
82% 79% 85% 22%; S13 12% 28%; S23 17%; S13 10% 22%; S23 21%; S13 17%
S21 S15 S22 S16 S21 S16 S16 S11 S11
42%; 10% 30%; 46% 38%; 28%; 27%; 44%; 44%
S22 20%; S8 11%: S18 16%; S16 14% S18 22%; S16 14% S9 19%; S10 18%; S7 11% S22 14%; S23 11% S21 17%; S22 15%
S14 37%; S8 13%; S18 13% S12 18%; S16 24%; S23 23% S20 39%; S12 21%; S19 19% S20 47%; S19 23% S12 41%; S23 25%; S19 11% S19 36%; S12 21%; S23 18% S19 46%; S20 26% S12 48% S8 22%; S11 14%; S19 17%; S21 17% S12 48% S12 48%; S23 18%; S16 10% S22 20%; S23 18%; S12 13% S14 12%; S8 10% S19 20%; S23 19%; S8 16%; S20 14% S10 21%; S8 20%; S21 12%; S7 10% S8 23%; S10 23%; S7 11%; S20 10% S7 29%; S19 23%; S10 11% S18 27%; S10 32%; S8 15% S10 32%; S18 15%; S8 10% S20 21%; S19 20%; S18 16%; S14 13%; S23 11% S9 35%; S7 29%; S10 15% S7 37%; S20 23% S7 49%; S9 16%; S22 10% S7 28%; S10 26%; S22 10% S9 20%; S1 14%; S10 14%; S9 43%; S10 20%; S8 13% S4 70%; S17 13%; S15 10% S5 69%; S15 12%
Table 6 (continued) Calc.
Scaled
Exp.
Description of the normal mode
748b 740 699 521 439 408 371
Cis Cis G1 G2 G2
522 440 409.3 373.0
364 355 212 210 204 108 97 90
G1 Cis G2 G1 Cis Cis G1 G2
365 356 212 210 204 108 97 90
520 440.4
221.0 221.0
S1 45%; S8 15%; S14 12% S1 30%; S24 29%; S14 17%; S17 10% S17 S24 S24 S24 S17 S15 S15 S14 S18 S22 S21
55%; 37%; 39%; 51%; 61%; 55%; 53%; 51%; 63%; 75% 76%
S15 14% S15 17% S17 21%; S4 13% S5 15% S22 13%; S18 10% S17 35% S24 18%; S17 21% S24 36% S22 17%
a Atom numbering is shown in Fig. 1 and internal coordinates definition is given in Table 1. b 37 Cl shift.
G3MP2 standard Gibbs free-energy differences values (DG°); both methods yield almost identical results (see Table 5). This information is of great importance because each rotamer contributes to the actual theoretical spectrum (the weighted sum of the spectra of the three rotamers) according to the respective relative abundance. The calculated infrared spectrum is shown in Fig. 6 together with the FTIR spectrum measured in Ar matrix. The calculated infrared spectrum predicts the overlap not just of bands calculated at the same wavenumber but even of some close-lying frequency bands. In the present work of the infrared spectrum of Ar matrix isolated epichlorhydrin is analyzed for the first time. As matrix isolation should reflect the stability in the gas phase and the vapor composition at the vaporization temperature, the experimental results are compared with previous data obtained for the vapor phase and for low-temperature Xe solutions. The summary of the observed frequencies, reported in Table 6, was based on the results of the theoretical results keeping into account the calculated frequencies of vibration and the respective IR band intensities, although the latter ones must be handled with particular care because they strongly depend on the computational level (method and basis set). Irrespective of the rotamer, all the antisymmetric CH stretchings of the CH2 and CH2Cl groups occur at higher frequencies than the corresponding symmetric modes. Thus, these modes are distributed as follows: mas CH2 > mas CH2Cl > m CH > ms CH2 > ms CH2Cl, confirming the previous assignment based on MP2/6–31G* calculations [31]. The assignment of the bands lying between 1500 and 1200 cm1 does not show appreciable differences with respect to the most recent study [33]. In particular, the bands observed in the range 1500–1400 cm1 are, in conformity with the results reported for the other oxyranes, due to the CH bending vibrations of the molecule, distributed as follows. m CH2 > m CH2Cl > m C2H7 bending. Concerning the CH bendings, the highest bending frequencies suggested by the calculations are the CH2 group bendings (calculated 1530 cm1; observed 1511.9 cm1), followed by the CH bendings of the CH2Cl group (calculated 1480 cm1; observed 1450 cm1) and by the bands around 1430 cm1, due to the C2H7 bendings (S23 and S16) strongly coupled with the S13 bending coordinate for the CH2 group. Our revised assignment accomplished from the B3LYP/6311++G(3df,3pd) P.E.D. analysis allows a better interpretation of the ring modes of the three rotamers. Aware of the unquestionable evidence of the high vibrational coupling characterizing all the ring modes, the bands of the Cis form calculated at 988
566
L. Gontrani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 558–567
Table 7 Adiabatic ionization energiesa of oxiranes (M (1 X;m00 ¼ 0 ! Mþ ð2 X;m00 ¼ 0Þ and assignment of the carbon–hydrogen stretching frequencies of the molecular ions. MeO/MeO+ 10.03 eV (80864 cm1) G1/G1+ 10.09 eV (81424 cm1) MeO+ 3139 MeO 3168 MeT+ 3177 MeT 3202 TFO+ 3162 TFO 3205 G1+ 3222 G1 3177 G2+ 3229 G2 3185 Cis+ 3162 Cis 3197
MeT/MeT+ 8.70 eV (70160 cm1) G2/G2+ 10.05 eV (81096 cm1) CH stretching modes (cm1)
TFO/TFO+ 10.95 eV (88317 cm1) Cis/Cis+ 9.98 eV (80498 cm1)
masCH3
3131
mas CH2
3098
mas CH3
3073 m CH
3031
ms CH2
3013 ms CH3
mas CH2
3113
mas CH3
3092
m CH
3087 mas CH3
3082
ms CH2
3029 ms CH3
mas CH2
3135
mas CH3
3123
m CH
3096 mas CH3
3088
ms CH2
3026 ms CH3
mas CH2
3139
m CH
3115
ms CH2
3106 mas CH3
3083
mas CH3
3019 ms CH3
mas CH2
3054
ms CH2
3035
m CH
mas CH2
314 2
m CH
3109
ms CH2
mas CH2
3157
mas CH2Cl 3121
m CH
3109 ms CH2
3053
ms CH2Cl
mas CH2
3150
mas CH2Cl 3123
m CH
3089 ms CH2
3084
ms CH2Cl
mas CH2
3200
mas CH2Cl 3121
ms CH2Cl
3119 m CH
3088
ms CH2
mas CH2
3154
mas CH2Cl 3124
m CH
3094 ms CH2
3091
ms CH2Cl
mas CH2Cl 3157
mas CH2
3098
m CH
3084 ms CH2Cl 2984
ms CH2
mas CH2
mas CH2Cl 3102
m CH
3084 ms CH2
ms CH2Cl
3129
3071
Other stretching modesb (cm1)
m CO MeO MeO+ MeT MeT+ TFO TFO+ G1 G1+ G2 G2+ Cis Cis+ a b
m CS
m CF
m CCl
741–632 974–844 639–602 578 882 688–596 791 841 863–812 841 795 844
788–1157-1147 813–1262-1250 726 782 739 803 699 740
The values include zero point energy corrections. The largest P.E.D. contributions are reported.
and 919 cm1, as well as those at 983 (G2), 931 (G1) cm1 and 863 cm1 (G2), have a high CO stretching character, most of them are highly coupled with the C1C2 and C2C3 stretchings (S8, S7) and even with some torsional modes. For instance, the band calculated at 919 cm1 for the Cis rotamer has a higher C3O4 stretching (S10) character (32%) than the band at 988 cm1 (21%), whereas the 983 cm1 band of G2, the internal coordinates for the C3O4 (S10) and C1C2 (S8) stretchings have actually both the same contribution. As regards the C2C3 stretching, the major contribution of the internal coordinate S7 is present in the bands computed at 851 cm1 (Cis), 863 cm1 (G1 and G2) as well as at 816 cm1 (G2) and 948 cm1 (G1). The most valuable C2O4 stretching contribution S9 is observed at 795 cm1 (Cis) and 791 cm1 (G1) and 863 cm1 (G2). At last, the CCl stretching modes, calculated in the narrow range of frequency for the three rotamers, are expected to occur within 700 and 740 cm1. The CCl stretching of the G2 and G1 rotamers were attributed to the bands measured at 784 and 754.1 cm1, while no band was observed in the expectation range for the Cis form (calculated value 699 cm1). Some low intensity bands, not corresponding to any fundamental vibration, were measured in the Ar matrix and the same conclusions reported elsewhere in this work are likely to hold for these bands.
Molecular ions The study of the corresponding ionic species of the molecules considered in this work might be of some interest as these ions are produced through particular experimental techniques. The calculations accomplished at the 6-311++G(3df,3pd) level are useful to predict adiabatic ionization energies of the process M (1 X;m00 ¼ 0 ! Mþ ð2 X;m00 ¼ 0Þ) and the vibrational spectra of the M+ species. The calculated adiabatic ionization energies are reported in Table 7. Another result was accomplished from B3LYP/6-311++G(3df,3pd) harmonic frequencies of vibrations of each ionic form in its doublet state. Occurrence of changes of the vibrational spectra of the cationic species was theoretically observed: these changes regard both infrared intensities and vibrational wavenumbers. Restricting the discussion to the carbon–hydrogen stretching modes, P.E.D. analysis indicates, with respect to the neutral precursors, the occurrence of a different assignment for these vibrations for most of the ionic species, as reported in Table 7. It is quite interesting to mention that the carbon–fluorine, as well as CCl, CO and CS stretching frequencies, of the ions undergo large frequency shifts. These calculated shifts are actually the largest ones for these species (see Table 6). Also the shifts of the CF3 bending frequencies are expected not to be entirely negligible, as
L. Gontrani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 558–567
these vibrations are calculated at 545, 508 and 375 cm1 in the ionic form of trifluoro-oxirane. Among the remaining vibrations, one may also see that the torsional frequency of the CX3 group (X = H, F, C) increases for methyltiirane (from 193 to 227 cm1), G1+ (from 84 to 91 cm1) and G2+ (from 87 to 92 cm1) rotamers but, on the contrary, the torsional mode frequency decreases for methyloxirane (from 263 to 211 cm1), trifluoro-oxirane (from 93 to 75 cm1) and for Cis+ (from 108 to 88 cm1). Conclusions This work is a reassessment of previous spectroscopic investigations on methyloxirane, methyltiirane and epichlorohydrin. The reinvestigation provides a more detailed and sometime new assignment of the infrared spectra measured in Ar matrix at 12 K. Furthermore, this study includes the study of the vibrational spectrum of trifluorooxirane, never studied before using matrix isolation spectroscopy. The results accomplished are actually expected to provide a more sound and detailed assignment of the observed FT-IR bands of the molecules since the vibrational analysis is based on high level density functional theory computations employing extended basis sets. Among the results reported of this study, some theoretical conclusions based on B3LYP/6311++G(3df,3pd) calculations are also reported for all the molecular ions of the oxiranes considered in this work.
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
[29] [30] [31] [32] [33] [34] [35] [36] [37]
[38] [39] [40] [41] [42]
Acknowledgements [43]
The Authors acknowledge CASPUR for CPU time grants (std12034-std12-011); L.G. acknowledges financial support from ‘‘FIRB’’ Futuro in ricerca (RBFR086BOQ_001); financial support by MIUR (PRIN 2009) is also acknowledged.
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Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy journal homepage: www.elsevier.com/locate/saa
FTIR spectra and density functional theory P.E.D. assignments of oxiranes in Ar matrix at 12 K L. Gontrani a,⇑, S. Nunziante Cesaro b, S. Stranges c,d, L. Bencivenni b, A. Pierett e,1 a
CNR-ISM, Istituto di Struttura della Materia, Via del Fosso del Cavaliere 100, 00133 Roma, Italy Dipartimento di Chimica, Università di Roma La Sapienza, P.le A. Moro 5, 00185 Roma, Italy c Dipartimento di Chimica e Tecnologie del Farmaco, Università Sapienza, P.le A. Moro 5, 00185 Roma, Italy d IOM-CNR, TASC Laboratory, Basovizza, Trieste, Italy e CASPUR, Via dei Tizii 6, 00185 Roma, Italy b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Trifluoroepoxypropane is studied in
argon matrix for the first time. Some vibrational modes were
reassigned. DFT calculations employed the
6-311++G(3df,3pd) basis set. Potential energy distribution (PED) is
performed with GAMESS and VIBCA.
a r t i c l e
i n f o
Article history: Received 25 July 2013 Received in revised form 3 December 2013 Accepted 4 December 2013 Available online 10 December 2013 Keywords: FTIR spectra of oxiranes in argon matrix DFT calculations Vibrational assignment P.E.D. analysis
a b s t r a c t The FTIR spectra of a series of oxiranes were studied in Ar matrix at 12 K. The interpretation of the spectra was accomplished on the basis of density functional theory calculations employing the 6-311++G(3df,3pd) basis set with the B3LYP functional. Potential energy distribution was carried out for each molecule employing the B3LYP/6-311++G(3df,3pd) force field and a non-redundant definition of internal coordinates. The study of the FTIR spectra led to the reassignment of some vibrational modes of the molecules. The FTIR spectrum of trifluoroepoxypropane measured in Ar matrix and its assignment is reported for the first time. Ó 2013 Elsevier B.V. All rights reserved.
Introduction Oxiranes, thiiranes and their derivatives present a variety of industrial applications since they are quite stable but their ring can be easily opened by acids or bases in catalytic reactions or by electrophiles and nucleophiles in stoichiometric reactions in order to synthesize various complex organic molecules and macro⇑ Corresponding author. Tel.: +39 3387615798. 1
E-mail address: [email protected] (L. Gontrani). Present address: CINECA, via dei Tizii 6, 00185 Roma, Italy.
1386-1425/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.saa.2013.12.005
molecules [1–4]. The ring can be also opened on solids and therefore employed for synthesis of epitaxial layers through Metal Organic Chemical Vapour Deposition (MOCVD) [5,6]. The increasing interest for practical applications of oxirane, thiiranes and derivates has been paralleled by an increasing attention to their fundamental properties. Among chiral molecules, methyloxirane is one of the smallest and is considered a good prototype for theoretical and experimental studies focused on chiral effects and chiroptical activities, due to the presence of a stereogenic center, that is bonded to the methyl group [7–13].
L. Gontrani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 558–567
The first determination of the equilibrium configuration of the molecule is due to Swalen and Herschbach [14] who, from microwave spectroscopy measurements, evaluated the height of the torsional barrier of the methyl group (2.71 kcal mol1). Afterwards, far infrared spectral data of methyoxirane in its gaseous and solid states provided an average torsional barrier of 2.573 ± 0.023 kcal mol1 and 3.05 kcal mol1, respectively [15]. Vibrational spectroscopy studies were also performed in all the aggregation states of the molecule [16,17], in CS2 and CCl4 solutions [18] or in N2 [17,18] and Ar matrix [18]. Lowe et al., however, did not report the spectrum of methyloxirane isolated in Ar matrix because of its complexity with respect to the N2 matrix [17,18]. Particular attention was always devoted to the high frequency region (3250–3100 cm1) where the carbon–hydrogen stretching modes are observed together with a number of combination bands of the methyl group torsion and asymmetric stretching vibrations whose presence makes their attribution difficult. The first assignment [13] was revised by Winter and Hummel [19] studying gaseous methyloxirane using high spectral resolution. Their assignment was confirmed in a recent paper where the low resolution infrared spectrum of room temperature and jet cooled methyloxirane was compared to results of high resolution spectroscopy [20]. Methylthiirane shares with methyloxirane the same conformational rigidity of the three-membered ring characterizing the molecule. In the past years, particular attention was devoted to the study of the methyl torsional vibrations [15,21–24]. The available spectroscopic data allowed the evaluation of the torsional barrier of the methyl group (3.26 kcal mol1), in agreement with previous microwave data providing 3.24 kcal mol1 [25]. Only later, the complete spectrum of liquid and Ar matrix-isolated methylthiirane was experimentally and theoretically studied [26,27]. More recently, gas phase electron diffraction data and theoretical calculations were used to determine the structure of methylthiirane in order to explain the ease of ring opening during polymerization [6]. Epichlorohydrin, at present largely employed for the preparation of epoxy resins [28] has been object of structural investigations mainly dealing with conformational equilibria in different physical states of the molecule. This conformationally flexible molecule was extensively studied in all its physical states using infrared spectroscopy [29–36], NIR-VCD [37], nuclear magnetic resonance (NMR) [38], microwave spectroscopy [39], electron diffraction [40], dipole moment measurement [41], angle resolved photoelectron spectroscopy [42], and electron velocity map imaging [43]; the optical rotatory dispersion was determined by a theoretical study [44]. All the quoted investigations were concerning with the determination of the stability of the conformers of the molecule, namely Gauche-2 (G2), Gauche-1 (G1) and Cis (Cis), their relative abundance in gaseous, liquid and solid states and just for their solutions, the influence of solvent polarity on the conformational population. The G2 conformer is the predominant gas-phase conformer, while the G1 and Cis rotamers are less abundant. Studies of the infrared spectra based their conclusion on the comparison of calculated and experimental relative intensities of multiplets (doublets and triplets) observed in the spectral ranges 980–930, 850–830, 795–760, and 530–400 cm1. As a further support, the torsional modes of both G-type conformers were observed and assigned in the far infrared spectrum of gaseous epichlorohydrin. Infrared and Raman studies performed on solid and liquid epichlorohydrin, confirmed the presence of G1 in the former physical state. Furthermore, the observation of doublets and triplets in the spectrum of the molecule in its liquid phase, suggested the simultaneous presence of G1 and G2 rotamers together with a minor amount of Cis form [32,36]. These results were afterwards confirmed by spectroscopic studies in liquid state at variable temperature or in liquid
559
xenon solutions [33]. Analogous conclusions were achieved studying the molecule in different solvents [35]. The populations of G2 and G1 conformers are strongly dependent on the polarity of the solvent, which, in contrast, does not affect that of the Cis conformer. Quite recently, infrared and Raman spectroscopy have been utilized to determine the fundamental vibrations of solid epichlorohydrin whose assignment is supported by theoretical calculations [44]. Studies on chiroptical properties and electronic structure of the rotamers of epichlorohydrin have recently been carried out in the valence ionization [42,43] and near infrared regimes [44]. Surprisingly, no fundamental data are found in literature concerning the structural and vibrational behaviour of 1,1,1-trifluoroepoxypropane despite its applicative importance in the synthesis of chain-end functionalized polymers [45]. For this reason, the vibrational absorptions of 1,1,1-trifluoroepoxypropane isolated in Ar matrix were measured in the 4000–400 cm1 spectral range. In addition, the complete vibrational spectra of methyloxirane, methyltiirane and epichlorohydrin in their pseudo-gaseous phase have been obtained and the data compared with previous ones when available. The experimental data reported in this work were interpreted by a theoretical treatment based on density functional theory for geometry optimizations and harmonic frequency calculations carried out for all the molecular species using extended basis sets. Materials and methods The experimental apparatus consists of a IFS 113v Fourier Transform Interferometer (Bruker) and a Cryostat (Displex CSA-202, Air Products and Chemicals) equipped with a gold plated copper coldfinger, free to rotate in a high vacuum shroud. The vacuum chamber is connected under rotary vacuum (103 torr) to the interferometer. Infrared spectra (range 4000–50 cm1) were recorded in reflection cumulating at least 200 scans at a resolution of 1 cm1 or better. High purity samples of methyloxirane, epichlohydrin, methylthiirane and 1,1,1-trifluoroepoxypropane were supplied by Sigma–Aldrich. The samples were vaporized in the temperature range 273–363 K from a suitable device and vapours were deposited onto a reflecting copper surface kept at 12 K. High purity argon (Linde) was employed as isolating gas. Annealing cycles, performed at 35 K, did not produce appreciable spectral changes ruling out presence of matrix aggregates. Computational section Calculations were run at CASPUR on an HP cluster, based on AMD Opteron CPUs. The lowest energy equilibrium structures and harmonic frequencies of vibration of all the molecules were calculated at the B3LYP and MP2 levels using the 6-311++G**, 6-311G++(3df,3pd) and aug-cc-PVDZ basis sets employing the GAUSSIAN-03 program package [46]. Calculated infrared spectra depend on the theoretical level as well as on the basis set because both influence vibrational frequencies and band intensities of molecular normal modes. The most satisfactory and homogeneous results were obtained for all the molecules at the B3LYP/6-311G++(3df,3pd) level providing the best agreement between calculated vibrational wavenumbers and relative infrared band intensities. Vibrational assignments of the computed harmonic frequencies were accomplished on the ground of potential energy distribution analysis carried out for each molecule at the B3LYP/6-311++G(3df,3pd) level. Harmonic frequencies of vibration were scaled adopting the scale factors suggested in literature [47,48].
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Fig. 1. Oxiranes: atom numbering.
Fig. 2. (red) FT-IR spectrum of methyloxirane (Ar matrix, 12 K); (blue) DFT spectrum. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 Internal coordinate definition.a S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 S19 S20 S21 S22 S23 S24 a
Stretching Stretching Stretching Stretching Stretching Stretching Stretching Stretching Stretching Stretching Bending Bending Bending Bending Bending Bending Bending Torsion Torsion Torsion Torsion Torsion Torsion out-Plane
C1H8 C3H5 C3H6 C1H9 C1H10 C2H7 C2C3 C1C2 C2O4 C3O4 H8C1H10 H5C3O4 H5C3H6 H9C1H8 H10C1H9 H7C2O4 C1C2O4 H8C1C2C3 H5C3O4C2 H6C3O4C2 H9C1C2C3 H10C1C2C3 H7C2O4C3 C1C3O4C2
(C1F8)
(C1Cl8)
(C1F9) (C1F10)
(C1Cl9) (C1Cl10)
(C2S4) (C3S4) F8C1F10 H5C3S4 F9C1F8 F10C1F9 H7C2S4 C1C2S4 F8C1C2C3 H5C3S4C2 H6C3S4C2 F9C1C2C3 F10C1C2C3 H7C2S4C3 H7C2S4C3
Cl8C1H10
Cl9C1H8 Cl10C1H9
Cl8C1C2C3
Cl9C1C2C3 Cl10C1C2C3
Atom numbering is given in Fig. 1.
P.E.D. analysis was calculated using VIBCA program [49] implementing the force constant matrix and coordinate transformations described in Califano’s book [50]. Force constants and coordinate sets were obtained from GAMESS software [51] after re-optimizing the molecular geometry and re-calculating the vibrational frequencies of each species. P.E.D. contributions below 10% are not reported in the paper. The molecular model of the molecules, including atom numbering, is shown in Fig. 1 and the set of not redundant internal coordinates is given in Table 1. Results and discussion
Table 2 FT-IR spectra of methyloxirane: experimental (Ar matrix, 12 K), B3LYP/6311++G(3df,3pd) vibrational frequencies (cm1) and vibrational assignment based on P.E.D. analysis.a Calc.
Scaled
Exp.
Description of the normal mode
3168 3113 3092 3087 3082 3029
3064 3011 2991 2986 2981 2930
S3 S1 S6 S5 S2 S4
1529
1479
1498 1484
1449 1435
1438 1408 1294 1188 1168 1158 1130
1391 1362 1252 1149 1130 1120 1093
1043 974 908
1009 976 910
844 773 411 368 211
846 774 412 369 211
3055.4 3001.6 2972.1 2935.5 2907.3 2874.4 2805.0 1500.5 1475.7 1459.1 1447.0 1440 1410.2 1370.5 1269.7 1174.7 1147.3 1137.7 1107.0 1056.4 1046.9 1027.0 953.4 898.0 879.8 833.3 745.8 411.6 375.5 213.1
52% 52% 68% 51% 50% 41%
S2 S6 S1 S4 S3 S5
47% 29% S5 10% 24% 48% 47% 35%; S1 23%
S13 69% S11 41%; S15 33%; S22 13% S14 46%; S11 20%; S15 10%; S18 10% S16 S14 S16 S23 S19 S20 S12
20%; 32%; 39%; 28%; 26%; 34%; 47%;
S23 17%; S15 16%; S8 12% S1519%; S11 18% S9 15%; S7 13%; S10 10% S20 13%; S16 11%; S12 10% S12 14%; S21 10%; S24 10% S19 27% S18 17%
S12 22%; S18 21%; S23 12%; S16 10% S10 39%; S8 27%; S21 10% S19 25%; S20 21% S7 56%, S9 19% S9 36%; S10 21%; S8 17%; S21 10% S24 41%; S17 35%; S22 10% S17 46%; S24 39% S21 38%; S22 35%; S18 21%
a Atom numbering is shown in Fig. 1 and internal coordinates definition is given in Table 1.
Methyloxirane The infrared spectra of methyloxirane in gaseous, liquid, solution and solid state have been extensively studied in the previous years [14–19]. The spectra of the single molecule in N2 [17,18] and argon low temperature matrices [18] were investigated. Lowe, however, did not report in his paper the spectrum of methyloxirane in argon matrix because of its complexity with respect to the nitrogen environment, attributed to site heterogeneity [18]. In
the present paper the vibrational spectrum of methyloxirane isolated in argon matrix measured in the range 4000–50 cm1 is reported and discussed for the first time. Surprisingly enough, frequency values and relative intensity of the spectrum of solid methyloxirane studied by Kirchner [16] presents a very good matching with the results of the spectrum in argon matrix. The infrared spectrum of methyloxirane isolated in argon matrix is shown in Fig. 2, where it is compared with the DFT spectrum
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calculated at the B3LYP/6-311++G(3df,3pd) level, and the experimental frequencies are listed in Table 2 along with the theoretical results (calculated frequencies and corresponding P.E.D. analysis). It is worth mentioning that comparison among spectra obtained from different experimental conditions shows a very good agreement in spite of the physical state of the molecule. The spectra of methyloxirane in all its physical states, in solution as well as in pseudo gaseous phase are indeed quite similar, as only minor frequency shifts and relative band intensity changes are observed, thus excluding occurrence of whatsoever intermolecular interaction. The assignment of some of the fundamental bands needs sometimes to be reconsidered in the light of more sophisticated calculations, in particular for the modes falling in the high frequency range where the CH stretching modes are expected. The present calculations do not fully agree with those reported by Lowe et al. [18] carried out at the HF/4–31G level. The disagreement mainly regards the assignment of some of the CH stretchings of the molecule, as one may realize from comparing our revised assignment with the previous one [18]. This fact is evidently due to the HF-SCF approximation and to the use of rather small basis sets. Furthermore, the not entirely matching between our conclusions and the earlier ones is also due to scaling procedure adopted by Lowe et al. [18]. The combined effect of both these facts might be responsible of the failure of the HF approximation when vibrational coupling is considerable as for this molecule. According to the revised assignment, the peaks at 3055.4 and 2907.3 cm1 are attributed to the antisymmetric and symmetric stretchings of the CH2 group, respectively, the band 3001.6 cm1 to the antisymmetric stretching of the CH3 group, strongly coupled with the CH bond stretching. Among the remaining bands, that measured at 2972.1 cm1 is attributed to the CH bond stretching, coupled with the CH3 group stretchings, and those at 2935.5 and 2874.4 cm1 are related to the antisymmetric and symmetric stretching vibrations of the methyl group, respectively. Therefore, for this molecule the carbon–hydrogen stretching frequencies are distributed as follows mas CH2 > mas CH3 > m CH > mas CH3 > ms CH2 > ms CH3. The matrix spectrum under consideration shows a number of weak and very weak intensity bands quite likely attributable to combinations or overtones, having no importance for the purpose of this work. On the other hand, no conclusive assignment can be reached for these absorptions because several assignments might be proposed for them. An improvement of the description of the molecular vibrations is reached for several bending and ring stretching modes (see Table 2) often showing vibrational coupling among them. For instance, the band measured at 1500.5 cm1 is described as the bending of the CH2 group of the molecule having small vibrational coupling ( m CH (3048.5 cm1) > ms CH2 (2994.8 cm1) instead of mas CH2 > m CH3 > m CH as established for methyloxirane. The CH3 group stretching frequencies of methyltiirane associated with the internal coordinates S1, S5 and S4 are observed in the matrix spectrum at 2988.1, 2971.3 and 2932.7 cm1. The two antisymmetric strechings of the methyl are predicted to occur at higher wavenumbers than the corresponding symmetric mode and show high vibrational coupling among them. In summary, the revised assignment for these vibrations reproduces that reported by Polavarapu et al. [26] based on HF/6–31G calculations, as well as that proposed by Alper and Lowe [52] and Polavarapu et al. [26]. Surprisingly enough, our B3LYP/6-311++G(3df,3pd) P.E.D. analysis does not confirm the conclusions reported for the carbon–hydrogen stretchings in a later investigation performed at the MP2 level [53]. As far as the remaining fundamentals are concerned, there are some differences between our P.E.D. analysis and the interpretation proposed by other authors [26,53]. The different conclusions reached in the previous studies evidently depend on the computational level, which, as assessed in advance in this work, is particularly sensitive to the combination of the method and of the basis set, resulting in not harmless consequences when some vibrational modes are closely lying. Actually, this fact occurs for the CH2 and CH3 bendings, calculated, and as expected observed, between 1500 and 1420 cm1. The present calculations show, in conformity with the results accomplished for methyloxirane and with a few exception, strong vibrational coupling among most of the modes. Thus, the carbon–sulphur stretchings are meaningfully coupled with the ring deformations. In particular, the C2S4 (S9) and C3S4 (S10) stretchings calculated at 639 and 602 cm1 and corresponding to the bands measured at 634.5 and 608.7 cm1, are strongly coupled with one another. For this molecule, the C1C2 stretching calculated at 881 cm1 and measured at 872.7 cm1 is largely mixed to the C2C3
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563
Table 4 FT-IR spectra of 1,1,1-trifluoroepoxypropane: experimental (Ar matrix, 12 K). B3LYP/ 6-311++G(3df,3pd) vibrational frequencies (cm1) and vibrational assignment based on P.E.D. analysis.a Calc.
Scaled
Exp.
Description of the normal mode
3205
3100
S2 56% S3 43%
3142
3039
3109
3007
1528
1478
1449
1402
1298
1256
1276 1171 1167 1157 1147
1234 1133 1129 1119 1109
1094 1011
1058 978
928 882
930 884
788
789
647 582 523
648 583 524
3093.0 3089.1 3087.8 3053.4 3023.6 2956.3 1490.2 1486.2 1452.5 1440.2 1433.2 1410.4 1402.0 1321.4 1294.6 1277.3 1261.0 1178.0 1173.2 1166.1 1141.6 1079.4 1074.2 1000.8 971.6 965.2 936.6 918.2 856.0 846.2 799.4 788.5 772.9 758.6 752 650.6 589.2 530.4
436 382 238 218 75
437 383 238 218 75
440.7 385.7 221.2
S6 98% S3 57% S2 42% S13 73%
S16 19% S8 16% S13 16% S23 16% S9 10%
S16 35% S1 17% S8 11% S23 10% S8 13%; S10 12%; S20 12%; S1 10% S19 50%; S20 16% S12 38%; S23 27%; S16 17% S5 45%; S20 11% S4 53% S12 47%; S16 12%; S23 10%; S20 10% S20 25%; S19 23%; S1 15%; S23 11%
S7 48%; S9 16%; S23 12% S10 60%; S9 23%
S1 23%; S8 15%; S5 14%
S15 S21 S11 S15 S14 S15 S24 S17 S18
23%; 31%; 32%; 11%; 45%; 43%; 47%; 46%; 96%
S4 14%; S17 10% S5 15%; S11 11% S14 16%; S22 15% S8 10% S17 16%; S11 22% S8 16%; S24 12% S22 35% S21 31%
a Atom numbering is shown in Fig. 1 and internal coordinates definition is given in Table 1.
carbon–carbon stretching (S7) and to the molecular torsion described by the internal coordinate S20. The matrix spectrum of the molecule shows a large number of weak and very weak absorptions for which the same conclusions already reported for methyloxirane are likely to hold. 1,1,1-Trifluoroepoxypropane In this paper we discuss the Ar matrix spectrum of trifluoroepoxypropane for the first time since no report of the vibrational spectrum of this molecule was found both in the mid- and farinfrared regions. Absorption bands of trifluoroepoxypropane isolated in Ar matrix at 12 K are reported in Table 4 together with the theoretical frequencies of vibration calculated at the B3LYP/ 6-311++G(3df,3pd) level and the corresponding P.E.D. analysis. It might be interesting to mention that the theoretical vibrational spectrum of this molecule is more affected than any other molecule of the series by the combination of computational method and basis set. This observation comes out when the DFT and
Fig. 4. (red) FT-IR spectrum of trifluorooxirane (Ar matrix, 12 K); (blue) DFT spectrum. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
MP2 theoretical frequencies and band intensities calculated from the different levels of theory are compared. The effect is particularly appreciable in the region 1270–1130 cm1 of the spectrum. Improvement between theory and experiment was at any rate achieved from B3LYP/6-311++G(3df,3pd) calculations, that reproduce nicely the measured FT-IR spectrum. The Ar matrix spectrum and the calculated one are shown in Fig. 4. P.E.D. analysis performed at the B3LYP/6-311++G(3df,3pd) level shows that the higher wavenumber bands measured in the carbon–hydrogen stretching region are due to the antisymmetric CH2 (3093.0 cm1), CH (3087.8 cm1) and symmetric CH2 (3023.6 cm1) stretching modes respectively, following thus the same order established for methyloxirane and methyltiirane. The CF stretching vibrations are strongly coupled among them as well as with other vibrations (see Table 4). The antisymmetric CF3 stretchings (1166.1 and 1141.6 cm1) occur at much higher wavenumbers than the corresponding symmetric stretching (788.5 cm1); the corresponding theoretical values for these stretching frequencies are 1157 cm1, 1147 cm1 and 788 cm1. The higher frequency antisymmetric mode is coupled with the torsional mode described by the internal coordinate S20 (H6C3O4C2 torsion) and the symmetric stretching, associated with the S1 and S5 internal coordinates, shows large coupling with the C1C2 stretching (S8) (see Table 4). All the frequencies ranging between 647 and 382 cm1 are related to S11, S14 and S15 bending coordinates of the CF3 group. In particular, the largest contributions of these coordinates are found in the frequencies computed at 523 cm1 (S11, 32%; S14, 16% and S15, 11%), 436 cm1 (S14, 45% and S11, 22%) and 382 cm1 (S15, 43%) and for this reason the CF3 bendings should be consequently attributed as follows: 530.4, 440.7 and 385.7 cm1. Also the bands calculated at 647 cm1 (exp. 650.6 cm1) and 582 cm1 (exp. 589.2 cm1) have a moderately large CF3 bending character, that is S15 (23%) for 647 cm1 and S11 (11%) for 582 cm1. The 647 cm1 frequency would thus be associated to a mode where the S15 internal coordinate predominates over S4 (CF stretching) and S17 (C1C2O4 bending) and the frequency 582 cm1 would be related to torsional mode of the CF bond with respect to the C1C2C3 axis. Concerning some frequencies lying between 1500 and 1200 cm1, the band at 1490.2 cm1 is without any doubt assigned
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Fig. 5. (a) G2, (b) G1 and (c) Cis rotamers of epichlorohydrin and (d) TS1 and (e) TS2 transition states.
to the CH2 bending involving H5 and H6 (S13 = 73%) and the band at 1433.2 cm1 to a complex mode of the molecule wherein the role of the S16 internal coordinate (OCH bending involving H7) is comparable to that of S8 (C1C2 stretching), S13 and S23 (see Table 4). The highest amount of the S16 coordinate is found in correspondence of the frequency 1294.6 cm1: this OCH bending mode is appreciably coupled with the C1C2 stretching as well as with the CH2 group bending. On these premises, the carbon–hydrogen bending mode of the CH2 group (calculated 1449 cm1, exp. 1433.2 cm1) is predicted at higher wavenumbers than the CH bending vibration (1298 cm1, exp. 1294.6 cm1). The frequency measured at 1261.0 cm1 would be related to the C1C2 stretching, coupled with the CF, CO stretchings and the torsional mode H6C3O4C2 (S20 internal coordinate). Among the bands just below 1200 cm1, those at 1178.0, 1173.2, 1079.4 and 1000.8 cm1 should be assigned to bending and torsional modes involving the CH2 and CH groups of the molecule (see details in Table 4). The further modes to be discussed are the CO and C2C3 stretchings, being the former mode attributed at 856.0 cm1 and the latter at 918.2 cm1 (coupled with the C2O4 stretching and the H7C2O4C3 torsion). The low frequencies calculated at 238, 218 (exp. 221.2 cm1) and 75 cm1 correspond to complex bending modes involving the carbon–fluorine bonds and that at 221.2 cm1 (calculated at 218 cm1) to an oxygen–carbon–carbon bending, strongly interacting with the FCC bending vibration. Epichlorohydrin Among oxirane derivatives, epichlorohydrin is one of the most extensively studied molecule in all its physical states
Fig. 6. (red) FT-IR spectrum of epichlorohydrin (Ar matrix, 12 K); (blue) DFT spectrum (the overall spectrum is the weighted sum of the theoretical spectra of the three rotamers). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
employing a number of techniques aiming to verify the presence of its conformers and their relative abundance. Only the most polar conformer G1 is present in solid phase [32,36] while all of them are simultaneously present in gas, liquid phases or in solutions. The G2 conformer should predominate over the G1 and Cis rotamers in gas phase and the amount of the Cis form is indeed small [33,43]. The same trend was confirmed diluting the molecule in liquid Xe at variable temperature [33]. Our MP4/6-311++G(3df,3pd), G2MP2 and G3MP2 calculations provide some further information about the Gauche-2, (G2) Gauche-1 (G1) and Cis conformers (see Fig. 5) suggesting that the three rotamers are not separated by high energy differences (see Table 5). These calculations agree that the G2 conformer is the lowest energy form and that the less stable G1 and Cis rotamers are 2.5 kJ mol1 and 5.4 kJ mol1 respectively higher in energy than G2. Moreover, the transition state TS1 between G2 and G1 and the corresponding one TS2, between G1 and Cis (see Fig. 5) are estimated relatively low from MP4/6-311++G(3df,3pd) single-point energy calculations. The conversion of the less stable G1 conformer into the most stable one G2 would require 8 kJ mol1, whereas the conversion of the Cis into the G1 form would require a higher amount, that is 15 kJ mol1. Gas-phase population at 298.15 K was estimated using the G2MP2 and
Table 5 Stability study of the stable conformers of epichlorohydrin at the MP4/6-311++G(3df,3pd) level for total electronic energy and B3LYP/&-311++G(3df,3pd) for zero-point energy and thermal corrections. DEe and DEo energy differences (kJ mol1) are the electronic energy differences and electronic plus zero-point energy differences. G2
G1
DE e 0 2.8 DE 0 0 2.7 DEe(G2 ? TS1) 17.7 14.1 DEe(G1 ? TS1) 11.3 DEe(G1 ? TS2) DE0(G2 ? TS1) (cis ? TS2) 14.6 10.5DE0(G1 ? TS1) 7.9 DE0(G1 ? TS2) G2MP2 and G3MP2 energy differences, calculated with respect to the G2 conformer and gas-phase conformer population at 298.15 K
Cis 5.5 5.4 20.4 DEe(cis ? TS2) 17.3 DE0
Energy differences (kJ mol1) G2MP2 (0 K) G2MP2 G3MP2(0 K) G3MP2
0 0 0 0
2.4 2.4 2.5 2.2
5.6 5.5 4.9 4.7
Gas-phase conformer population at 298.15 K G2MP2 G3MP2 Average
70% 68% 69%
24% 24.5% 24.25%
6% 7.5% 6.75%
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L. Gontrani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 558–567 Table 6 FT-IR spectra of epichlorohydrin: experimental (Ar matrix, 12 K), B3LYP/6311++G(3df,3pd) vibrational frequencies (cm1) and vibrational assignment based on P.E.D. analysis.a Calc.
Scaled
Exp.
Description of the normal mode
3197 3185 3177 3154 3150 3129 3124 3123 3102 3094 3089 3091 3084 3084 3071
Cis G2 G1 G2 G1 Cis G2 G1 Cis G2 G1 G2 G1 Cis Cis
3092 3081 3073 3051 3047 3027 3022 3021 3001 2993 2988 2990 2983 2983 2971
3071 3059.2 3032 3032 3032 3012.5 3012.5 3012.5
S2 S3 S3 S1 S1 S4 S6 S6 S3 S2 S2 S5 S4 S6 S6
1528 1526 1529
G1 Cis G2
1478 1476 1479
1490 1470 1472 1439 1430 1432
G2 G1 Cis G2 G1 Cis
1441 1422 1424 1392 1383 1385
1322 1304 1298 1294 1285 1277
Cis G2 Cis G1 G1 G2
1261 1256 1252 1243 1235
1307.7 1307.7 1277.5 1277.5 1269.1
1233 1228
Cis G1
1193 1188
1211 1176 1174 1168 1162 1157 1158 1113 1107 1096
G2 G2 Cis G1 G1 Cis G2 G2 G1 Cis
1171 1138 1136 1130 1124 1119 1120 1077 1071 1060
1256.6 1256.6 1249 1193 1147.4 1147.4 1147.4 1134.8 1134.8 1134.8 1107.1 1107.1 1107.1
1095 1072
G1 G2
1037 1037
1096.7 1027.1
1041 988 983 948 931 919 890
Cis Cis G2 G1 G1 Cis G2
990 985 950 933 921 892
970.4 966.4 952.7 934.8 914 894.3
863
G2
865
856.3
863 851 816 795 791 739
G1 Cis G2 Cis G1 G2
865 853 818 797 793 740
726
G1
727
846.2 840.1 833.1–830 797.7 797.7 784 771.1 754.1
2990.7 2972.6 2972.6 2972.6 2972.6 2936.3 2906.4 2894 1511.9 1511.9 1511.9 1486.2 1476.6 1458.5 1452.4 1452.4 1447.6 1410.0 1410.0 1403
64%; 54%; 53%; 55%; 59%; 51%; 90% 90% 63%; 55%; 52%; 59%; 62%; 50%; 47%;
S3 S2 S2 S5 S4 S5
35% 44% 44% 36% 32% 49%
S2 S3 S3 S1 S1 S5 S4
35% 45% 45% 40% 35% 25%; S4 22% 27%; S5 26%
S13 78% S13 70% S13 75%
S11 S14 S15 S23 S16 S16
82% 79% 85% 22%; S13 12% 28%; S23 17%; S13 10% 22%; S23 21%; S13 17%
S21 S15 S22 S16 S21 S16 S16 S11 S11
42%; 10% 30%; 46% 38%; 28%; 27%; 44%; 44%
S22 20%; S8 11%: S18 16%; S16 14% S18 22%; S16 14% S9 19%; S10 18%; S7 11% S22 14%; S23 11% S21 17%; S22 15%
S14 37%; S8 13%; S18 13% S12 18%; S16 24%; S23 23% S20 39%; S12 21%; S19 19% S20 47%; S19 23% S12 41%; S23 25%; S19 11% S19 36%; S12 21%; S23 18% S19 46%; S20 26% S12 48% S8 22%; S11 14%; S19 17%; S21 17% S12 48% S12 48%; S23 18%; S16 10% S22 20%; S23 18%; S12 13% S14 12%; S8 10% S19 20%; S23 19%; S8 16%; S20 14% S10 21%; S8 20%; S21 12%; S7 10% S8 23%; S10 23%; S7 11%; S20 10% S7 29%; S19 23%; S10 11% S18 27%; S10 32%; S8 15% S10 32%; S18 15%; S8 10% S20 21%; S19 20%; S18 16%; S14 13%; S23 11% S9 35%; S7 29%; S10 15% S7 37%; S20 23% S7 49%; S9 16%; S22 10% S7 28%; S10 26%; S22 10% S9 20%; S1 14%; S10 14%; S9 43%; S10 20%; S8 13% S4 70%; S17 13%; S15 10% S5 69%; S15 12%
Table 6 (continued) Calc.
Scaled
Exp.
Description of the normal mode
748b 740 699 521 439 408 371
Cis Cis G1 G2 G2
522 440 409.3 373.0
364 355 212 210 204 108 97 90
G1 Cis G2 G1 Cis Cis G1 G2
365 356 212 210 204 108 97 90
520 440.4
221.0 221.0
S1 45%; S8 15%; S14 12% S1 30%; S24 29%; S14 17%; S17 10% S17 S24 S24 S24 S17 S15 S15 S14 S18 S22 S21
55%; 37%; 39%; 51%; 61%; 55%; 53%; 51%; 63%; 75% 76%
S15 14% S15 17% S17 21%; S4 13% S5 15% S22 13%; S18 10% S17 35% S24 18%; S17 21% S24 36% S22 17%
a Atom numbering is shown in Fig. 1 and internal coordinates definition is given in Table 1. b 37 Cl shift.
G3MP2 standard Gibbs free-energy differences values (DG°); both methods yield almost identical results (see Table 5). This information is of great importance because each rotamer contributes to the actual theoretical spectrum (the weighted sum of the spectra of the three rotamers) according to the respective relative abundance. The calculated infrared spectrum is shown in Fig. 6 together with the FTIR spectrum measured in Ar matrix. The calculated infrared spectrum predicts the overlap not just of bands calculated at the same wavenumber but even of some close-lying frequency bands. In the present work of the infrared spectrum of Ar matrix isolated epichlorhydrin is analyzed for the first time. As matrix isolation should reflect the stability in the gas phase and the vapor composition at the vaporization temperature, the experimental results are compared with previous data obtained for the vapor phase and for low-temperature Xe solutions. The summary of the observed frequencies, reported in Table 6, was based on the results of the theoretical results keeping into account the calculated frequencies of vibration and the respective IR band intensities, although the latter ones must be handled with particular care because they strongly depend on the computational level (method and basis set). Irrespective of the rotamer, all the antisymmetric CH stretchings of the CH2 and CH2Cl groups occur at higher frequencies than the corresponding symmetric modes. Thus, these modes are distributed as follows: mas CH2 > mas CH2Cl > m CH > ms CH2 > ms CH2Cl, confirming the previous assignment based on MP2/6–31G* calculations [31]. The assignment of the bands lying between 1500 and 1200 cm1 does not show appreciable differences with respect to the most recent study [33]. In particular, the bands observed in the range 1500–1400 cm1 are, in conformity with the results reported for the other oxyranes, due to the CH bending vibrations of the molecule, distributed as follows. m CH2 > m CH2Cl > m C2H7 bending. Concerning the CH bendings, the highest bending frequencies suggested by the calculations are the CH2 group bendings (calculated 1530 cm1; observed 1511.9 cm1), followed by the CH bendings of the CH2Cl group (calculated 1480 cm1; observed 1450 cm1) and by the bands around 1430 cm1, due to the C2H7 bendings (S23 and S16) strongly coupled with the S13 bending coordinate for the CH2 group. Our revised assignment accomplished from the B3LYP/6311++G(3df,3pd) P.E.D. analysis allows a better interpretation of the ring modes of the three rotamers. Aware of the unquestionable evidence of the high vibrational coupling characterizing all the ring modes, the bands of the Cis form calculated at 988
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Table 7 Adiabatic ionization energiesa of oxiranes (M (1 X;m00 ¼ 0 ! Mþ ð2 X;m00 ¼ 0Þ and assignment of the carbon–hydrogen stretching frequencies of the molecular ions. MeO/MeO+ 10.03 eV (80864 cm1) G1/G1+ 10.09 eV (81424 cm1) MeO+ 3139 MeO 3168 MeT+ 3177 MeT 3202 TFO+ 3162 TFO 3205 G1+ 3222 G1 3177 G2+ 3229 G2 3185 Cis+ 3162 Cis 3197
MeT/MeT+ 8.70 eV (70160 cm1) G2/G2+ 10.05 eV (81096 cm1) CH stretching modes (cm1)
TFO/TFO+ 10.95 eV (88317 cm1) Cis/Cis+ 9.98 eV (80498 cm1)
masCH3
3131
mas CH2
3098
mas CH3
3073 m CH
3031
ms CH2
3013 ms CH3
mas CH2
3113
mas CH3
3092
m CH
3087 mas CH3
3082
ms CH2
3029 ms CH3
mas CH2
3135
mas CH3
3123
m CH
3096 mas CH3
3088
ms CH2
3026 ms CH3
mas CH2
3139
m CH
3115
ms CH2
3106 mas CH3
3083
mas CH3
3019 ms CH3
mas CH2
3054
ms CH2
3035
m CH
mas CH2
314 2
m CH
3109
ms CH2
mas CH2
3157
mas CH2Cl 3121
m CH
3109 ms CH2
3053
ms CH2Cl
mas CH2
3150
mas CH2Cl 3123
m CH
3089 ms CH2
3084
ms CH2Cl
mas CH2
3200
mas CH2Cl 3121
ms CH2Cl
3119 m CH
3088
ms CH2
mas CH2
3154
mas CH2Cl 3124
m CH
3094 ms CH2
3091
ms CH2Cl
mas CH2Cl 3157
mas CH2
3098
m CH
3084 ms CH2Cl 2984
ms CH2
mas CH2
mas CH2Cl 3102
m CH
3084 ms CH2
ms CH2Cl
3129
3071
Other stretching modesb (cm1)
m CO MeO MeO+ MeT MeT+ TFO TFO+ G1 G1+ G2 G2+ Cis Cis+ a b
m CS
m CF
m CCl
741–632 974–844 639–602 578 882 688–596 791 841 863–812 841 795 844
788–1157-1147 813–1262-1250 726 782 739 803 699 740
The values include zero point energy corrections. The largest P.E.D. contributions are reported.
and 919 cm1, as well as those at 983 (G2), 931 (G1) cm1 and 863 cm1 (G2), have a high CO stretching character, most of them are highly coupled with the C1C2 and C2C3 stretchings (S8, S7) and even with some torsional modes. For instance, the band calculated at 919 cm1 for the Cis rotamer has a higher C3O4 stretching (S10) character (32%) than the band at 988 cm1 (21%), whereas the 983 cm1 band of G2, the internal coordinates for the C3O4 (S10) and C1C2 (S8) stretchings have actually both the same contribution. As regards the C2C3 stretching, the major contribution of the internal coordinate S7 is present in the bands computed at 851 cm1 (Cis), 863 cm1 (G1 and G2) as well as at 816 cm1 (G2) and 948 cm1 (G1). The most valuable C2O4 stretching contribution S9 is observed at 795 cm1 (Cis) and 791 cm1 (G1) and 863 cm1 (G2). At last, the CCl stretching modes, calculated in the narrow range of frequency for the three rotamers, are expected to occur within 700 and 740 cm1. The CCl stretching of the G2 and G1 rotamers were attributed to the bands measured at 784 and 754.1 cm1, while no band was observed in the expectation range for the Cis form (calculated value 699 cm1). Some low intensity bands, not corresponding to any fundamental vibration, were measured in the Ar matrix and the same conclusions reported elsewhere in this work are likely to hold for these bands.
Molecular ions The study of the corresponding ionic species of the molecules considered in this work might be of some interest as these ions are produced through particular experimental techniques. The calculations accomplished at the 6-311++G(3df,3pd) level are useful to predict adiabatic ionization energies of the process M (1 X;m00 ¼ 0 ! Mþ ð2 X;m00 ¼ 0Þ) and the vibrational spectra of the M+ species. The calculated adiabatic ionization energies are reported in Table 7. Another result was accomplished from B3LYP/6-311++G(3df,3pd) harmonic frequencies of vibrations of each ionic form in its doublet state. Occurrence of changes of the vibrational spectra of the cationic species was theoretically observed: these changes regard both infrared intensities and vibrational wavenumbers. Restricting the discussion to the carbon–hydrogen stretching modes, P.E.D. analysis indicates, with respect to the neutral precursors, the occurrence of a different assignment for these vibrations for most of the ionic species, as reported in Table 7. It is quite interesting to mention that the carbon–fluorine, as well as CCl, CO and CS stretching frequencies, of the ions undergo large frequency shifts. These calculated shifts are actually the largest ones for these species (see Table 6). Also the shifts of the CF3 bending frequencies are expected not to be entirely negligible, as
L. Gontrani et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 120 (2014) 558–567
these vibrations are calculated at 545, 508 and 375 cm1 in the ionic form of trifluoro-oxirane. Among the remaining vibrations, one may also see that the torsional frequency of the CX3 group (X = H, F, C) increases for methyltiirane (from 193 to 227 cm1), G1+ (from 84 to 91 cm1) and G2+ (from 87 to 92 cm1) rotamers but, on the contrary, the torsional mode frequency decreases for methyloxirane (from 263 to 211 cm1), trifluoro-oxirane (from 93 to 75 cm1) and for Cis+ (from 108 to 88 cm1). Conclusions This work is a reassessment of previous spectroscopic investigations on methyloxirane, methyltiirane and epichlorohydrin. The reinvestigation provides a more detailed and sometime new assignment of the infrared spectra measured in Ar matrix at 12 K. Furthermore, this study includes the study of the vibrational spectrum of trifluorooxirane, never studied before using matrix isolation spectroscopy. The results accomplished are actually expected to provide a more sound and detailed assignment of the observed FT-IR bands of the molecules since the vibrational analysis is based on high level density functional theory computations employing extended basis sets. Among the results reported of this study, some theoretical conclusions based on B3LYP/6311++G(3df,3pd) calculations are also reported for all the molecular ions of the oxiranes considered in this work.
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The Authors acknowledge CASPUR for CPU time grants (std12034-std12-011); L.G. acknowledges financial support from ‘‘FIRB’’ Futuro in ricerca (RBFR086BOQ_001); financial support by MIUR (PRIN 2009) is also acknowledged.
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