Article pubs.acs.org/JPCB
Rydberg and π−π* Transitions in Film Surfaces of Various Kinds of Nylons Studied by Attenuated Total Reflection Far-Ultraviolet Spectroscopy and Quantum Chemical Calculations: Peak Shifts in the Spectra and Their Relation to Nylon Structure and Hydrogen Bondings Yusuke Morisawa,*,† Manaka Yasunaga,‡ Harumi Sato,§ Ryoichi Fukuda,∥,⊥ Masahiro Ehara,*,∥,⊥ and Yukihiro Ozaki‡ †
Department of Chemistry, School of Science and Engineering, Kinki University, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Gakuen, Sanda, Hyogo 699-1337, Japan § Graduate School of Human Development and Environment, Kobe University, Tsurukabuto, Nada-ku, Kobe, Japan ∥ Institute of Molecular Science and Research Center for Computation of Science, Nishigo-Naka, Myodaiji, Okazaki, Aichi 444-8585, Japan ⊥ Elements Strategy Initiative for Catalysis and Batteries (ESICB), Kyoto University, Kyoto 615-8520, Japan ‡
S Supporting Information *
ABSTRACT: Attenuated total reflection far-ultraviolet (ATRFUV) spectra in the 145−260 nm region were measured for surfaces (thickness 50−200 nm) of various kinds of nylons in cast films to explore their electronic transitions in the FUV region. ATR-FUV spectra show two major bands near 150 and 200 nm in the surface condensed phase of nylons. Transmittance (Tr) spectra were also observed in particular for the analysis of valence excitations. Time-dependent density functional theory (TD-DFT/CAM-B3LYP) calculations were carried out using the model systems to provide the definitive assignments of their absorption spectra and to elucidate their peak shifts in several nylons, in particular, focusing on their crystal alignment structures and intermolecular hydrogen bondings. Two major bands of nylon films near 150 and 200 nm are characterized as σ-Rydberg 3p and π−π* transitions of nylons, respectively. These assignments are also coherent with those of liquid n-alkanes (n = 5−14) and liquid amides observed previously. The Rydberg transitions are delocalized over the hydrocarbon chains, while the π−π* transitions are relatively localized at the amide group. Differences in the peak positions and intensity were found in both ATR- and Tr-FUV spectra for different nylons. A red-shift of the π−π* amide band in the FUV spectra of nylon-6 and nylon-6/6 models in α-form is attributed to the crystal structure pattern and the intermolecular hydrogen bondings, which result in the different delocalization character of the π−π* transitions and transition dipole coupling.
1. INTRODUCTION It has become clear that far-ultraviolet (FUV) spectroscopy holds great potential in elucidating electronic transitions and structure of a wide range of molecules in condensed phases.1−4 FUV spectroscopy in the 145−200 nm region has recently been a matter of keen interest because many kinds of organic5−9 and inorganic10−15 materials in the condensed phase, even liquid alkanes, show bands due to electronic transitions in the FUV region. Importantly, the spectroscopy is powerful not only for basic studies but also for various applications such as monitoring of the quality of semiconductor wafer cleaning.3,16,17 More recently, the electronic states and photocatalytic activities of metal-modified TiO2 were examined by using ATRFUV spectroscopy.14,15 Such rapid progress in FUV spectroscopy has been stimulated by the development of an attenuated © 2014 American Chemical Society
total reflection (ATR)-FUV spectrometer, which has enabled us to measure the spectra in the complete FUV region for liquid and solid samples without facing problems such as peak saturation.3,4 In the previous studies we investigated the ATR-FUV spectra of water,10−12 alcohols,5 several kinds of n- and branched alkanes,6,7 ketones,8 and amides9 observed in the liquid phase. The FUV spectra of n-alkanes gave a broad feature near 150 nm, while the branched alkanes show an additional shoulder at around 180 nm. The 150 nm band shows a lower energy shift with a significant intensity increase as the alkyl chain length Received: July 31, 2014 Revised: September 7, 2014 Published: September 9, 2014 11855
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increases. We investigated assignments of absorption bands for the n-alkanes and branched alkanes in the liquid states and the cause of lower energy shift of the 150 nm feature by comparing the experimental results with quantum chemical calculations.7 It was found that the 150 nm band of n-alkanes arises from the σRydberg 3py transition, and its longer wavelength shift is caused by the destabilizing effect of next highest occupied molecular orbital (HOMO) level and the stabilization of Rydberg 3p level, which is expanded as the carbon chain increase. These studies of FUV spectra of alkanes have provided new insight into Rydberg states and transitions of liquid alkanes.7 The FUV region of amides in the liquid phase is also very important because a valence−Rydberg coupling is expected for a π−π* transition and a direct interaction occurs via a hydrogen bond. Thus, we studied the electronic transitions of five kinds of amides in the liquid phase, namely, formamide, Nmethylformamide, N-methylacetamide, N,N-dimethylformamide, and N,N-dimethylacetamide using ATR-FUV spectroscopy.9 It was revealed by combining the experimental results and the results of quantum chemical calculations based on density functional theory/time-dependent density functional theory (DFT/TD-DFT) and direct symmetry-adapted clusterconfiguration interaction (SAC-CI) method that the major band of the amides at around 190 nm can be attributed mainly to the π−π* transition, but several types of Rydberg transitions also exist in its vicinity and the mixing of orbitals with the same symmetry occurs, and that the valence−Rydberg coupling of the π−π* transition is more significant than the n−π* transition, which also holds in the pure liquid phase. The transmittance (Tr) absorption spectra of nylon-3, nylon4, nylon-6, nylon-6/6, and nylon-6/10 were originally reported by Onari.18 The π−π* amide band was observed at around 190 nm, and the intensity of the band in the wavelength shorter than 165 nm was found to increase as the number of methylene unit; however, the assignment of the latter band has not been clear in the solid state or condensed phase. In relation to nylon, the absorption spectra of α-polypeptides are of interest. Those spectra have a interpeptide charge-transfer (CT) transition at around 165−155 nm.19 It was theoretically shown that this CT transition is rapidly weakened in β-peptide. Therefore, in the βpeptides and polyamides with longer methylene unit, this CT transition should not exist.17 Unfortunately, most theoretical works on the solid amides and polyamides have not focused on the Rydberg transitions in the condensed phase/solid state. However, the recent studies by using ATR-FUV for liquids revealed that the Rydberg transitions do exist even in the liquid or condensed phase. In this work, we investigate the electronic transitions of five nylons (nylon-6, nylon-11, nylon-12, nylon-6/6, and nylon-6/ 12) in cast films using ATR- and Tr- FUV spectroscopy. We provide the definitive assignments in the 145−250 nm region of these nylons by the quantum-chemical calculations and also comparing with the spectra of n-alkanes and amides. By analyzing the difference in the FUV spectra of these five nylons, we clarified the effects of the nylon structures and intermolecular hydrogen bonding based on the transition dipole coupling in the possible nylon sheet.
Figure 1. Chemical structures of (a) nylon-6, (b) nylon-11, (c) nylon12, (d) nylon-6/6, and (e) nylon-6/12.
of nylons were prepared by dissolving them in 1,1,1,3,3,3hexafluoro-2-propanol (99.0%, Wako Pure Chemical Industries, Ltd., Osaka, Japan). The concentrations of nylons in the cast film solutions were ca. 0.1 and ca. 1 wt % for Tr and ATR-FUV measurements, respectively. Measurement of ATR-FUV Spectra. Details of the ATRFUV spectrometer for solid samples have been reported in ref 3, except for cast-film sample setting. The cast films are directly formed on the probed surface of the internal reflection element made by sapphire. The elimination of the solvent was confirmed by the observed spectra. All spectra were collected at room temperature (24−25 °C). The observed ATR spectra were not identical to the corresponding transmittance spectra, as the ATR technique changed the peak intensity as well as peak position. All the spectra reported in this paper are without any ATR correction. The ATR spectra of nylons cannot be subjected to the Kramers−Kroning transformation (KKT) because the referring refractive index of nylons was hard to measure.10 Measurement of Tr-FUV Spectra. Tr-FUV spectra were measured by same spectrometer of the ATR-FUV with different sample alignment. The cast films were formed on the CaF2 plate and dried in the vacuum oven at 60 °C for 6 h. The films have about 5 μm thickness. Computational Details. To discuss the relationship between the electronic structures and ATR-FUV spectra, the DFT/TD-DFT calculations were carried out for model compounds of nylon-6, nylon-11, and nylon-12, namely, C 6 H 1 3 NHCOCH 3 , C 1 1 H 2 3 NHCOCH 3 , and C 1 2 H 2 4 NHCOCH3, respectively, in their all transoid structure. The ground-state geometries were obtained by the optimization at B3LYP20,21/cc-pVTZ22 level of approximation followed by the vibrational analysis to confirm the structures are true local minimum. For calculating the vertical transition energies of these model systems, the TD-CAM-B3LYP23 calculations were carried out with cc-pVDZ basis sets with diffuse s and p functions; the exponents of diffuse functions are taken from aug-cc-pVDZ. The TD-CAM-B3LYP method is suitable for describing both valence and Rydberg excited states. For analyzing the effects of hydrogen bondings and the coupling of the excitations of sheet structure on the π−π* transition energy, the monomer, dimer, and trimer models of nylon-6 and nylon-6/6 were also examined, where the trimer models include the unit cell of the hydrogen-bonded sheets that can represent the π−π* transition,24 namely, double and single unit cells for nylon-6 and nylon-6/6, respectively. These model systems which were designed for the α phase and assumed to be planar, namely Cs structures, were adopted for both nylon-6 and nylon-6/6. The γ phase is thermally unstable and usually
2. EXPERIMENT AND THEORY Materials. Nylon-6, nylon-11, nylon-12, nylon-6/6, and nylon-6/12 were purchased from Scientific Polymer Products Co. Figure 1 shows their chemical structures. Nylons were used after they had been dried for 6 h in a vacuum dryer. Cast films 11856
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converted to the α phase,25 in particular, when the cast films are dried at room temperature as in the present case. Thus, we did not consider the γ phase in the calculations to interpret the present FUV spectra. The present cluster models provide qualitative interpretation of the spectral shape difference between nylon-6 and nylon-6/6, though the models do not include the periodic nature of nylon. The geometry optimization of these models was carried out at the B3LYP/ cc-pVDZ level followed by the calculations of the vertical transition energy at TD-CAM-B3LYP with cc-pVDZ plus diffuse s and p functions on N and O atoms [denoted as ccpVDZ + diffuse(N,O)]. All calculations were performed using the Gaussian09 suite of programs.26
latter. The intensity of the 150 nm band increases in the order of nylon-6, nylon-6/6, nylon-6/12, nylon-11, and nylon-12. The relative content of CH2 groups increases in the above order. Thus, the more CH2 groups, the more intense the 150 nm band. This order is the same as that for n-alkanes (n = 5− 14) where the intensity of 150 nm band increases with the number of carbon atoms. Figure 3a displays Tr spectra of cast films of nylon-6, nylon11, nylon-12, nylon-6/6, and nylon-6/12. The intensity in the
3. RESULTS AND DISCUSSION ATR-FUC Spectra of Five Nylons. Figure 2a compares ATR-FUV spectra in the 145−260 nm region of cast films of
Figure 3. (a) Tr spectra of cast films of nylon-6, nylon-11, nylon-12, nylon-6/6, and nylon-6/12. (b) Tr spectra of cast films of nylon-6, nylon-11, nylon-12, nylon-6/6, and nylon-6/12 normalized by the intensity of the band near 200 nm.
Tr spectra is influenced by the film thickness. The thickness of the nylon films was about 800−400 nm estimated from the absorbance of the Tr spectra and the absorption coefficients of nylon-6/6 in the literature,18 but it was difficult to measure it accurately. Thus, we used the spectra normalized by the intensity of 190 nm band shown in Figure 3b. The band near 190 nm is observed at a shorter wavelength by ca. 10 nm in the Tr spectra compared with the corresponding band in the ATRFUV spectra. The 150 nm band, which is clearly observed in the ATR-FUV spectra, can be seen only as a tail in the shorter wavelength side in the Tr spectra. This is because an absorption peak appears at a longer wavelength side in an ATR-FUV spectrum than in a Tr spectrum due to the effect of real part of reflective index. Figure 4 plots the absorption wavelength of the 195 nm peak of the Tr spectra versus that of the 205 nm peak of the ATR spectra. Nylon-m/n (nylon-6/6 and nylon-6/12) yields the band at a longer wavelength side for both Tr and ATR spectra. In contrast, nylon-m (nylon-6, nylon-11, and nylon-12) gives the peak at a shorter wavelength side in the Tr and ATR spectra except for the ATR spectrum of nylon-6. In general, nylon-m/n
Figure 2. (a) ATR-FUV spectra of cast films of nylon-6, nylon-11, nylon-12, nylon-6/6, nylon-6/12, and liquid NMA. (b) ATR-FUV spectra of nylon-6, nylon-11, nylon-12, nylon-6/6, and nylon-6/12 normalized by the intensity of the band near 200 nm.
nylon-6, nylon-11, nylon-12, nylon-6/6, nylon-6/12, and liquid N-methylacetamide (NMA). Figure 2b shows the corresponding spectra of nylons normalized by the intensity of the peak near 200 nm. All the nylons yield two intense features near 150 and 200 nm. However, liquid NMA (Figure 2a) and other amides give only the 190 nm band,12 while liquid n-alkanes (n = 5−14) show only the 160 nm feature.10,11 Thus, one can assume that the 150 and 200 nm features of nylons are due to the alkane and amide parts, respectively. Of note is that nylon11 and nylon-12, which have a longer CH2 chain, show significantly different spectra from other nylons. The formers yield the amide band at a higher energy side by several nanometers and the stronger alkyl band compared with the 11857
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bonding between the amide groups is not considered. The effects of hydrogen bonding in nylon-6 and nylon-6/6 will be addressed later. Figure S1b illustrates σ(HOMO−4) and Rydberg 3py orbitals of the nylon-6 model. These orbitals are delocalized over the hydrocarbon chain, which is similar to the case of nhexane.7 The σ-Rydberg 3py transition yields an intense band near 140 nm. Figure 5 shows that the intensity of the 140 nm band increases in the order of nylon-6, nylon-11, and nylon-12, and its position shifts toward a lower energy side in the same order. These results are in good agreement with the experimental observation and also the calculated results of liquid n-alkanes.6,7 This evidently shows that the Rydberg transitions exist even in the condensed phase or solid state. Thus, the two major absorption bands of nylons are ascribed to the amide group origin and the hydrocarbon group origin. As seen in Figure 4, the effects of the hydrogen bonding on the absorption spectra should be responsible for the longer wavelength shift of nylon-6 and nylon-6/6, etc. Therefore, we assessed the effects using the monomer, dimer, and trimer models. The trimer models of nylon-6 and nylon-6/6 are shown in Figure 6 with transition dipole strengths, and other
Figure 4. Peak positions of the band near 195 nm of the Tr spectra versus those of the band near 205 nm of the ATR-FUV spectra without KKT for the five kinds of nylons.
has stronger hydrogen bondings than nylon-m. Thus, the longer wavelength shift of the bands in both Tr and ATR spectra is in good agreement with this fact. However, the ATR spectrum of nylon-6 shows the longer wavelength shift of this band. Assignments and Effects of Intermolecular Hydrogen Bondings. In order to perform assignments of ATR-FUV spectra of nylons, we carried out quantum chemical calculations for monomer units of nylon-6, nylon-11, and nylon-12 using TD-CAM-B3LYP. Figure 5 shows the computed spectra with
Figure 5. Calculated spectra of model compounds of nylon-6, nylon11, and nylon-12.
bars and their convolution. The calculated spectra yield bands near 170 and 140 nm. Taking accounts of TD-CAM-B3LYP providing higher transition energy and also not considering the effects of surroundings, these bands should correspond to the observed bands near 200 and 150 nm, respectively. The band observed at around 200 nm is attributed to the π−π* transition. Molecular orbitals (MOs) relevant for transitions are shown in the Supporting Information. Figure S1a depicts the π and π* orbitals which give rise to the band near 170 nm. These orbitals are relatively localized in the amide group. The MOs or transitions in nylon-11 and nylon-12 models show similar results. Also, the π and π* orbitals of nylon-6 monomer are very similar to those of NMA;9 namely, the alkane chain does not affect this π−π* transition significantly. It should be noted that the peak position and intensity of the 170 nm band are nearly the same for the models of nylon-6, nylon-11, and nylon-12. This is because in the vacuum calculation of the monomers the strong hydrogen
Figure 6. Computational trimer models of hydrogen bonded systems for (a) nylon-6 and (b) nylon-6/6 with the transition dipole strengths.
models are in Figures S2 and S3. In nylon-6, hydrogen bonds are perfectly constituted by inverting alternate chains24 in the α-form and the progressive shear structure is adopted in the αform of nylon-6/6. Note that these trimer models represent the essential part of the hydrogen-bonded system of unit cell19 regarding the π−π* transitions, and therefore, the monomer unit with three amide groups is different from above models. The crystalline structure and its IR spectrum of nylon-6 11858
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Table 1. Excitation Energy (ΔE), Wavelength (λ), and Oscillator Strength ( f) of States in the ππ* Region for Monomer, Dimer, and Trimer Models for Nylon-6 and Nylon-6/6 Calculated by TD-CAM-B3LYP system nylon-6 monomer
dimer
trimer
nylon-6/6 monomer
dimer
trimer
transition no.
ΔE (eV)
λ (nm)
fb
character
8 9 10 11 suma 11 12 15 17 18 suma 13 18 19 22 24 33 suma
7.214 7.254 7.355 7.372 7.267 7.033 7.045 7.215 7.250 7.254 7.209 7.005 7.137 7.154 7.222 7.260 7.646 7.174
171.9 170.9 168.6 168.2 170.6 176.3 176.0 171.9 171.0 170.9 172.0 177.0 173.7 173.3 171.7 170.8 162.2 172.8
0.261 0.143 0.103 0.125 0.958 0.287 0.154 0.578 0.141 0.129 0.887 0.662 0.111 0.177 0.667 0.414 0.139 0.978
π → π* π → π* n → Ryd, π → π* n → Ryd, π → π*
8 10 11 suma 10 12 13 24 suma 11 13 16 23 suma
7.223 7.348 7.377 7.273 7.018 7.037 7.056 7.591 7.105 7.009 7.019 7.051 7.215 7.052
171.7 168.7 168.1 170.6 176.7 176.2 175.7 163.3 174.5 176.9 176.7 175.8 171.8 175.8
0.313 0.113 0.136 0.960 0.237 0.643 0.107 0.167 0.911 0.186 1.395 0.269 0.112 1.018
π π π π π
→ → → → →
π* π* π* π* π*
π π π π π n
→ → → → → →
π* π* π* π* π* Ryd, π → π*
π → π* n → Ryd, π → π* n → Ryd, π → π* π π π n
→ → → →
π* π* π* Ryd, π → π*
π π π π
→ → → →
π* π* π* π*
The peak position of the ππ* band is calculated by summing over the convoluted bands of the calculated peaks with the fwhm of 0.59 eV. Theoretical convoluted spectra are shown in Figure S5. bTransitions whose oscillator strength is larger than 0.1 are listed. The sum of the oscillator strengths is normalized with respect to the number of molecules in the dimer and trimer models. a
177.0 nm with similar dipole strength exist because of its crystal structure, namely, the inversion of alternate chain. This different transition dipole coupling scheme is the origin of the difference in the red-shift of nylon-6 and nylon-6/6. To interpret the difference of amide π−π* transition more clearly, the distribution of π−π* transitions are compared for nylon-6 and nylon-6/6 trimer models in Figure 7. Typical MOs relevant for the π−π* transition of these models are also shown in Figure 8. Some other MOs and those of monomer models are given in Figure S4. In the nylon-6/6 model, first three transitions with considerable oscillator strength are characterized as the transitions from/to the relatively delocalized MOs; namely, as seen in Figure 8b, π and π* amplitudes are located in two or more amide groups. The intensities of the low-lying transitions are enhanced by the linear combination of the delocalized π−π* configurations, while those of higher energy transitions are canceled out. The most intense transition at 176.7 nm with f = 1.395 is weighted by many configurations. Orbital energies of both occupied and unoccupied MOs are affected by about
polymorphs were theoretically studied using DFT calculations.27 The results of the π−π* and few n-Rydberg transitions by TD-CAM-B3LYP are given in Table 1, in which only the strong transitions with oscillator strength larger than 0.1 are listed. The complete lists of the transitions are given in Tables S1 and S2. The peak positions of the π−π* bands which are calculated by summing over the convoluted bands of the calculated peaks with the fwhm of 0.59 eV show a red-shift as 171 (171), 172 (175), and 173 (176) nm for the monomer, dimer, and trimer models of nylon-6 (nylon-6/6), respectively. The oscillator strength is considerably distributed to many transitions in the wide range of spectra in the dimer or trimer models via hydrogen bonding effects. The red-shift (4−5 nm) almost converges at the trimer model. The difference in the red-shift between nylon-6 and nylon-6/6 is about 3 nm (0.12 eV), which agrees well with the experimental value of 0.06 eV. In the case of nylon-6/6, an intense transition at 176.7 nm with significant dipole strength is enhanced in the trimer model as seen in Figure 6 (for details, see Figure S3). However, in the nylon-6 trimer model, three dominant transitions at 170.8, 171.7, and 11859
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for nylon-6 (C−C distance), which is in good agreement with the experimental value of 9.56 Å.24
4. CONCLUSION The ATR-FUV and Tr spectra in the 145−260 nm region were measured for cast films of five kinds of nylons (nylon-6, nylon11, nylon-12, nylon-6/6, and nylon-6/12). Nylons consist of hydrocarbon chains and amide groups, and the latter form strong intermolecular hydrogen bondings. In the ATR-FUV spectra nylons show two major bands near 150 and 200 nm due to the hydrocarbon chains and the amide groups, respectively. The intensity of the 150 nm band increases in the order of nylon-6, nylon-11, and nylon-12, which have 5, 10, and 11 hydrocarbon chains, respectively. Nylon-6/6 and nylon-6/12, which have strong intermolecular hydrogen bondings, give rise to the 200 nm band at a lower energy side (ca. 205 nm) compared with nylon-11 and nylon-12 (ca. 200 nm). For the detailed assignments of FUV spectra of nylons, we carried out the TD-CAM-B3LYP calculations using nylon-6, nylon-11, and nylon-12 model units. The σ and Rydberg 3p orbitals are delocalized to the hydrocarbon chain, while the π and π* transitions are relatively localized in the amide group. The effects of hydrogen bondings are considered with the monomer to trimer nylon-6 and nylon-6/6 models. The calculated energy shift well reproduces the experimental trend. A red-shift of the π−π* amide band in the FUV spectra of nylon-6 and nylon-6/6 models in the α form is attributed to the different crystal structure pattern and intermolecular hydrogen bondings, which result in the different transition dipole coupling. The π−π* transitions in nylon-6/6 are delocalized with being coupled to each other, and the oscillator strengths are accumulated to the lowest state, while in nylon-6, the π−π* states are localized and the oscillator strengths are distributed to some states. The present study has demonstrated that one can investigate the electronic structure and transitions of polymers and other solid samples which cannot be explored by ordinary UV spectroscopy by combining FUV spectroscopy and quantumchemical calculations.
Figure 7. Distribution of the π−π* transitions with relatively strong ( f ≥ 0.1, red bar) and weak (f < 0.1, black bar) intensity calculated with three models of nylon-6 and nylon-6/6.
Figure 8. Typical MOs relevant for the π−π* transition in the trimer model of (a) nylon-6 and (b) nylon-6/6 (H = HOMO, L = LUMO).
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ASSOCIATED CONTENT
S Supporting Information *
0.08 eV via hydrogen bondings, and the delocalization over hydrogen bond is observed. On the other hand, valence π orbitals of nylon-6 are well localized in each amide group as typically seen in monomer case (Figure 4S). In the dimer and trimer models, the transitions from these π orbitals independently generate a series of π−π* states; however, the coupling between these states is small. The MOs are localized even in the dimer and trimer models as shown in Figure 8a. Consequently, some intense π−π* states are generated in the dimer and trimer models of nylon-6. The calculated interaction energies of the hydrogen bondings were 22.2 (22.7) and 48.5 (49.4) kcal/mol for the dimer and trimer models of nylon-6 (nylon-6/6), respectively. These energies are not additive, and some nonlinear effect may exist. Though the present results agree with the experimental trend, the calculated hydrogen bond interaction energies with these models may not be enough to explain the difference of hydrogen bond interactions of nylon-6 and nylon-6/6; the present models only consider the local structure and the global structure of these two polymers should be important. Within this model system, the unit cell distance was calculated as 9.7 Å
Detailed results for monomer, dimer, and trimer of nylon-6 and nylon-6/6. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail [email protected] (Y.M.). *E-mail [email protected] (M.E.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by a Grant-in-Aid (KAKENHI) for the Japan Society for the Promotion of Science (JSPS) and the Joint Studies Program of the Institute for Molecular Science. The work was also supported by Nanotechnology Platform Program (Molecule and Material Synthesis) of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. The calculations were partly performed using the Research Center for Computational Science in Okazaki, Japan. 11860
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(20) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-Salvetti Correlation-Energy Formula into a Functional of the Electron Density. Phys. Rev. B 1988, 37, 785−789. (21) Becke, A. D. Density-Functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652. (22) Dunning, T. H., Jr. Gaussian Basis Sets for Use in Correlated Molecular Calculations. I. The Atoms Boron Through Neon and Hydrogen. J. Chem. Phys. 1989, 90, 1007−1023. (23) Yanai, T.; Tew, D. P.; Handy, N. C. A New Hybrid ExchangeCorrelation Functional Using the Coulomb-Attenuating Method (CAM-B3LYP). Chem. Phys. Lett. 2004, 393, 51−57. (24) Holmes, D. R.; Bunn, C. W.; Smith, D. J. The Crystal Structure of Polycaproamide: Nylon 6. J. Polym. Sci. 1955, 17, 159−177. (25) Dasgupta, S.; Hammond, W. B.; Goddard, W. A., III Crystal Structures and Properties of Nylon Polymers from Theory. J. Am. Chem. Soc. 1996, 118, 122991−12301 and references therein.. (26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson et al. GAUSSIAN 09, Revision B.01; Gaussian, Inc.: Wallingford, CT, 2010. (27) Quarti, C.; Milani, A.; Civalleri, B.; Orlando, R.; Castiglioni, C. Ab Initio Calculation of the Crystalline Structure and IR Spectrum of Polymers: Nylon 6 Polymorphs. J. Phys. Chem. B 2012, 116, 8299− 8311.
REFERENCES
(1) Robin, M. B. Higher Excited States of Polyatomic Molecules; Academic Press: New York, 1974; Vol. II. (2) Chergui, M.; Schwentner, N. Experimental Evidence to Rydbergization of Antibonding Molecular Orbitals. Chem. Phys. Lett. 1994, 219, 237−242. (3) Higashi, N.; Ikehata, A.; Ozaki, Y. An Attenuated Total Reflectance Far-UV Spectrometer. Rev. Sci. Instrum. 2007, 78, 103107. (4) Ozaki, Y.; Morisawa, Y.; Higashi, N.; Ikehata, A. Far-Ultraviolet Spectroscopy in the Solid and Liquid States: A Review. Appl. Spectrosc. 2012, 66, 1−25. (5) Morisawa, Y.; Ikehata, A.; Higashi, N.; Ozaki, Y. Attenuated Total Reflectance−Far Ultraviolet (ATR−FUV) Spectra of CH3OH, CH3OD, CD3OH and CD3OD in a Liquid Phase ∼Rydberg States∼. Chem. Phys. Lett. 2009, 476, 205−208. (6) Tachibana, S.; Morisawa, Y.; Ikehata, A.; Higashi, N.; Ozaki, Y. Far-Ultraviolet Spectra of N-Alkanes and Branched Alkanes in the Liquid Phase Observed Using an Attenuated Total Reflection−Far Ultraviolet (ATR-FUV) Spectrometer. Appl. Spectrosc. 2011, 65, 221− 226. (7) Morisawa, Y.; Tachibana, S.; Ehara, M.; Ozaki, Y. Elucidating Electronic Transitions from Sigma Orbitals of Liquid N- and Branched Alkanes by Far-Ultraviolet Spectroscopy and Quantum Chemical Calculations. J. Phys. Chem. A 2012, 116, 11957−11964. (8) Morisawa, Y.; Ikehata, A.; Higashi, N.; Ozaki, Y. Low-N Rydberg Transitions of Liquid Ketones Studied by Attenuated Total Reflection Far-Ultraviolet Spectroscopy. J. Phys. Chem. A 2011, 115, 562−568. (9) Morisawa, Y.; Yasunaga, M.; Fukuda, R.; Ehara, M.; Ozaki, Y. Electronic Transitions in Liquid Amides Studied by Using Attenuated Total Reflection Far-Ultraviolet Spectroscopy and Quantum Chemical Calculations. J. Chem. Phys. 2013, 139, 154301. (10) Ikehata, A.; Higashi, N.; Ozaki, Y. Direct Observation of the Absorption Bands of the First Electronic Transition in Liquid H2O and D2O by Attenuated Total Reflectance Far-UV Spectroscopy. J. Chem. Phys. 2008, 129, 234510. (11) Ikehata, A.; Mitsuoka, M.; Morisawa, Y.; Kariyama, N.; Higashi, N.; Ozaki, Y. Effect of Cations on Absorption Bands of First Electronic Transition of Liquid Water. J. Phys. Chem. A 2010, 114, 8319−8322. (12) Goto, T.; Ikehata, A.; Morisawa, Y.; Higashi, N.; Ozaki, Y. The Effect of Metal Cations on the Nature of the First Electronic Transition of Liquid Water As Studied by Attenuated Total Reflection Far-Ultraviolet Spectroscopy. Phys. Chem. Chem. Phys. 2012, 14, 8097−8104. (13) Goto, T.; Ikehata, A.; Morisawa, Y.; Higashi, N.; Ozaki, Y. Effects of Lanthanoid Cations on the First Electronic Transition of Liquid Water Studied Using Attenuated Total Reflection FarUltraviolet Spectroscopy: Ligand Field Splitting of Lanthanoid Hydrates in Aqueous Solutions. Inorg. Chem. 2012, 51, 10650−10656. (14) Tanabe, I.; Ozaki, Y. Consistent Changes in Electronic States and Photocatalytic Activities of Metal (Au, Pd, Pt)-Modified TiO2 Studied by Far-Ultraviolet Spectroscopy. Chem. Commun. 2014, 50, 2117−2119. (15) Tanabe, I.; Ryoki, T.; Ozaki, Y. Significant Enhancement of Photocatalytic Activity of Rutile TiO2 Compared with Anatase TiO2 upon Pt Nanoparticle Deposition Studied by Far-Ultraviolet Spectroscopy. Phys. Chem. Chem. Phys. 2014, 16, 7749−7753. (16) Morisawa, Y.; Higashi, N.; Takaba, K.; Kariyama, N.; Goto, T.; Ikehata, A.; Ozaki, Y. Development of a Time-Resolved Attenuated Total Reflectance Spectrometer in Far-Ultraviolet Region. Rev. Sci. Instrum. 2012, 83, 073103. (17) Goto, T.; Morisawa, Y.; Higashi, N.; Ikehata, A.; Ozaki, Y. Pulse Laser Photolysis of Aqueous Ozone in the Microsecond Range Studied by Time-Resolved Far-Ultraviolet Absorption Spectroscopy. Anal. Chem. 2013, 85, 4500−4506. (18) Onari, S. Vacuum Ultraviolet Absorption Spectra of Polyamides. Jpn. J. Appl. Phys. 1970, 9, 227. (19) Serrano-Andres, L.; Fulscher, M. P. Theoretical Study of the Electronic Spectroscopy of Peptides. III. Charge-Transfer Transitions in Polypeptides. J. Am. Chem. Soc. 1998, 120, 10912−10920. 11861
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Rydberg and π−π* Transitions in Film Surfaces of Various Kinds of Nylons Studied by Attenuated Total Reflection Far-Ultraviolet Spectroscopy and Quantum Chemical Calculations: Peak Shifts in the Spectra and Their Relation to Nylon Structure and Hydrogen Bondings Yusuke Morisawa,*,† Manaka Yasunaga,‡ Harumi Sato,§ Ryoichi Fukuda,∥,⊥ Masahiro Ehara,*,∥,⊥ and Yukihiro Ozaki‡ †
Department of Chemistry, School of Science and Engineering, Kinki University, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan Department of Chemistry, School of Science and Technology, Kwansei Gakuin University, Gakuen, Sanda, Hyogo 699-1337, Japan § Graduate School of Human Development and Environment, Kobe University, Tsurukabuto, Nada-ku, Kobe, Japan ∥ Institute of Molecular Science and Research Center for Computation of Science, Nishigo-Naka, Myodaiji, Okazaki, Aichi 444-8585, Japan ⊥ Elements Strategy Initiative for Catalysis and Batteries (ESICB), Kyoto University, Kyoto 615-8520, Japan ‡
S Supporting Information *
ABSTRACT: Attenuated total reflection far-ultraviolet (ATRFUV) spectra in the 145−260 nm region were measured for surfaces (thickness 50−200 nm) of various kinds of nylons in cast films to explore their electronic transitions in the FUV region. ATR-FUV spectra show two major bands near 150 and 200 nm in the surface condensed phase of nylons. Transmittance (Tr) spectra were also observed in particular for the analysis of valence excitations. Time-dependent density functional theory (TD-DFT/CAM-B3LYP) calculations were carried out using the model systems to provide the definitive assignments of their absorption spectra and to elucidate their peak shifts in several nylons, in particular, focusing on their crystal alignment structures and intermolecular hydrogen bondings. Two major bands of nylon films near 150 and 200 nm are characterized as σ-Rydberg 3p and π−π* transitions of nylons, respectively. These assignments are also coherent with those of liquid n-alkanes (n = 5−14) and liquid amides observed previously. The Rydberg transitions are delocalized over the hydrocarbon chains, while the π−π* transitions are relatively localized at the amide group. Differences in the peak positions and intensity were found in both ATR- and Tr-FUV spectra for different nylons. A red-shift of the π−π* amide band in the FUV spectra of nylon-6 and nylon-6/6 models in α-form is attributed to the crystal structure pattern and the intermolecular hydrogen bondings, which result in the different delocalization character of the π−π* transitions and transition dipole coupling.
1. INTRODUCTION It has become clear that far-ultraviolet (FUV) spectroscopy holds great potential in elucidating electronic transitions and structure of a wide range of molecules in condensed phases.1−4 FUV spectroscopy in the 145−200 nm region has recently been a matter of keen interest because many kinds of organic5−9 and inorganic10−15 materials in the condensed phase, even liquid alkanes, show bands due to electronic transitions in the FUV region. Importantly, the spectroscopy is powerful not only for basic studies but also for various applications such as monitoring of the quality of semiconductor wafer cleaning.3,16,17 More recently, the electronic states and photocatalytic activities of metal-modified TiO2 were examined by using ATRFUV spectroscopy.14,15 Such rapid progress in FUV spectroscopy has been stimulated by the development of an attenuated © 2014 American Chemical Society
total reflection (ATR)-FUV spectrometer, which has enabled us to measure the spectra in the complete FUV region for liquid and solid samples without facing problems such as peak saturation.3,4 In the previous studies we investigated the ATR-FUV spectra of water,10−12 alcohols,5 several kinds of n- and branched alkanes,6,7 ketones,8 and amides9 observed in the liquid phase. The FUV spectra of n-alkanes gave a broad feature near 150 nm, while the branched alkanes show an additional shoulder at around 180 nm. The 150 nm band shows a lower energy shift with a significant intensity increase as the alkyl chain length Received: July 31, 2014 Revised: September 7, 2014 Published: September 9, 2014 11855
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increases. We investigated assignments of absorption bands for the n-alkanes and branched alkanes in the liquid states and the cause of lower energy shift of the 150 nm feature by comparing the experimental results with quantum chemical calculations.7 It was found that the 150 nm band of n-alkanes arises from the σRydberg 3py transition, and its longer wavelength shift is caused by the destabilizing effect of next highest occupied molecular orbital (HOMO) level and the stabilization of Rydberg 3p level, which is expanded as the carbon chain increase. These studies of FUV spectra of alkanes have provided new insight into Rydberg states and transitions of liquid alkanes.7 The FUV region of amides in the liquid phase is also very important because a valence−Rydberg coupling is expected for a π−π* transition and a direct interaction occurs via a hydrogen bond. Thus, we studied the electronic transitions of five kinds of amides in the liquid phase, namely, formamide, Nmethylformamide, N-methylacetamide, N,N-dimethylformamide, and N,N-dimethylacetamide using ATR-FUV spectroscopy.9 It was revealed by combining the experimental results and the results of quantum chemical calculations based on density functional theory/time-dependent density functional theory (DFT/TD-DFT) and direct symmetry-adapted clusterconfiguration interaction (SAC-CI) method that the major band of the amides at around 190 nm can be attributed mainly to the π−π* transition, but several types of Rydberg transitions also exist in its vicinity and the mixing of orbitals with the same symmetry occurs, and that the valence−Rydberg coupling of the π−π* transition is more significant than the n−π* transition, which also holds in the pure liquid phase. The transmittance (Tr) absorption spectra of nylon-3, nylon4, nylon-6, nylon-6/6, and nylon-6/10 were originally reported by Onari.18 The π−π* amide band was observed at around 190 nm, and the intensity of the band in the wavelength shorter than 165 nm was found to increase as the number of methylene unit; however, the assignment of the latter band has not been clear in the solid state or condensed phase. In relation to nylon, the absorption spectra of α-polypeptides are of interest. Those spectra have a interpeptide charge-transfer (CT) transition at around 165−155 nm.19 It was theoretically shown that this CT transition is rapidly weakened in β-peptide. Therefore, in the βpeptides and polyamides with longer methylene unit, this CT transition should not exist.17 Unfortunately, most theoretical works on the solid amides and polyamides have not focused on the Rydberg transitions in the condensed phase/solid state. However, the recent studies by using ATR-FUV for liquids revealed that the Rydberg transitions do exist even in the liquid or condensed phase. In this work, we investigate the electronic transitions of five nylons (nylon-6, nylon-11, nylon-12, nylon-6/6, and nylon-6/ 12) in cast films using ATR- and Tr- FUV spectroscopy. We provide the definitive assignments in the 145−250 nm region of these nylons by the quantum-chemical calculations and also comparing with the spectra of n-alkanes and amides. By analyzing the difference in the FUV spectra of these five nylons, we clarified the effects of the nylon structures and intermolecular hydrogen bonding based on the transition dipole coupling in the possible nylon sheet.
Figure 1. Chemical structures of (a) nylon-6, (b) nylon-11, (c) nylon12, (d) nylon-6/6, and (e) nylon-6/12.
of nylons were prepared by dissolving them in 1,1,1,3,3,3hexafluoro-2-propanol (99.0%, Wako Pure Chemical Industries, Ltd., Osaka, Japan). The concentrations of nylons in the cast film solutions were ca. 0.1 and ca. 1 wt % for Tr and ATR-FUV measurements, respectively. Measurement of ATR-FUV Spectra. Details of the ATRFUV spectrometer for solid samples have been reported in ref 3, except for cast-film sample setting. The cast films are directly formed on the probed surface of the internal reflection element made by sapphire. The elimination of the solvent was confirmed by the observed spectra. All spectra were collected at room temperature (24−25 °C). The observed ATR spectra were not identical to the corresponding transmittance spectra, as the ATR technique changed the peak intensity as well as peak position. All the spectra reported in this paper are without any ATR correction. The ATR spectra of nylons cannot be subjected to the Kramers−Kroning transformation (KKT) because the referring refractive index of nylons was hard to measure.10 Measurement of Tr-FUV Spectra. Tr-FUV spectra were measured by same spectrometer of the ATR-FUV with different sample alignment. The cast films were formed on the CaF2 plate and dried in the vacuum oven at 60 °C for 6 h. The films have about 5 μm thickness. Computational Details. To discuss the relationship between the electronic structures and ATR-FUV spectra, the DFT/TD-DFT calculations were carried out for model compounds of nylon-6, nylon-11, and nylon-12, namely, C 6 H 1 3 NHCOCH 3 , C 1 1 H 2 3 NHCOCH 3 , and C 1 2 H 2 4 NHCOCH3, respectively, in their all transoid structure. The ground-state geometries were obtained by the optimization at B3LYP20,21/cc-pVTZ22 level of approximation followed by the vibrational analysis to confirm the structures are true local minimum. For calculating the vertical transition energies of these model systems, the TD-CAM-B3LYP23 calculations were carried out with cc-pVDZ basis sets with diffuse s and p functions; the exponents of diffuse functions are taken from aug-cc-pVDZ. The TD-CAM-B3LYP method is suitable for describing both valence and Rydberg excited states. For analyzing the effects of hydrogen bondings and the coupling of the excitations of sheet structure on the π−π* transition energy, the monomer, dimer, and trimer models of nylon-6 and nylon-6/6 were also examined, where the trimer models include the unit cell of the hydrogen-bonded sheets that can represent the π−π* transition,24 namely, double and single unit cells for nylon-6 and nylon-6/6, respectively. These model systems which were designed for the α phase and assumed to be planar, namely Cs structures, were adopted for both nylon-6 and nylon-6/6. The γ phase is thermally unstable and usually
2. EXPERIMENT AND THEORY Materials. Nylon-6, nylon-11, nylon-12, nylon-6/6, and nylon-6/12 were purchased from Scientific Polymer Products Co. Figure 1 shows their chemical structures. Nylons were used after they had been dried for 6 h in a vacuum dryer. Cast films 11856
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converted to the α phase,25 in particular, when the cast films are dried at room temperature as in the present case. Thus, we did not consider the γ phase in the calculations to interpret the present FUV spectra. The present cluster models provide qualitative interpretation of the spectral shape difference between nylon-6 and nylon-6/6, though the models do not include the periodic nature of nylon. The geometry optimization of these models was carried out at the B3LYP/ cc-pVDZ level followed by the calculations of the vertical transition energy at TD-CAM-B3LYP with cc-pVDZ plus diffuse s and p functions on N and O atoms [denoted as ccpVDZ + diffuse(N,O)]. All calculations were performed using the Gaussian09 suite of programs.26
latter. The intensity of the 150 nm band increases in the order of nylon-6, nylon-6/6, nylon-6/12, nylon-11, and nylon-12. The relative content of CH2 groups increases in the above order. Thus, the more CH2 groups, the more intense the 150 nm band. This order is the same as that for n-alkanes (n = 5− 14) where the intensity of 150 nm band increases with the number of carbon atoms. Figure 3a displays Tr spectra of cast films of nylon-6, nylon11, nylon-12, nylon-6/6, and nylon-6/12. The intensity in the
3. RESULTS AND DISCUSSION ATR-FUC Spectra of Five Nylons. Figure 2a compares ATR-FUV spectra in the 145−260 nm region of cast films of
Figure 3. (a) Tr spectra of cast films of nylon-6, nylon-11, nylon-12, nylon-6/6, and nylon-6/12. (b) Tr spectra of cast films of nylon-6, nylon-11, nylon-12, nylon-6/6, and nylon-6/12 normalized by the intensity of the band near 200 nm.
Tr spectra is influenced by the film thickness. The thickness of the nylon films was about 800−400 nm estimated from the absorbance of the Tr spectra and the absorption coefficients of nylon-6/6 in the literature,18 but it was difficult to measure it accurately. Thus, we used the spectra normalized by the intensity of 190 nm band shown in Figure 3b. The band near 190 nm is observed at a shorter wavelength by ca. 10 nm in the Tr spectra compared with the corresponding band in the ATRFUV spectra. The 150 nm band, which is clearly observed in the ATR-FUV spectra, can be seen only as a tail in the shorter wavelength side in the Tr spectra. This is because an absorption peak appears at a longer wavelength side in an ATR-FUV spectrum than in a Tr spectrum due to the effect of real part of reflective index. Figure 4 plots the absorption wavelength of the 195 nm peak of the Tr spectra versus that of the 205 nm peak of the ATR spectra. Nylon-m/n (nylon-6/6 and nylon-6/12) yields the band at a longer wavelength side for both Tr and ATR spectra. In contrast, nylon-m (nylon-6, nylon-11, and nylon-12) gives the peak at a shorter wavelength side in the Tr and ATR spectra except for the ATR spectrum of nylon-6. In general, nylon-m/n
Figure 2. (a) ATR-FUV spectra of cast films of nylon-6, nylon-11, nylon-12, nylon-6/6, nylon-6/12, and liquid NMA. (b) ATR-FUV spectra of nylon-6, nylon-11, nylon-12, nylon-6/6, and nylon-6/12 normalized by the intensity of the band near 200 nm.
nylon-6, nylon-11, nylon-12, nylon-6/6, nylon-6/12, and liquid N-methylacetamide (NMA). Figure 2b shows the corresponding spectra of nylons normalized by the intensity of the peak near 200 nm. All the nylons yield two intense features near 150 and 200 nm. However, liquid NMA (Figure 2a) and other amides give only the 190 nm band,12 while liquid n-alkanes (n = 5−14) show only the 160 nm feature.10,11 Thus, one can assume that the 150 and 200 nm features of nylons are due to the alkane and amide parts, respectively. Of note is that nylon11 and nylon-12, which have a longer CH2 chain, show significantly different spectra from other nylons. The formers yield the amide band at a higher energy side by several nanometers and the stronger alkyl band compared with the 11857
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bonding between the amide groups is not considered. The effects of hydrogen bonding in nylon-6 and nylon-6/6 will be addressed later. Figure S1b illustrates σ(HOMO−4) and Rydberg 3py orbitals of the nylon-6 model. These orbitals are delocalized over the hydrocarbon chain, which is similar to the case of nhexane.7 The σ-Rydberg 3py transition yields an intense band near 140 nm. Figure 5 shows that the intensity of the 140 nm band increases in the order of nylon-6, nylon-11, and nylon-12, and its position shifts toward a lower energy side in the same order. These results are in good agreement with the experimental observation and also the calculated results of liquid n-alkanes.6,7 This evidently shows that the Rydberg transitions exist even in the condensed phase or solid state. Thus, the two major absorption bands of nylons are ascribed to the amide group origin and the hydrocarbon group origin. As seen in Figure 4, the effects of the hydrogen bonding on the absorption spectra should be responsible for the longer wavelength shift of nylon-6 and nylon-6/6, etc. Therefore, we assessed the effects using the monomer, dimer, and trimer models. The trimer models of nylon-6 and nylon-6/6 are shown in Figure 6 with transition dipole strengths, and other
Figure 4. Peak positions of the band near 195 nm of the Tr spectra versus those of the band near 205 nm of the ATR-FUV spectra without KKT for the five kinds of nylons.
has stronger hydrogen bondings than nylon-m. Thus, the longer wavelength shift of the bands in both Tr and ATR spectra is in good agreement with this fact. However, the ATR spectrum of nylon-6 shows the longer wavelength shift of this band. Assignments and Effects of Intermolecular Hydrogen Bondings. In order to perform assignments of ATR-FUV spectra of nylons, we carried out quantum chemical calculations for monomer units of nylon-6, nylon-11, and nylon-12 using TD-CAM-B3LYP. Figure 5 shows the computed spectra with
Figure 5. Calculated spectra of model compounds of nylon-6, nylon11, and nylon-12.
bars and their convolution. The calculated spectra yield bands near 170 and 140 nm. Taking accounts of TD-CAM-B3LYP providing higher transition energy and also not considering the effects of surroundings, these bands should correspond to the observed bands near 200 and 150 nm, respectively. The band observed at around 200 nm is attributed to the π−π* transition. Molecular orbitals (MOs) relevant for transitions are shown in the Supporting Information. Figure S1a depicts the π and π* orbitals which give rise to the band near 170 nm. These orbitals are relatively localized in the amide group. The MOs or transitions in nylon-11 and nylon-12 models show similar results. Also, the π and π* orbitals of nylon-6 monomer are very similar to those of NMA;9 namely, the alkane chain does not affect this π−π* transition significantly. It should be noted that the peak position and intensity of the 170 nm band are nearly the same for the models of nylon-6, nylon-11, and nylon-12. This is because in the vacuum calculation of the monomers the strong hydrogen
Figure 6. Computational trimer models of hydrogen bonded systems for (a) nylon-6 and (b) nylon-6/6 with the transition dipole strengths.
models are in Figures S2 and S3. In nylon-6, hydrogen bonds are perfectly constituted by inverting alternate chains24 in the α-form and the progressive shear structure is adopted in the αform of nylon-6/6. Note that these trimer models represent the essential part of the hydrogen-bonded system of unit cell19 regarding the π−π* transitions, and therefore, the monomer unit with three amide groups is different from above models. The crystalline structure and its IR spectrum of nylon-6 11858
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Table 1. Excitation Energy (ΔE), Wavelength (λ), and Oscillator Strength ( f) of States in the ππ* Region for Monomer, Dimer, and Trimer Models for Nylon-6 and Nylon-6/6 Calculated by TD-CAM-B3LYP system nylon-6 monomer
dimer
trimer
nylon-6/6 monomer
dimer
trimer
transition no.
ΔE (eV)
λ (nm)
fb
character
8 9 10 11 suma 11 12 15 17 18 suma 13 18 19 22 24 33 suma
7.214 7.254 7.355 7.372 7.267 7.033 7.045 7.215 7.250 7.254 7.209 7.005 7.137 7.154 7.222 7.260 7.646 7.174
171.9 170.9 168.6 168.2 170.6 176.3 176.0 171.9 171.0 170.9 172.0 177.0 173.7 173.3 171.7 170.8 162.2 172.8
0.261 0.143 0.103 0.125 0.958 0.287 0.154 0.578 0.141 0.129 0.887 0.662 0.111 0.177 0.667 0.414 0.139 0.978
π → π* π → π* n → Ryd, π → π* n → Ryd, π → π*
8 10 11 suma 10 12 13 24 suma 11 13 16 23 suma
7.223 7.348 7.377 7.273 7.018 7.037 7.056 7.591 7.105 7.009 7.019 7.051 7.215 7.052
171.7 168.7 168.1 170.6 176.7 176.2 175.7 163.3 174.5 176.9 176.7 175.8 171.8 175.8
0.313 0.113 0.136 0.960 0.237 0.643 0.107 0.167 0.911 0.186 1.395 0.269 0.112 1.018
π π π π π
→ → → → →
π* π* π* π* π*
π π π π π n
→ → → → → →
π* π* π* π* π* Ryd, π → π*
π → π* n → Ryd, π → π* n → Ryd, π → π* π π π n
→ → → →
π* π* π* Ryd, π → π*
π π π π
→ → → →
π* π* π* π*
The peak position of the ππ* band is calculated by summing over the convoluted bands of the calculated peaks with the fwhm of 0.59 eV. Theoretical convoluted spectra are shown in Figure S5. bTransitions whose oscillator strength is larger than 0.1 are listed. The sum of the oscillator strengths is normalized with respect to the number of molecules in the dimer and trimer models. a
177.0 nm with similar dipole strength exist because of its crystal structure, namely, the inversion of alternate chain. This different transition dipole coupling scheme is the origin of the difference in the red-shift of nylon-6 and nylon-6/6. To interpret the difference of amide π−π* transition more clearly, the distribution of π−π* transitions are compared for nylon-6 and nylon-6/6 trimer models in Figure 7. Typical MOs relevant for the π−π* transition of these models are also shown in Figure 8. Some other MOs and those of monomer models are given in Figure S4. In the nylon-6/6 model, first three transitions with considerable oscillator strength are characterized as the transitions from/to the relatively delocalized MOs; namely, as seen in Figure 8b, π and π* amplitudes are located in two or more amide groups. The intensities of the low-lying transitions are enhanced by the linear combination of the delocalized π−π* configurations, while those of higher energy transitions are canceled out. The most intense transition at 176.7 nm with f = 1.395 is weighted by many configurations. Orbital energies of both occupied and unoccupied MOs are affected by about
polymorphs were theoretically studied using DFT calculations.27 The results of the π−π* and few n-Rydberg transitions by TD-CAM-B3LYP are given in Table 1, in which only the strong transitions with oscillator strength larger than 0.1 are listed. The complete lists of the transitions are given in Tables S1 and S2. The peak positions of the π−π* bands which are calculated by summing over the convoluted bands of the calculated peaks with the fwhm of 0.59 eV show a red-shift as 171 (171), 172 (175), and 173 (176) nm for the monomer, dimer, and trimer models of nylon-6 (nylon-6/6), respectively. The oscillator strength is considerably distributed to many transitions in the wide range of spectra in the dimer or trimer models via hydrogen bonding effects. The red-shift (4−5 nm) almost converges at the trimer model. The difference in the red-shift between nylon-6 and nylon-6/6 is about 3 nm (0.12 eV), which agrees well with the experimental value of 0.06 eV. In the case of nylon-6/6, an intense transition at 176.7 nm with significant dipole strength is enhanced in the trimer model as seen in Figure 6 (for details, see Figure S3). However, in the nylon-6 trimer model, three dominant transitions at 170.8, 171.7, and 11859
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for nylon-6 (C−C distance), which is in good agreement with the experimental value of 9.56 Å.24
4. CONCLUSION The ATR-FUV and Tr spectra in the 145−260 nm region were measured for cast films of five kinds of nylons (nylon-6, nylon11, nylon-12, nylon-6/6, and nylon-6/12). Nylons consist of hydrocarbon chains and amide groups, and the latter form strong intermolecular hydrogen bondings. In the ATR-FUV spectra nylons show two major bands near 150 and 200 nm due to the hydrocarbon chains and the amide groups, respectively. The intensity of the 150 nm band increases in the order of nylon-6, nylon-11, and nylon-12, which have 5, 10, and 11 hydrocarbon chains, respectively. Nylon-6/6 and nylon-6/12, which have strong intermolecular hydrogen bondings, give rise to the 200 nm band at a lower energy side (ca. 205 nm) compared with nylon-11 and nylon-12 (ca. 200 nm). For the detailed assignments of FUV spectra of nylons, we carried out the TD-CAM-B3LYP calculations using nylon-6, nylon-11, and nylon-12 model units. The σ and Rydberg 3p orbitals are delocalized to the hydrocarbon chain, while the π and π* transitions are relatively localized in the amide group. The effects of hydrogen bondings are considered with the monomer to trimer nylon-6 and nylon-6/6 models. The calculated energy shift well reproduces the experimental trend. A red-shift of the π−π* amide band in the FUV spectra of nylon-6 and nylon-6/6 models in the α form is attributed to the different crystal structure pattern and intermolecular hydrogen bondings, which result in the different transition dipole coupling. The π−π* transitions in nylon-6/6 are delocalized with being coupled to each other, and the oscillator strengths are accumulated to the lowest state, while in nylon-6, the π−π* states are localized and the oscillator strengths are distributed to some states. The present study has demonstrated that one can investigate the electronic structure and transitions of polymers and other solid samples which cannot be explored by ordinary UV spectroscopy by combining FUV spectroscopy and quantumchemical calculations.
Figure 7. Distribution of the π−π* transitions with relatively strong ( f ≥ 0.1, red bar) and weak (f < 0.1, black bar) intensity calculated with three models of nylon-6 and nylon-6/6.
Figure 8. Typical MOs relevant for the π−π* transition in the trimer model of (a) nylon-6 and (b) nylon-6/6 (H = HOMO, L = LUMO).
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ASSOCIATED CONTENT
S Supporting Information *
0.08 eV via hydrogen bondings, and the delocalization over hydrogen bond is observed. On the other hand, valence π orbitals of nylon-6 are well localized in each amide group as typically seen in monomer case (Figure 4S). In the dimer and trimer models, the transitions from these π orbitals independently generate a series of π−π* states; however, the coupling between these states is small. The MOs are localized even in the dimer and trimer models as shown in Figure 8a. Consequently, some intense π−π* states are generated in the dimer and trimer models of nylon-6. The calculated interaction energies of the hydrogen bondings were 22.2 (22.7) and 48.5 (49.4) kcal/mol for the dimer and trimer models of nylon-6 (nylon-6/6), respectively. These energies are not additive, and some nonlinear effect may exist. Though the present results agree with the experimental trend, the calculated hydrogen bond interaction energies with these models may not be enough to explain the difference of hydrogen bond interactions of nylon-6 and nylon-6/6; the present models only consider the local structure and the global structure of these two polymers should be important. Within this model system, the unit cell distance was calculated as 9.7 Å
Detailed results for monomer, dimer, and trimer of nylon-6 and nylon-6/6. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail [email protected] (Y.M.). *E-mail [email protected] (M.E.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was supported by a Grant-in-Aid (KAKENHI) for the Japan Society for the Promotion of Science (JSPS) and the Joint Studies Program of the Institute for Molecular Science. The work was also supported by Nanotechnology Platform Program (Molecule and Material Synthesis) of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. The calculations were partly performed using the Research Center for Computational Science in Okazaki, Japan. 11860
dx.doi.org/10.1021/jp5077005 | J. Phys. Chem. B 2014, 118, 11855−11861
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