Article pubs.acs.org/JPCA
Methyl Substitution Effect on the Jet-Cooled Laser-Induced Fluorescence Spectrum of Cyclohexoxy Radical Haiyan Hao, Lingxuan Wang, and Lily Zu* College of Chemistry, Beijing Normal University, Beijing, 100875, People’s Republic of China S Supporting Information *
ABSTRACT: Understanding the structure and properties of cyclohexoxy radical and its substitutes is important because of their presence in combustion processes, in atmospheric chemistry, and as intermediates in the hydrocarbon reactions. In this work, jet-cooled laser-induced fluorescence (LIF) spectra of five dimethyl substituted cyclohexoxy radicals are obtained for the first time. The correlation between the spectral variations and the radical structural changes is studied with the assistance of theoretical calculations at the B3LYP/6-31+G(d) and CASSCF/631+G(d) levels. The results show that the spectral characters of the dimethylcyclohexoxy radicals and their dissociation kinetics are predominantly affected by the methyl substitution position related to the C−O group. The spectral effect of the two methyl groups will add up if they locate on asymmetric carbons of the cyclohexoxy ring. Methyl substitution on β carbon weakens the six-member ring of cyclohexoxy and results in unimolecular dissociation via β C−C bond cleavage on the methyl group side and forms vinoxy variants. This study clearly shows that the LIF spectra can be used to identify cyclohexoxy and the isomers of its methyl substitutes. The results will help to understand the photochemistry of cyclic hydrocarbons in the atmospheric and combustion processes.
1. INTRODUCTION Cyclohexoxy radical and its substitutes are of special interest in atmospheric chemistry because of their important role in the oxidation reaction of cycloaliphatic hydrocarbons from modern fuels.1,2 Recently, we reported the vibrationally resolved laserinduced fluorescence (LIF) spectra of 2-, 3-, and 4methylcyclohexoxy radicals, and we observed different spectral characters.3 The C−O stretch progression with v′ up to 3 was observed in the LIF spectra when methyl substitution occurred on γ (3-methylcyclohexoxy) and δ (4-methylcyclohexoxy) carbons of the six-member ring of cyclohexoxy. In contrast, methyl substitution on β carbon (2-methylcyclcohexoxy) resulted in a significantly simplified spectrum with no C−O stretch band appearing. The structural changes were reflected in the spectra. The addition of a second methyl group onto the six-member ring will introduce extra gauche-butane type steric interactions.4,5 The intramolecular hydrogen bonding formed between the O atom and the methyl H will also affect the radical stability depending on the location of the methyl groups. Investigating the spectroscopic behavior of dimethyl substituted cyclohexoxy radicals will reveal the correlation between the radical structure and its spectral property and thus provide a basis for isomeric analysis of cyclic molecules. In this work, supersonic jet-cooled laser-induced fluorescence spectra of five dimethyl substituted cyclohexoxy radicals are studied. The correlation between spectral variation and the radical structural changes is investigated assisted by theoretical calculations using B3LYP/6-31+G(d) and CASSCF/631+G(d) methods. The spectral interpretation of the six© 2015 American Chemical Society
member rings with substituted methyl groups offers an opportunity to measure the steric perturbations in the cyclohexoxy ring, and can help the monitoring of cyclic alkoxy radicals by spectroscopic method.
2. EXPERIMENTAL AND COMPUTATIONAL METHODS The LIF experiments were conducted on a system described in refs 3 and 6−8. Basicly, the probe beam was provided by a dye laser (Narrowscan, Radiant Dyes) pumped by 532 or 355 nm of a Nd:YAG laser (Surelite III, Continuum). Five dyes, Stylry 8, Pyridine 1, Pyridine 2, Exlite 389/398, and Exlite 384 (Exciton, U.S.) were used in this study. The third harmonic (355 nm) of another Nd:YAG laser (Surelite II, Continuum) acted as the photolysis laser source with ∼30 mJ power per pulse. The alkyl nitrite precursor was synthesized according to a well-known procedure9 and was verified by UV and IR spectra.10 All alcohols for synthesis (2-methylcyclohexanol, 3methylcyclohexanol, 4-methylcyclohexanol, 2,3-dimethylcyclohexanol, 2,5-dimethylcyclohexanol, 2,6-dimethylcyclohexanol, 3,4-dimethylcyclohexanol, and 3,5-methylcyclohexanol with purity ≥97%) were purchased from Sigma-Aldrich and TCI Shanghai. The nitrite sample was introduced into the vacuum chamber via a 0.5 mm general valve and was photolized immediately. The total fluorescence generated by the excitation of the radical was collected using a photomultiplier tube Received: January 29, 2015 Revised: March 17, 2015 Published: March 17, 2015 3384
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Table 1. Calculated Ground State Energies (in kcal/mol) of Dimethylcyclohexoxy Radicals at the B3LYP/6-31+G(d) Level Ebrel + ΔZPE
radical stereomer chair conformer
stereomer 1 a
3,5-dimethylcyclohexoxy 3,4-dimethylcyclohexoxy 2,5-dimethylcyclohexoxy 2,3-dimethylcyclohexoxy 2,6-dimethylcyclohexoxy
stereomer 2
stereomer 3
stereomer 4
eee
aaa
aee
eaa
aae
eea
aea
eae
0 0 0 0 0
6.96 3.43 4.63 2.79 6.61
0.31 0.41 0.56 0.21 0.89
6.32 3.53 5.24 2.66 6.76
2.41 2.41 2.40 1.86 2.71
2.53 2.24 2.24 1.75 2.38
2.40 2.76 2.55 1.68 2.71
2.53 2.49 2.57 2.11 2.38
The conformers are named according to the substitution positions of oxygen, then methyl groups (attached to low number carbon first). bRelative energies (with Zero-point energy (ZPE) correction) to the eee conformer of each dimethylcyclohexoxy radical are shown. a
Table 2. Calculated and Experimental Data of Chair eee Conformers of All Five Dimethylcyclohexoxy Isomers
The energies are relative energies to 3,5-dimethylcyclohexoxy at the B3LYP/6-31+G(d) level. bThe B̃ − X̃ excitation energies are calculated at the TD-UB3LYP/6-31+G(d) level. cCalculated B̃ − X̃ adiabatic transition energies using CASSCF(9,7)/6-31+G(d) method with ZPE correction (scale factor 0.95). dObserved band maxima relative to experimental origin. a
Figure 1. Jet-cooled LIF spectra of 3-methylcyclohexoxy, 3,5-dimethylcyclohexoxy, and 3,4-dimethylcyclohexoxy radicals.
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Figure 2. Jet-cooled LIF spectra of 2-methylcyclohexoxy, 2,5-dimethylcyclohexoxy, 2,3-dimethylcyclohexoxy, and 2,6-dimethylcyclohexoxy radicals.
(Hamamasu, CR110) and was digitized by an oscilloscope (Tektronics, TBS3032B).7 A LabView program was used to control the operation of the experimental system and to record the fluorescence signal.3,6−8 The ground state geometry optimization of the chair conformers was conducted at the B3LYP/6-31+G(d) level for five dimethyl substituted cyclohexoxy radicals, that is, 3,4dimethylcyclohexoxy, 3,5-dimethylcyclohexoxy, 2,3-dimethylcyclohexoxy, 2,5-dimethylcyclohexoxy, and 2,6-dimethylcyclohexoxy. The CASSCF/6-31+G(d) method was also used to calculate the ground state and the B̃ excited state of dimethylcyclohexoxy radicals. Two nonbonding orbitals of O atom and the σ orbital of the C−O bond were included in the CAS(9,7) active space while other active orbitals and electrons were chosen by the program automatically.3,6−8 Calculated vibrational frequencies were used to help the experimental spectral assignment. The B̃ − X̃ excitation energies were calculated using the time-dependent density functional theory (TDDFT) method and the 6-31+G(d) basis set. The adiabatic transition energies between the B̃ state and the X̃ state were derived from the CASSCF calculations. The Gaussian 03 program was used in all calculations.11
Figure 3. Vibrational progressions in the LIF spectrum of 3,5dimethylcyclohexoxy.
their relative energies.4,5 Hence, for dimethyl substituted cyclohexoxy radicals, the lowest energy conformer, chair eee, is the primary species which will be considered in the analysis of the spectral carrier.12 Relative ground state (X̃ state) energies and the B̃ − X̃ transition energies of the chair eee conformers of the five dimethylcyclohexoxy radicals are presented in Table 2. 3.2. LIF Spectra of Dimethylcyclohexoxy Radicals. Vibrationally resolved LIF spectra of 3,4-dimethylcyclohexoxy, 3,5-dimethylcyclohexoxy, 2,3-dimethylcyclohexoxy, 2,5-dimethylcyclohexoxy, and 2,6-dimethylcyclohexoxy radicals were obtained in the supersonic jet-cooled condition. The experimental results showed that the spectra of these dimethyl substituted cyclohexoxy radicals fell into two groups. The 3,4and 3,5-dimethylcyclohexoxy followed the pattern of the spectrum of 3-methylcyclohexoxy (Figure 1). The other three dimethylcyclohexoxy radicals containing a methyl group on β carbon had a simple spectrum like 2-methylcyclohexoxy (Figure 2). This observation suggested that the substituents on the nearest carbon to the C−O group played the dominant role in the spectral variation. Upon this, more structural effect was revealed when the spectra were examined in detail. 3.3. Assignment of the Spectrum of 3,5-Dimethylcyclohexoxy Radical. Comparing to the LIF spectrum of 3-
3. RESULTS AND DISCUSSION 3.1. Stereomers and Conformers of Dimethylcyclohexoxy Radicals. The five dimethylcyclohexoxy radicals, that is, 3,4-dimethylcyclohexoxy, 3,5-dimethylcyclohexoxy, 2,3-dimethylcyclohexoxy, 2,5-dimethylcyclohexoxy, and 2,6-dimethylcyclohexoxy radicals, are positional isomers. Each isomer has four sets of stereomers according to the cis or trans relations between the oxygen atom and the two methyl groups. Each stereomer in turn has two interconvertible chair conformers defined by the equatorial (e) or axial (a) conformation of the substituents (Table 1). The calculation results revealed that the eee conformer in which all substituents were at the equatorial position always had the lowest energy (Table 1). Because the separation of the stereomers and the conformers is practically impossible, the dimethylcyclohexanols used to synthesize the precursors are the mixture of stereomers. However, the nature abundances of the dimethylcyclohexanol stereomers and conformers depend on 3386
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cyclohexoxy and its substitutes resulted from the electronic transition between the ground state (X̃ state) and the second excited state (B̃ state), with an electron excitation from the C− O σ bond to the O lone pair.13,14 Therefore, relatively strong intensities were expected if the vibrational mode had a component along the C−O stretch direction. From the structural view, the symmetry of the molecule increased from C1 to Cs when a second methyl group substituted H on the symmetric γ carbon (C5) of the 3-methylcyclohexoxy. For Cs geometry, the single-quantum excitation of asymmetric a″ vibrational modes was symmetry forbidden.13 Hence, less modes would be observed in the spectrum of 3,5dimethylcyclohexoxy which had Cs symmetry. Vibrational modes with a′ symmetry, band B (ν34, C−CH3 twist), band C (ν32, ring stretch along O−C1−C4), and band A* (ν28, C−O stretch), were all observed in the spectrum of 3,5dimethylcyclohexoxy with high intensity. On the other hand, band a and band b recorded in the spectrum of 3methylcyclohexoxy which corresponded to the C5 ring-bending and the C3−CH3 swing modes (see Scheme S1 of the Supporting Information) were not observed in the spectrum of 3,5-dimethylcyclohexoxy (Figure 3). These two vibrational modes were all related to asymmetric γ carbons of 3methylcyclohexoxy. For 3,5-dimethylcyclohexoxy, both γ carbons had methyl groups and became symmetric so that the C5 ring-bending mode did not exist anymore and the symmetric C3−CH3 and C5−CH3 swing would not result in the distortion of the ring, and thus, there was no component along the C−O stretch direction. However, recent studies by Liu and co-workers15,16 pointed out that some asymmetric vibronic bands could gain intensity via the pseudo-Jahn−Teller (JT) effect because of the low-lying first excited state (à state) of cyclohexoxy radical and its variants. Our B3LYP/6-31+G(d) calculation results showed that the potential energy curves of 3,5-dimethylcyclohexoxy were similar to that of cyclohexoxy,13,15 that is, the Cs geometry of ground-state 3,5dimethylcyclohexoxy was at the maximum of the X̃ 2A″ state JT barrier, a saddle point with an imaginary vibrational frequency. When the molecule was allowed to distort from Cs symmetry in geometry optimization, the symmetry of the X̃ state was lowered to C1 with a stabilization energy of 98 cm−1. The low-lying first excited state (à state) had A′ symmetry. The energy separation between the minima of à and X̃ states was 194 cm−1 for 3,5-dimethylcyclohexoxy compared to 215 cm−1 for cyclohexoxy radical. For 3,5-dimethylcyclohexoxy, one a″ band (ν62) was observed. The detailed assignment of the 3,5dimethylcyclohexoxy spectrum is shown in Table 3. The progression of the C−O stretch dominated in the 3,5dimethylcyclohexoxy spectrum as in 3-methylcyclohexoxy. The C−O stretch frequency 680 cm−1 was close to the value 683 cm−1 observed for 3-methylcyclohexoxy. Calculated C−O stretch frequency of 3,5-dimethylcyclohexoxy agreed well with the experimental observation. The lowest 25 vibrational frequencies of 3,5-dimethylcyclohexoxy on the B̃ state are presented in the Supporting Information (Table S1). 3.4. Gauche Steric Interaction in 3,4-Dimethylcyclohexoxy Radical. When the second methyl substitution occurred on the δ carbon (C4) of 3-methylcyclohexoxy instead of on the γ carbon, as in the case of 3,4-dimethylcyclohexoxy, the LIF excitation spectrum became more complicated compared to that of 3-methylcyclohexoxy. Interestingly, the origin band of 3,4-dimethylcyclohexoxy shifted to further red (106 cm−1 to 3-methylcyclohexoxy origin) than 3,5-dimethyl-
Table 3. Assignment of the Low-Frequency Experimental Bands in the LIF Spectrum of 3,5-Dimethylcyclohexoxy Radical; Bands Are Named as in Figure 3
̃
̃
Experimental band maxima (cm−1) relative to ν00B − X at 26611.1 cm−1. Calculated CASSCF(9,7)/6-31+G(d) frequencies (scale factor 0.95) of the fundamental and the combinational bands.
a b
Scheme 1. Newman Projection of Five Dimethylcyclohexoxy Radicals Showing the Additional Gauche Steric Interaction in 3,4- and 2,3-Dimethylcyclohexoxy Radicals Introduced by the Vicinal Methyl Groups
methylcyclohexoxy, the spectral origin of 3,5-dimethylcyclohexoxy shifted to the red for 66 cm−1. If the horizontal axis was adjusted to overlap the origin bands of 3-methylcyclohexoxy and 3,5-dimethylcyclohexoxy (Figure 3), the two spectra displayed the same mainframe except that the latter looked more cleaner with less small features. The LIF spectra of 3387
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Figure 4. LIF spectra of (a) 3,4-dimethylcyclohexoxy, (b) 3-methylcyclohexoxy, (c) 4-methylcyclohexoxy, and (d) the combination of b and c.
Table 4. Vibronic Band Assignment of 3,4Dimethylcyclohexoxy Radical; Bands Are Labeled as in Figure 4
̃
Scheme 2. Structures of 3,5-Dimethylcyclohexoxy and 2,6Dimethylcyclohexoxy Radicals Showing Intramolecular Hydrogen-Bonding Interaction and β C−C Bond Length (in Å)
substitution on the γ carbon. Unlike 3,5-dimethylcyclohexoxy, the 3,4-dimethylcyclohexoxy radical had C1 symmetry. The vicinal methyl substituent increased the energy of 1.53 kcal/mol above 3,5-dimethylcyclohexoxy because of the additional gauche-butane type steric interaction5,17−19 introduced by the two methyl groups substituted on adjacent carbons of the ring (Scheme 1), and thus, the X̃ state of 3,4-dimethylcyclohexoxy was destabilized. Overlapping the origin bands of the two spectra via adjusting the horizontal axis revealed that the 3,4dimethylcyclohexoxy spectrum (Figure 4a) contained all strong bands in the 3-methylcyclohexoxy spectrum (Figure 4b). The
̃
Experimental band frequency (cm−1) shifted from ν00B − X at 26570.8 cm−1. bThe CASSCF(9,7)/6-31+G(d) calculated frequencies (cm−1) with a scale factor of 0.95. a
cyclohexoxy (Figure 1). This phenomenon implied that the methyl substitution on the δ carbon added to the effect of 3388
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Figure 5. Expanded view of the high frequency spectral region of (a) 2-methylcyclohexoxy, (b) 2,5-dimethylcyclohexoxy, (c) 2,3dimethylcyclohexoxy, and (d) 2,6-dimethylcyclohexoxy radicals.
O stretch mode agreed well with the experimental observation (see Table S2 of the Supporting Information). 3.5. Dimethylcyclohexoxy Radicals with β Carbon Substitution. Red shift of the origin band was also recorded in the LIF spectra of dimethylcyclohexoxy radicals with β carbon substitution (Figure 2). The red-shifts relative to the 2methylcyclohexoxy origin were 66, 129, and 516 cm−1 for 2,5-, 2,3-, and 2,6-dimethylcyclohexoxy radicals, respectively. For 2,5- and 2,3-dimethylcyclohexoxy, the second methyl group was substituted on the γ carbon (C5 and C3, respectively). However, the extra gauche-butane type steric effect caused by the two methyl goups on the adjacent carbons (Scheme 1) destabilized the X̃ state of 2,3-dimethylcyclohexoxy for 1.69 kcal/mol compared to 2,5-dimethylcyclohexoxy; hence, the B̃ − X̃ transition shifted further to red. For 2,6-dimethylcyclohexoxy radical, the two methyl groups were substituted on β carbons (C2 and C6) separated on each side of the oxygen so that the radical had Cs symmetry. Unlike 3,5-dimethylcyclohexoxy which also had a Cs symmetry compared to 3-methylcyclohexoxy, the scale of red shift (516 cm−1) was much bigger for 2,6dimethylcyclohexoxy relative to 2-methylcyclohexoxy. Calculated B̃ − X̃ adiabatic transition energy of 2,6-dimethylcyclohexoxy (27 689 cm−1) and 3,5-dimethylcyclohexoxy (28 218 cm−1) (Table 2) supported the experimental observation of the difference between their origin bands (25 759.0 and 26611.1 cm−1). In the spectra studies of chain alkoxy radicals, Gopalakrishnan and co-workers contributed the origin band red-shift of gauche conformer related to trans conformer of 1propoxy as the B̃ state stabilization because of the nonclassical C−H···O hydrogen bonding between the O atom and one of the γ H’s which was possible in the G conformer but not in the T conformer of 1-propoxy.20,21 Calculated structural parameters of dimethyl substituted cyclohexoxy radicals agreed with this suggestion. The hydrogen-bonding interaction of C(H3)− H···O (2.589 Å) in 2,6-dimethylcyclohexoxy was stronger than that of β C−H···O (2.723 Å) in 3,5-dimethylcyclohexoxy (Scheme 2), especially in the B̃ state. Therefore, the stabilization of the B̃ state of 2,6-dimethylcyclohexoxy would lower the B̃ − X̃ transition energy. At the same time, the β C−
Figure 6. Lifetime measurements of (a) the origin band of 2,6dimethylcyclohexoxy, (b) band g of 2-methylvinoxy, and (c) band v3 of vinoxy.
C−O stretch progression with a frequency of 681 cm−1 similar to 3-methylcyclohexoxy stood out in the LIF spectrum. However, more bands appeared in the fundamental vibrational frequency region. The puzzle was solved when we red-shifted the spectrum of 4-methylcyclohexoxy 216 cm−1 and compared it with the extra bands of 3,4-dimethylcyclohexoxy (Figure 4c). The spectra combination of 3-methylcyclohexoxy and 4methylcyclohexoxy resembled the spectrum of 3,4-dimethylcyclohexoxy quite well (Figure 4d). However, the intensity of the vibronic bands corresponding to 4-methylcyclohexoxy decreased, probably because the Cs symmetry of 4-methylcyclohexoxy no longer existed. The assignment of the vibronic bands of 3,4-dimethylcyclohexoxy is listed in Table 4. Generally, the prediction of vibrational frequencies of 3,4-dimethylcyclohexoxy was not as good as that of 3,5-dimethylcyclohexoxy. The reason might be the complicated vibrational modes of 3,4dimethylcyclohexoxy because of the asymmetric substitution of the two methyl groups. Nevertheless, the prediction of the C− 3389
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Table 5. High-Frequency Bands (in cm−1, Labeled as in Figure 5d) in the LIF Spectrum of 2,6-Dimethylcyclohexoxy Are Assigneda According to Ref 22
a
The last two columns of band positions and assignment are taken from ref 22.
Table 6. Assignment of the High-Frequency Bands (in cm−1, Figure 5) Originated from the Dissociation of 2,5Dimethylcyclohexoxy, 2,3-Dimethylcyclohexoxy, and 2Methylcyclohexoxy
opening via β C−C bond fission was the major process competing with the photoexcitation of the radical. The experimental observation also showed that instead of remaining as a 6-oxo-1-hexyl radical which was predicted in the theoretical studies of cyclohexoxy,24−29 further dissociation occurred after the ring-opening and 2-methylvinoxy was formed. Bands with a short lifetime were also obseved in the highfrequency region of 2-methylcyclohexoxy, 2,5-dimethylcyclohexoxy, and 2,3-dimethylcyclohexoxy spectra but did not belong to 2-methylvinoxy radicals (Figure 5). The majority of these bands can be assigned to vinoxy30,31 (Table 6). Therefore, we can conclude that the β C−C bond fission preferred to occur at the C1−C2(CH3) bond instead of at the C1−C6 bond. The fact that no spectrum from dissociation products was observed in the high-frequency region of γ (3-) and δ (4-) methyl substitutes of cyclohexoxy radicals supported our hyperthesis that the methyl substitution on β carbon destabilized the cyclohexoxy ring. However, assignment of bands around 29 300 cm−1 cannot be made at this stage. They might come from other dissociation products of the ringopening, probably alkyl substitutes of vinoxy radical.32 Vibronic assignment of 2,3-, 2,5-, and 2,6-dimethylcyclohexoxy radicals was difficult as the vibronic bands that appeared in the spectra were very weak (Figure 2). One difference between the 2,5-dimethylcyclohexoxy and other β carbon substituted methylcyclohexoxy radicals must be noted here. A lowfrequency band appeared at 59 cm−1 above the origin band in the spectrum of 2,5-dimethylcyclohexoxy. The B̃ state vibrational frequency calculation did not give such a vibrational mode for 2,5-dimethylcyclohexoxy (see Tables S3−S5 of the Supporting Information). The origin bands of B̃ − Ã and B̃ − X̃ transitions were both observed previously in the moderate expansion condition for cyclohexoxy at such spacing.13,15 However, the possibility of observing both origin bands from B̃ − Ã and B̃ − X̃ transition was unlikely in this study as no B̃ − Ã transition origin was observed for cyclohexoxy and its single methyl substitutes under our experimental condition.3,8 On the other hand, assigning the two closely spaced bands of 2,5dimethylcyclohexoxy to origins of two conformers also faced a challenge as the spectra of cyclohexoxy,13 single methyl substituted cyclohexoxy,3 and 3,4-, and 3,5-dimethylcyclohex-
a
Band position and assignment are taken from ref 30. bBand position and assignment are taken from ref 31.
C bond in 2,6-dimethylcyclohexoxy was elongated because of the rigid ring strain as the hydrogen-bonding interaction between the oxygen and the methyl H formed a five-member ring (Scheme 2). The β C−C bond elongation was more pronounced in the X̃ state than in the B̃ state because the longer C−O bond length in the B̃ state compensated for the ring strain. Thus, the destabilized ground state and stabilized excited state resulted in the far red-shift of the spectral origin of 2,6-dimethylcyclohexoxy. No C−O stretch vibronic band appeared in the LIF spectrum of 2,6-dimethylcyclohexoxy radical, and the spectrum was limited in a short frequency range (25 500−26 500 cm−1) (Figure 2). On the other hand, a series of bands with significant intensity were observed in the 28 500−30 000 cm−1 region after ∼2000 cm−1 silence (Figure 5). The lifetime of these high frequency bands was much shorter than the origin band of 2,6dimethylcyclohexoxy radical (Figure 6). These vibronic bands can be assigned to 2-methylvinoxy radicals (Table 5) according to the reported studies by Williams and co-workers.22,23 This experimental observation suggested that ground-state ring3390
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oxy radicals could all be assigned using calculated results from one conformer which had the lowest energy. However, the observation of the spectra of cis and trans 2-methylvinoxy radicals as the dissociation products of 2,6-dimethylcyclohexoxy suggested that the CH3 group could be in cis or trans position relative to the oxygen, which implied that two stereomers might exist in the sample (Table 1). The aee conformer of stereomer 2 (CH3 groups in trans relative to O) is only 0.56 kcal/mol higher in energy than the eee conformer of stereomer 1 (CH3 groups in cis relative to O). Therefore, there is a possibilty to observe the origin bands from two stereomers of 2,5dimethylcyclohexoxy. However, reliable assignment of the vibronic bands could only be made by further studies of disperse fluorescence and rotationally resolved spectra of these radicals.15,33
ASSOCIATED CONTENT
* Supporting Information S
Vibrational modes of band a and band b in the LIF spectrum of 3-methylcyclohexoxy radical. Calculated vibrational frequencies of the eee conformer of 3,5-, 3,4-, 2,5-, 2,3-, and 2,6dimethylcyclohexoxy radicals in the B̃ excited state. This material is available free of charge via the Internet at http:// pubs.acs.org.
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REFERENCES
(1) Schauer, J. J.; Kleeman, M. J.; Cass, G. R.; Simoneit, B. R. T. Measurement of Emissions from Air Pollution Sources. 5. C1-C32 Organic Compounds from Gasoline-Powered Motor Vehicles. Environ. Sci. Technol. 2002, 36, 1169−1180. (2) Platz, J.; Sehested, J.; Nielsen, O. J. Atmospheric Chemistry of Cyclohexane: UV Spectra of c-C6H11· and (c-C6H11)O2· Radicals, Kinetics of the Reactions of (c-C6H11)O2· Radicals with NO and NO2, and the Fate of the Alkoxy Radical (c-C6H11)O·. J. Phys. Chem. A 1999, 103, 2688−2695. (3) Lin, J.; Wu, Q.; Liang, G.; Zu, L.; Fang, W. Jet-Cooled LaserInduced Fluorescence Spectroscopy of Methylcyclohexoxy Radicals. RSC Adv. 2012, 2, 583−589. (4) Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of Organic Compounds; Wiley Press: New York, 1997. (5) Nasipuri, D. Stereochemistry in Organic Compounds, Principles and Applications; New Age International (P) Ltd. Press: New Delhi, 1994. (6) Lin, J.; Zu, L.; Fang, W. Conformation and Spectroscopy Study of Cycloheptoxy Radical. J. Phys. Chem. A 2011, 115, 274−279. (7) Liang, G.; Liu, C.; Hao, H.; Zu, L.; Fang, W. Laser-Induced Fluorescence of Isobutoxy in Competition with Ground State Decomposition. J. Phys. Chem. A 2013, 117, 13229−13235. (8) Wu, Q.; Liang, G.; Zu, L.; Fang, W. Vibrationally Resolved LIF Spectrum of Tertiary Methylcyclohexoxy Radical. J. Phys. Chem. A 2012, 116, 3156−3162. (9) Blatt, A. H. Organic Syntheses; Wiley Press: New York, 1966. (10) Rosenberg, M.; SØlling, T. I. Computational Investigation of Photo Induced Processes in Alkyl Nitrites and the Product Alkoxy Radicals. Chem. Phys. Lett. 2010, 484, 113−118. (11) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, revision D.02; Gaussian, Inc.: Wallingford, CT, 2004. (12) Miller, T. A. Spectroscopic Probing and Diagnostics of the Geometric Structure of the Alkoxy and Alkyl Peroxy Radical Intermediates. Mol. Phys. 2006, 104, 2581−2593. (13) Zu, L.; Liu, J.; Tarczay, G.; Dupré, P.; Miller, T. A. Jet-Cooled Laser Spectroscopy of the Cyclohexoxy Radical. J. Chem. Phys. 2004, 120, 10579−10593. (14) Tarczay, G.; Gopalakrishnan, S.; Miller, T. A. Theoretical Prediction of Spectroscopic Constants of 1-Alkoxy Radicals. J. Mol. Spectrosc. 2003, 220, 276−290. (15) Liu, J.; Miller, T. A. Jet-Cooled Laser-Induced Fluorescence Spectroscopy of Cyclohexoxy: Rotational and Fine Structure of Molecules in Nearly Degenerate Electronic States. J. Phys. Chem. A 2014, 118, 11871−11890. (16) Chhantyal-Pun, R.; Roudjane, M.; Melnik, D. G.; Miller, T. A.; Liu, J. Jet-Cooled Laser-Induced Fluorescence Spectroscopy of Isopropoxy Radical: Vibronic Analysis of B̃ − X̃ and B̃ − X̃ Band Systems. J. Phys. Chem. A 2014, 118, 11852−11870. (17) Gonçalves, K. M. S.; Garcia, D. R.; Ramalho, T. C.; FigueroaVillar, J. D.; Freitas, M. P. Conformational Analysis of 1-Chloro- and 1Bromo-2-propanol. J. Phys. Chem. A 2013, 117, 10980−10984. (18) Dalling, D. K.; Grant, D. M. Carbon-13 Magnetic Resonance. XXI. Steric Interactions in the Methylcyclohexanes. J. Am. Chem. Soc. 1972, 94, 5318−5324. (19) Curtis, J.; Dalling, D. K.; Grant, D. M. Deuterium Chemical Shifts and Chemical Shift Parameters in Methylcyclohexanes. J. Org. Chem. 1986, 51, 136−142. (20) Gopalakrishnan, S.; Carter, C. C.; Zu, L.; Stakhursky, V.; Tarczay, G.; Miller, T. A. Rotationally Resolved B̃ − X̃ Electronic Spectra of Both Conformers of the 1-Propoxy Radical. J. Chem. Phys. 2003, 118, 4954−4969. (21) Gopalakrishnan, S.; Zu, L.; Miller, T. A. Rotationally Resolved Electronic Spectra of the B̃ − X̃ Transition in Multiple Conformers of 1-Butoxy and 1-Pentoxy Radicals. J. Phys. Chem. A 2003, 107, 5189− 5201. (22) Williams, S.; Harding, L. B.; Stanton, J. F.; Weisshaar, J. C. Barrier to Methyl Internal Rotation of Cis- and Trans-2-Methylvinoxy
4. CONCLUSIONS In this work, vibrationally resolved LIF spectra of five double methyl substituted cyclohexoxy radicals were obtained in the supersonic jet-cooled condition. The spectral character of the dimethylcyclohexoxy radical was predominantly affected by the nearest substituent related to the C−O group. The spectral effect of the two methyl groups added up when they located on asymmetric carbons of the cyclohexoxy ring. When two methyl groups located on adjacent ring carbons, the additional gauche interaction destabilized the radical and resulted in a further redshift of the B̃ − X̃ transition origin. Detection of the dissociation products 2-methylvinoxy radicals for 2,6-dimethylcyclohexoxy and vinoxy radical for 2,3- and 2,5-dimethylcyclohexoxy radicals revealed that the β C−C bond fission preferred to occur on the side with β methyl substitution, that is, the methyl group on β carbon weakened the six-member ring of cyclohexoxy. The experimental results also showed that the radical further dissociated after the ringopening. This study clearly shows that the LIF spectra can be used to identify cyclohexoxy radical and the isomers of its methyl substitutes. The results will help to understand the photochemistry of cyclic hydrocarbons in the atmosphere.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Fax: +86-10-58802075. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are pleased to acknowledge the financial support of this research by National Natural Science Foundation of China (grant nos. 21373033 and 21173024). We also acknowledge the grant from the Fundamental Research Funds for the Central Universities. 3391
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The Journal of Physical Chemistry A Radical in the X̃ 2(A″) and B̃ 2(A″) States: Experiment and Theory. J. Phys. Chem. A 2000, 104, 9906−9913. (23) Williams, S.; Harding, L. B.; Stanton, J. F.; Weisshaar, J. C. Barrier to Methyl Internal Rotation of 1-Methylvinoxy Radical in the X̃ 2(A″) and B̃ 2(A″) States: Experiment and Theory. J. Phys. Chem. A 2000, 104, 10131−10138. (24) Zhang, L.; Callahan, K. M.; Derbyshire, D.; Dibble, T. S. LaserInduced Fluorescence Spectra of 4-Methylcyclohexoxy Radical and Perdeuterated Cyclohexoxy Radical and Direct Kinetic Studies of Their Reactions with O2. J. Phys. Chem. A 2005, 109, 9232−9240. (25) Welz, O.; Striebelz, F.; Olzmann, M. On the Thermal Unimolecular Decomposition of the Cyclohexoxy Radicalan Experimental and Theoretical Study. Phys. Chem. Chem. Phys. 2008, 10, 320−329. (26) Aschmann, S. M.; Chew, A. A.; Arey, J.; Atkinson, R. Products of the Gas-Phase Reaction of OH Radicals with Cyclohexane: Reactions of the Cyclohexoxy Radical. J. Phys. Chem. A 1997, 101, 8042−8048. (27) Orlando, J. J.; Iraci, L. T.; Tyndall, G. S. Chemistry of the Cyclopentoxy and Cyclohexoxy Radicals at Subambient Temperatures. J. Phys. Chem. A 2000, 104, 5072−5079. (28) Zhang, L.; Kitney, K. A.; Ferenac, M. A.; Deng, W.; Dibble, T. S. LIF Spectra of Cyclohexoxy Radical and Direct Kinetic Studies of Its Reaction with O2. J. Phys. Chem. A 2004, 108, 447−454. (29) Vereecken, L.; Peeters, J. A Decomposition of Substituted Alkoxy Radicals − Part I: A Generalized Structure-Activity Relationship for Reaction Barrier Heights. Phys. Chem. Chem. Phys. 2009, 11, 9062−9074. (30) Chhantyal-Pun, R.; Chen, M.; Sun, D.; Miller, T. A. Detection and Characterization of Products from Photodissociation of XCH2CH2ONO (X = F, Cl, Br, OH). J. Phys. Chem. A 2012, 116, 12032−12040. (31) Brock, L. R.; Rohlfing, E. A. Spectroscopic Studies of the B̃ 2A″ − X̃ 2A″ System of the Jet-Cooled Vinoxy Radical. J. Chem. Phys. 1997, 106, 10048−10065. (32) Washida, N.; Inomata, S.; Furubayashi, M. Laser-Induced Fluorescence of Methyl Substituted Vinoxy Radicals and Reactions of Oxygen Atoms with Olefins. J. Phys. Chem. A 1998, 102, 7924−7930. (33) Jin, J.; Sioutis, I.; Tarczay, G.; Gopalakrishnan, S.; Bezant, A.; Miller, T. A. Dispersed Fluorescence Spectroscopy of Primary and Secondary Alkoxy Radicals. J. Chem. Phys. 2004, 121, 11780−11797.
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Methyl Substitution Effect on the Jet-Cooled Laser-Induced Fluorescence Spectrum of Cyclohexoxy Radical Haiyan Hao, Lingxuan Wang, and Lily Zu* College of Chemistry, Beijing Normal University, Beijing, 100875, People’s Republic of China S Supporting Information *
ABSTRACT: Understanding the structure and properties of cyclohexoxy radical and its substitutes is important because of their presence in combustion processes, in atmospheric chemistry, and as intermediates in the hydrocarbon reactions. In this work, jet-cooled laser-induced fluorescence (LIF) spectra of five dimethyl substituted cyclohexoxy radicals are obtained for the first time. The correlation between the spectral variations and the radical structural changes is studied with the assistance of theoretical calculations at the B3LYP/6-31+G(d) and CASSCF/631+G(d) levels. The results show that the spectral characters of the dimethylcyclohexoxy radicals and their dissociation kinetics are predominantly affected by the methyl substitution position related to the C−O group. The spectral effect of the two methyl groups will add up if they locate on asymmetric carbons of the cyclohexoxy ring. Methyl substitution on β carbon weakens the six-member ring of cyclohexoxy and results in unimolecular dissociation via β C−C bond cleavage on the methyl group side and forms vinoxy variants. This study clearly shows that the LIF spectra can be used to identify cyclohexoxy and the isomers of its methyl substitutes. The results will help to understand the photochemistry of cyclic hydrocarbons in the atmospheric and combustion processes.
1. INTRODUCTION Cyclohexoxy radical and its substitutes are of special interest in atmospheric chemistry because of their important role in the oxidation reaction of cycloaliphatic hydrocarbons from modern fuels.1,2 Recently, we reported the vibrationally resolved laserinduced fluorescence (LIF) spectra of 2-, 3-, and 4methylcyclohexoxy radicals, and we observed different spectral characters.3 The C−O stretch progression with v′ up to 3 was observed in the LIF spectra when methyl substitution occurred on γ (3-methylcyclohexoxy) and δ (4-methylcyclohexoxy) carbons of the six-member ring of cyclohexoxy. In contrast, methyl substitution on β carbon (2-methylcyclcohexoxy) resulted in a significantly simplified spectrum with no C−O stretch band appearing. The structural changes were reflected in the spectra. The addition of a second methyl group onto the six-member ring will introduce extra gauche-butane type steric interactions.4,5 The intramolecular hydrogen bonding formed between the O atom and the methyl H will also affect the radical stability depending on the location of the methyl groups. Investigating the spectroscopic behavior of dimethyl substituted cyclohexoxy radicals will reveal the correlation between the radical structure and its spectral property and thus provide a basis for isomeric analysis of cyclic molecules. In this work, supersonic jet-cooled laser-induced fluorescence spectra of five dimethyl substituted cyclohexoxy radicals are studied. The correlation between spectral variation and the radical structural changes is investigated assisted by theoretical calculations using B3LYP/6-31+G(d) and CASSCF/631+G(d) methods. The spectral interpretation of the six© 2015 American Chemical Society
member rings with substituted methyl groups offers an opportunity to measure the steric perturbations in the cyclohexoxy ring, and can help the monitoring of cyclic alkoxy radicals by spectroscopic method.
2. EXPERIMENTAL AND COMPUTATIONAL METHODS The LIF experiments were conducted on a system described in refs 3 and 6−8. Basicly, the probe beam was provided by a dye laser (Narrowscan, Radiant Dyes) pumped by 532 or 355 nm of a Nd:YAG laser (Surelite III, Continuum). Five dyes, Stylry 8, Pyridine 1, Pyridine 2, Exlite 389/398, and Exlite 384 (Exciton, U.S.) were used in this study. The third harmonic (355 nm) of another Nd:YAG laser (Surelite II, Continuum) acted as the photolysis laser source with ∼30 mJ power per pulse. The alkyl nitrite precursor was synthesized according to a well-known procedure9 and was verified by UV and IR spectra.10 All alcohols for synthesis (2-methylcyclohexanol, 3methylcyclohexanol, 4-methylcyclohexanol, 2,3-dimethylcyclohexanol, 2,5-dimethylcyclohexanol, 2,6-dimethylcyclohexanol, 3,4-dimethylcyclohexanol, and 3,5-methylcyclohexanol with purity ≥97%) were purchased from Sigma-Aldrich and TCI Shanghai. The nitrite sample was introduced into the vacuum chamber via a 0.5 mm general valve and was photolized immediately. The total fluorescence generated by the excitation of the radical was collected using a photomultiplier tube Received: January 29, 2015 Revised: March 17, 2015 Published: March 17, 2015 3384
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Table 1. Calculated Ground State Energies (in kcal/mol) of Dimethylcyclohexoxy Radicals at the B3LYP/6-31+G(d) Level Ebrel + ΔZPE
radical stereomer chair conformer
stereomer 1 a
3,5-dimethylcyclohexoxy 3,4-dimethylcyclohexoxy 2,5-dimethylcyclohexoxy 2,3-dimethylcyclohexoxy 2,6-dimethylcyclohexoxy
stereomer 2
stereomer 3
stereomer 4
eee
aaa
aee
eaa
aae
eea
aea
eae
0 0 0 0 0
6.96 3.43 4.63 2.79 6.61
0.31 0.41 0.56 0.21 0.89
6.32 3.53 5.24 2.66 6.76
2.41 2.41 2.40 1.86 2.71
2.53 2.24 2.24 1.75 2.38
2.40 2.76 2.55 1.68 2.71
2.53 2.49 2.57 2.11 2.38
The conformers are named according to the substitution positions of oxygen, then methyl groups (attached to low number carbon first). bRelative energies (with Zero-point energy (ZPE) correction) to the eee conformer of each dimethylcyclohexoxy radical are shown. a
Table 2. Calculated and Experimental Data of Chair eee Conformers of All Five Dimethylcyclohexoxy Isomers
The energies are relative energies to 3,5-dimethylcyclohexoxy at the B3LYP/6-31+G(d) level. bThe B̃ − X̃ excitation energies are calculated at the TD-UB3LYP/6-31+G(d) level. cCalculated B̃ − X̃ adiabatic transition energies using CASSCF(9,7)/6-31+G(d) method with ZPE correction (scale factor 0.95). dObserved band maxima relative to experimental origin. a
Figure 1. Jet-cooled LIF spectra of 3-methylcyclohexoxy, 3,5-dimethylcyclohexoxy, and 3,4-dimethylcyclohexoxy radicals.
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Figure 2. Jet-cooled LIF spectra of 2-methylcyclohexoxy, 2,5-dimethylcyclohexoxy, 2,3-dimethylcyclohexoxy, and 2,6-dimethylcyclohexoxy radicals.
(Hamamasu, CR110) and was digitized by an oscilloscope (Tektronics, TBS3032B).7 A LabView program was used to control the operation of the experimental system and to record the fluorescence signal.3,6−8 The ground state geometry optimization of the chair conformers was conducted at the B3LYP/6-31+G(d) level for five dimethyl substituted cyclohexoxy radicals, that is, 3,4dimethylcyclohexoxy, 3,5-dimethylcyclohexoxy, 2,3-dimethylcyclohexoxy, 2,5-dimethylcyclohexoxy, and 2,6-dimethylcyclohexoxy. The CASSCF/6-31+G(d) method was also used to calculate the ground state and the B̃ excited state of dimethylcyclohexoxy radicals. Two nonbonding orbitals of O atom and the σ orbital of the C−O bond were included in the CAS(9,7) active space while other active orbitals and electrons were chosen by the program automatically.3,6−8 Calculated vibrational frequencies were used to help the experimental spectral assignment. The B̃ − X̃ excitation energies were calculated using the time-dependent density functional theory (TDDFT) method and the 6-31+G(d) basis set. The adiabatic transition energies between the B̃ state and the X̃ state were derived from the CASSCF calculations. The Gaussian 03 program was used in all calculations.11
Figure 3. Vibrational progressions in the LIF spectrum of 3,5dimethylcyclohexoxy.
their relative energies.4,5 Hence, for dimethyl substituted cyclohexoxy radicals, the lowest energy conformer, chair eee, is the primary species which will be considered in the analysis of the spectral carrier.12 Relative ground state (X̃ state) energies and the B̃ − X̃ transition energies of the chair eee conformers of the five dimethylcyclohexoxy radicals are presented in Table 2. 3.2. LIF Spectra of Dimethylcyclohexoxy Radicals. Vibrationally resolved LIF spectra of 3,4-dimethylcyclohexoxy, 3,5-dimethylcyclohexoxy, 2,3-dimethylcyclohexoxy, 2,5-dimethylcyclohexoxy, and 2,6-dimethylcyclohexoxy radicals were obtained in the supersonic jet-cooled condition. The experimental results showed that the spectra of these dimethyl substituted cyclohexoxy radicals fell into two groups. The 3,4and 3,5-dimethylcyclohexoxy followed the pattern of the spectrum of 3-methylcyclohexoxy (Figure 1). The other three dimethylcyclohexoxy radicals containing a methyl group on β carbon had a simple spectrum like 2-methylcyclohexoxy (Figure 2). This observation suggested that the substituents on the nearest carbon to the C−O group played the dominant role in the spectral variation. Upon this, more structural effect was revealed when the spectra were examined in detail. 3.3. Assignment of the Spectrum of 3,5-Dimethylcyclohexoxy Radical. Comparing to the LIF spectrum of 3-
3. RESULTS AND DISCUSSION 3.1. Stereomers and Conformers of Dimethylcyclohexoxy Radicals. The five dimethylcyclohexoxy radicals, that is, 3,4-dimethylcyclohexoxy, 3,5-dimethylcyclohexoxy, 2,3-dimethylcyclohexoxy, 2,5-dimethylcyclohexoxy, and 2,6-dimethylcyclohexoxy radicals, are positional isomers. Each isomer has four sets of stereomers according to the cis or trans relations between the oxygen atom and the two methyl groups. Each stereomer in turn has two interconvertible chair conformers defined by the equatorial (e) or axial (a) conformation of the substituents (Table 1). The calculation results revealed that the eee conformer in which all substituents were at the equatorial position always had the lowest energy (Table 1). Because the separation of the stereomers and the conformers is practically impossible, the dimethylcyclohexanols used to synthesize the precursors are the mixture of stereomers. However, the nature abundances of the dimethylcyclohexanol stereomers and conformers depend on 3386
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cyclohexoxy and its substitutes resulted from the electronic transition between the ground state (X̃ state) and the second excited state (B̃ state), with an electron excitation from the C− O σ bond to the O lone pair.13,14 Therefore, relatively strong intensities were expected if the vibrational mode had a component along the C−O stretch direction. From the structural view, the symmetry of the molecule increased from C1 to Cs when a second methyl group substituted H on the symmetric γ carbon (C5) of the 3-methylcyclohexoxy. For Cs geometry, the single-quantum excitation of asymmetric a″ vibrational modes was symmetry forbidden.13 Hence, less modes would be observed in the spectrum of 3,5dimethylcyclohexoxy which had Cs symmetry. Vibrational modes with a′ symmetry, band B (ν34, C−CH3 twist), band C (ν32, ring stretch along O−C1−C4), and band A* (ν28, C−O stretch), were all observed in the spectrum of 3,5dimethylcyclohexoxy with high intensity. On the other hand, band a and band b recorded in the spectrum of 3methylcyclohexoxy which corresponded to the C5 ring-bending and the C3−CH3 swing modes (see Scheme S1 of the Supporting Information) were not observed in the spectrum of 3,5-dimethylcyclohexoxy (Figure 3). These two vibrational modes were all related to asymmetric γ carbons of 3methylcyclohexoxy. For 3,5-dimethylcyclohexoxy, both γ carbons had methyl groups and became symmetric so that the C5 ring-bending mode did not exist anymore and the symmetric C3−CH3 and C5−CH3 swing would not result in the distortion of the ring, and thus, there was no component along the C−O stretch direction. However, recent studies by Liu and co-workers15,16 pointed out that some asymmetric vibronic bands could gain intensity via the pseudo-Jahn−Teller (JT) effect because of the low-lying first excited state (à state) of cyclohexoxy radical and its variants. Our B3LYP/6-31+G(d) calculation results showed that the potential energy curves of 3,5-dimethylcyclohexoxy were similar to that of cyclohexoxy,13,15 that is, the Cs geometry of ground-state 3,5dimethylcyclohexoxy was at the maximum of the X̃ 2A″ state JT barrier, a saddle point with an imaginary vibrational frequency. When the molecule was allowed to distort from Cs symmetry in geometry optimization, the symmetry of the X̃ state was lowered to C1 with a stabilization energy of 98 cm−1. The low-lying first excited state (à state) had A′ symmetry. The energy separation between the minima of à and X̃ states was 194 cm−1 for 3,5-dimethylcyclohexoxy compared to 215 cm−1 for cyclohexoxy radical. For 3,5-dimethylcyclohexoxy, one a″ band (ν62) was observed. The detailed assignment of the 3,5dimethylcyclohexoxy spectrum is shown in Table 3. The progression of the C−O stretch dominated in the 3,5dimethylcyclohexoxy spectrum as in 3-methylcyclohexoxy. The C−O stretch frequency 680 cm−1 was close to the value 683 cm−1 observed for 3-methylcyclohexoxy. Calculated C−O stretch frequency of 3,5-dimethylcyclohexoxy agreed well with the experimental observation. The lowest 25 vibrational frequencies of 3,5-dimethylcyclohexoxy on the B̃ state are presented in the Supporting Information (Table S1). 3.4. Gauche Steric Interaction in 3,4-Dimethylcyclohexoxy Radical. When the second methyl substitution occurred on the δ carbon (C4) of 3-methylcyclohexoxy instead of on the γ carbon, as in the case of 3,4-dimethylcyclohexoxy, the LIF excitation spectrum became more complicated compared to that of 3-methylcyclohexoxy. Interestingly, the origin band of 3,4-dimethylcyclohexoxy shifted to further red (106 cm−1 to 3-methylcyclohexoxy origin) than 3,5-dimethyl-
Table 3. Assignment of the Low-Frequency Experimental Bands in the LIF Spectrum of 3,5-Dimethylcyclohexoxy Radical; Bands Are Named as in Figure 3
̃
̃
Experimental band maxima (cm−1) relative to ν00B − X at 26611.1 cm−1. Calculated CASSCF(9,7)/6-31+G(d) frequencies (scale factor 0.95) of the fundamental and the combinational bands.
a b
Scheme 1. Newman Projection of Five Dimethylcyclohexoxy Radicals Showing the Additional Gauche Steric Interaction in 3,4- and 2,3-Dimethylcyclohexoxy Radicals Introduced by the Vicinal Methyl Groups
methylcyclohexoxy, the spectral origin of 3,5-dimethylcyclohexoxy shifted to the red for 66 cm−1. If the horizontal axis was adjusted to overlap the origin bands of 3-methylcyclohexoxy and 3,5-dimethylcyclohexoxy (Figure 3), the two spectra displayed the same mainframe except that the latter looked more cleaner with less small features. The LIF spectra of 3387
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Figure 4. LIF spectra of (a) 3,4-dimethylcyclohexoxy, (b) 3-methylcyclohexoxy, (c) 4-methylcyclohexoxy, and (d) the combination of b and c.
Table 4. Vibronic Band Assignment of 3,4Dimethylcyclohexoxy Radical; Bands Are Labeled as in Figure 4
̃
Scheme 2. Structures of 3,5-Dimethylcyclohexoxy and 2,6Dimethylcyclohexoxy Radicals Showing Intramolecular Hydrogen-Bonding Interaction and β C−C Bond Length (in Å)
substitution on the γ carbon. Unlike 3,5-dimethylcyclohexoxy, the 3,4-dimethylcyclohexoxy radical had C1 symmetry. The vicinal methyl substituent increased the energy of 1.53 kcal/mol above 3,5-dimethylcyclohexoxy because of the additional gauche-butane type steric interaction5,17−19 introduced by the two methyl groups substituted on adjacent carbons of the ring (Scheme 1), and thus, the X̃ state of 3,4-dimethylcyclohexoxy was destabilized. Overlapping the origin bands of the two spectra via adjusting the horizontal axis revealed that the 3,4dimethylcyclohexoxy spectrum (Figure 4a) contained all strong bands in the 3-methylcyclohexoxy spectrum (Figure 4b). The
̃
Experimental band frequency (cm−1) shifted from ν00B − X at 26570.8 cm−1. bThe CASSCF(9,7)/6-31+G(d) calculated frequencies (cm−1) with a scale factor of 0.95. a
cyclohexoxy (Figure 1). This phenomenon implied that the methyl substitution on the δ carbon added to the effect of 3388
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Figure 5. Expanded view of the high frequency spectral region of (a) 2-methylcyclohexoxy, (b) 2,5-dimethylcyclohexoxy, (c) 2,3dimethylcyclohexoxy, and (d) 2,6-dimethylcyclohexoxy radicals.
O stretch mode agreed well with the experimental observation (see Table S2 of the Supporting Information). 3.5. Dimethylcyclohexoxy Radicals with β Carbon Substitution. Red shift of the origin band was also recorded in the LIF spectra of dimethylcyclohexoxy radicals with β carbon substitution (Figure 2). The red-shifts relative to the 2methylcyclohexoxy origin were 66, 129, and 516 cm−1 for 2,5-, 2,3-, and 2,6-dimethylcyclohexoxy radicals, respectively. For 2,5- and 2,3-dimethylcyclohexoxy, the second methyl group was substituted on the γ carbon (C5 and C3, respectively). However, the extra gauche-butane type steric effect caused by the two methyl goups on the adjacent carbons (Scheme 1) destabilized the X̃ state of 2,3-dimethylcyclohexoxy for 1.69 kcal/mol compared to 2,5-dimethylcyclohexoxy; hence, the B̃ − X̃ transition shifted further to red. For 2,6-dimethylcyclohexoxy radical, the two methyl groups were substituted on β carbons (C2 and C6) separated on each side of the oxygen so that the radical had Cs symmetry. Unlike 3,5-dimethylcyclohexoxy which also had a Cs symmetry compared to 3-methylcyclohexoxy, the scale of red shift (516 cm−1) was much bigger for 2,6dimethylcyclohexoxy relative to 2-methylcyclohexoxy. Calculated B̃ − X̃ adiabatic transition energy of 2,6-dimethylcyclohexoxy (27 689 cm−1) and 3,5-dimethylcyclohexoxy (28 218 cm−1) (Table 2) supported the experimental observation of the difference between their origin bands (25 759.0 and 26611.1 cm−1). In the spectra studies of chain alkoxy radicals, Gopalakrishnan and co-workers contributed the origin band red-shift of gauche conformer related to trans conformer of 1propoxy as the B̃ state stabilization because of the nonclassical C−H···O hydrogen bonding between the O atom and one of the γ H’s which was possible in the G conformer but not in the T conformer of 1-propoxy.20,21 Calculated structural parameters of dimethyl substituted cyclohexoxy radicals agreed with this suggestion. The hydrogen-bonding interaction of C(H3)− H···O (2.589 Å) in 2,6-dimethylcyclohexoxy was stronger than that of β C−H···O (2.723 Å) in 3,5-dimethylcyclohexoxy (Scheme 2), especially in the B̃ state. Therefore, the stabilization of the B̃ state of 2,6-dimethylcyclohexoxy would lower the B̃ − X̃ transition energy. At the same time, the β C−
Figure 6. Lifetime measurements of (a) the origin band of 2,6dimethylcyclohexoxy, (b) band g of 2-methylvinoxy, and (c) band v3 of vinoxy.
C−O stretch progression with a frequency of 681 cm−1 similar to 3-methylcyclohexoxy stood out in the LIF spectrum. However, more bands appeared in the fundamental vibrational frequency region. The puzzle was solved when we red-shifted the spectrum of 4-methylcyclohexoxy 216 cm−1 and compared it with the extra bands of 3,4-dimethylcyclohexoxy (Figure 4c). The spectra combination of 3-methylcyclohexoxy and 4methylcyclohexoxy resembled the spectrum of 3,4-dimethylcyclohexoxy quite well (Figure 4d). However, the intensity of the vibronic bands corresponding to 4-methylcyclohexoxy decreased, probably because the Cs symmetry of 4-methylcyclohexoxy no longer existed. The assignment of the vibronic bands of 3,4-dimethylcyclohexoxy is listed in Table 4. Generally, the prediction of vibrational frequencies of 3,4-dimethylcyclohexoxy was not as good as that of 3,5-dimethylcyclohexoxy. The reason might be the complicated vibrational modes of 3,4dimethylcyclohexoxy because of the asymmetric substitution of the two methyl groups. Nevertheless, the prediction of the C− 3389
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Table 5. High-Frequency Bands (in cm−1, Labeled as in Figure 5d) in the LIF Spectrum of 2,6-Dimethylcyclohexoxy Are Assigneda According to Ref 22
a
The last two columns of band positions and assignment are taken from ref 22.
Table 6. Assignment of the High-Frequency Bands (in cm−1, Figure 5) Originated from the Dissociation of 2,5Dimethylcyclohexoxy, 2,3-Dimethylcyclohexoxy, and 2Methylcyclohexoxy
opening via β C−C bond fission was the major process competing with the photoexcitation of the radical. The experimental observation also showed that instead of remaining as a 6-oxo-1-hexyl radical which was predicted in the theoretical studies of cyclohexoxy,24−29 further dissociation occurred after the ring-opening and 2-methylvinoxy was formed. Bands with a short lifetime were also obseved in the highfrequency region of 2-methylcyclohexoxy, 2,5-dimethylcyclohexoxy, and 2,3-dimethylcyclohexoxy spectra but did not belong to 2-methylvinoxy radicals (Figure 5). The majority of these bands can be assigned to vinoxy30,31 (Table 6). Therefore, we can conclude that the β C−C bond fission preferred to occur at the C1−C2(CH3) bond instead of at the C1−C6 bond. The fact that no spectrum from dissociation products was observed in the high-frequency region of γ (3-) and δ (4-) methyl substitutes of cyclohexoxy radicals supported our hyperthesis that the methyl substitution on β carbon destabilized the cyclohexoxy ring. However, assignment of bands around 29 300 cm−1 cannot be made at this stage. They might come from other dissociation products of the ringopening, probably alkyl substitutes of vinoxy radical.32 Vibronic assignment of 2,3-, 2,5-, and 2,6-dimethylcyclohexoxy radicals was difficult as the vibronic bands that appeared in the spectra were very weak (Figure 2). One difference between the 2,5-dimethylcyclohexoxy and other β carbon substituted methylcyclohexoxy radicals must be noted here. A lowfrequency band appeared at 59 cm−1 above the origin band in the spectrum of 2,5-dimethylcyclohexoxy. The B̃ state vibrational frequency calculation did not give such a vibrational mode for 2,5-dimethylcyclohexoxy (see Tables S3−S5 of the Supporting Information). The origin bands of B̃ − Ã and B̃ − X̃ transitions were both observed previously in the moderate expansion condition for cyclohexoxy at such spacing.13,15 However, the possibility of observing both origin bands from B̃ − Ã and B̃ − X̃ transition was unlikely in this study as no B̃ − Ã transition origin was observed for cyclohexoxy and its single methyl substitutes under our experimental condition.3,8 On the other hand, assigning the two closely spaced bands of 2,5dimethylcyclohexoxy to origins of two conformers also faced a challenge as the spectra of cyclohexoxy,13 single methyl substituted cyclohexoxy,3 and 3,4-, and 3,5-dimethylcyclohex-
a
Band position and assignment are taken from ref 30. bBand position and assignment are taken from ref 31.
C bond in 2,6-dimethylcyclohexoxy was elongated because of the rigid ring strain as the hydrogen-bonding interaction between the oxygen and the methyl H formed a five-member ring (Scheme 2). The β C−C bond elongation was more pronounced in the X̃ state than in the B̃ state because the longer C−O bond length in the B̃ state compensated for the ring strain. Thus, the destabilized ground state and stabilized excited state resulted in the far red-shift of the spectral origin of 2,6-dimethylcyclohexoxy. No C−O stretch vibronic band appeared in the LIF spectrum of 2,6-dimethylcyclohexoxy radical, and the spectrum was limited in a short frequency range (25 500−26 500 cm−1) (Figure 2). On the other hand, a series of bands with significant intensity were observed in the 28 500−30 000 cm−1 region after ∼2000 cm−1 silence (Figure 5). The lifetime of these high frequency bands was much shorter than the origin band of 2,6dimethylcyclohexoxy radical (Figure 6). These vibronic bands can be assigned to 2-methylvinoxy radicals (Table 5) according to the reported studies by Williams and co-workers.22,23 This experimental observation suggested that ground-state ring3390
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oxy radicals could all be assigned using calculated results from one conformer which had the lowest energy. However, the observation of the spectra of cis and trans 2-methylvinoxy radicals as the dissociation products of 2,6-dimethylcyclohexoxy suggested that the CH3 group could be in cis or trans position relative to the oxygen, which implied that two stereomers might exist in the sample (Table 1). The aee conformer of stereomer 2 (CH3 groups in trans relative to O) is only 0.56 kcal/mol higher in energy than the eee conformer of stereomer 1 (CH3 groups in cis relative to O). Therefore, there is a possibilty to observe the origin bands from two stereomers of 2,5dimethylcyclohexoxy. However, reliable assignment of the vibronic bands could only be made by further studies of disperse fluorescence and rotationally resolved spectra of these radicals.15,33
ASSOCIATED CONTENT
* Supporting Information S
Vibrational modes of band a and band b in the LIF spectrum of 3-methylcyclohexoxy radical. Calculated vibrational frequencies of the eee conformer of 3,5-, 3,4-, 2,5-, 2,3-, and 2,6dimethylcyclohexoxy radicals in the B̃ excited state. This material is available free of charge via the Internet at http:// pubs.acs.org.
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REFERENCES
(1) Schauer, J. J.; Kleeman, M. J.; Cass, G. R.; Simoneit, B. R. T. Measurement of Emissions from Air Pollution Sources. 5. C1-C32 Organic Compounds from Gasoline-Powered Motor Vehicles. Environ. Sci. Technol. 2002, 36, 1169−1180. (2) Platz, J.; Sehested, J.; Nielsen, O. J. Atmospheric Chemistry of Cyclohexane: UV Spectra of c-C6H11· and (c-C6H11)O2· Radicals, Kinetics of the Reactions of (c-C6H11)O2· Radicals with NO and NO2, and the Fate of the Alkoxy Radical (c-C6H11)O·. J. Phys. Chem. A 1999, 103, 2688−2695. (3) Lin, J.; Wu, Q.; Liang, G.; Zu, L.; Fang, W. Jet-Cooled LaserInduced Fluorescence Spectroscopy of Methylcyclohexoxy Radicals. RSC Adv. 2012, 2, 583−589. (4) Eliel, E. L.; Wilen, S. H.; Mander, L. N. Stereochemistry of Organic Compounds; Wiley Press: New York, 1997. (5) Nasipuri, D. Stereochemistry in Organic Compounds, Principles and Applications; New Age International (P) Ltd. Press: New Delhi, 1994. (6) Lin, J.; Zu, L.; Fang, W. Conformation and Spectroscopy Study of Cycloheptoxy Radical. J. Phys. Chem. A 2011, 115, 274−279. (7) Liang, G.; Liu, C.; Hao, H.; Zu, L.; Fang, W. Laser-Induced Fluorescence of Isobutoxy in Competition with Ground State Decomposition. J. Phys. Chem. A 2013, 117, 13229−13235. (8) Wu, Q.; Liang, G.; Zu, L.; Fang, W. Vibrationally Resolved LIF Spectrum of Tertiary Methylcyclohexoxy Radical. J. Phys. Chem. A 2012, 116, 3156−3162. (9) Blatt, A. H. Organic Syntheses; Wiley Press: New York, 1966. (10) Rosenberg, M.; SØlling, T. I. Computational Investigation of Photo Induced Processes in Alkyl Nitrites and the Product Alkoxy Radicals. Chem. Phys. Lett. 2010, 484, 113−118. (11) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, revision D.02; Gaussian, Inc.: Wallingford, CT, 2004. (12) Miller, T. A. Spectroscopic Probing and Diagnostics of the Geometric Structure of the Alkoxy and Alkyl Peroxy Radical Intermediates. Mol. Phys. 2006, 104, 2581−2593. (13) Zu, L.; Liu, J.; Tarczay, G.; Dupré, P.; Miller, T. A. Jet-Cooled Laser Spectroscopy of the Cyclohexoxy Radical. J. Chem. Phys. 2004, 120, 10579−10593. (14) Tarczay, G.; Gopalakrishnan, S.; Miller, T. A. Theoretical Prediction of Spectroscopic Constants of 1-Alkoxy Radicals. J. Mol. Spectrosc. 2003, 220, 276−290. (15) Liu, J.; Miller, T. A. Jet-Cooled Laser-Induced Fluorescence Spectroscopy of Cyclohexoxy: Rotational and Fine Structure of Molecules in Nearly Degenerate Electronic States. J. Phys. Chem. A 2014, 118, 11871−11890. (16) Chhantyal-Pun, R.; Roudjane, M.; Melnik, D. G.; Miller, T. A.; Liu, J. Jet-Cooled Laser-Induced Fluorescence Spectroscopy of Isopropoxy Radical: Vibronic Analysis of B̃ − X̃ and B̃ − X̃ Band Systems. J. Phys. Chem. A 2014, 118, 11852−11870. (17) Gonçalves, K. M. S.; Garcia, D. R.; Ramalho, T. C.; FigueroaVillar, J. D.; Freitas, M. P. Conformational Analysis of 1-Chloro- and 1Bromo-2-propanol. J. Phys. Chem. A 2013, 117, 10980−10984. (18) Dalling, D. K.; Grant, D. M. Carbon-13 Magnetic Resonance. XXI. Steric Interactions in the Methylcyclohexanes. J. Am. Chem. Soc. 1972, 94, 5318−5324. (19) Curtis, J.; Dalling, D. K.; Grant, D. M. Deuterium Chemical Shifts and Chemical Shift Parameters in Methylcyclohexanes. J. Org. Chem. 1986, 51, 136−142. (20) Gopalakrishnan, S.; Carter, C. C.; Zu, L.; Stakhursky, V.; Tarczay, G.; Miller, T. A. Rotationally Resolved B̃ − X̃ Electronic Spectra of Both Conformers of the 1-Propoxy Radical. J. Chem. Phys. 2003, 118, 4954−4969. (21) Gopalakrishnan, S.; Zu, L.; Miller, T. A. Rotationally Resolved Electronic Spectra of the B̃ − X̃ Transition in Multiple Conformers of 1-Butoxy and 1-Pentoxy Radicals. J. Phys. Chem. A 2003, 107, 5189− 5201. (22) Williams, S.; Harding, L. B.; Stanton, J. F.; Weisshaar, J. C. Barrier to Methyl Internal Rotation of Cis- and Trans-2-Methylvinoxy
4. CONCLUSIONS In this work, vibrationally resolved LIF spectra of five double methyl substituted cyclohexoxy radicals were obtained in the supersonic jet-cooled condition. The spectral character of the dimethylcyclohexoxy radical was predominantly affected by the nearest substituent related to the C−O group. The spectral effect of the two methyl groups added up when they located on asymmetric carbons of the cyclohexoxy ring. When two methyl groups located on adjacent ring carbons, the additional gauche interaction destabilized the radical and resulted in a further redshift of the B̃ − X̃ transition origin. Detection of the dissociation products 2-methylvinoxy radicals for 2,6-dimethylcyclohexoxy and vinoxy radical for 2,3- and 2,5-dimethylcyclohexoxy radicals revealed that the β C−C bond fission preferred to occur on the side with β methyl substitution, that is, the methyl group on β carbon weakened the six-member ring of cyclohexoxy. The experimental results also showed that the radical further dissociated after the ringopening. This study clearly shows that the LIF spectra can be used to identify cyclohexoxy radical and the isomers of its methyl substitutes. The results will help to understand the photochemistry of cyclic hydrocarbons in the atmosphere.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Fax: +86-10-58802075. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are pleased to acknowledge the financial support of this research by National Natural Science Foundation of China (grant nos. 21373033 and 21173024). We also acknowledge the grant from the Fundamental Research Funds for the Central Universities. 3391
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The Journal of Physical Chemistry A Radical in the X̃ 2(A″) and B̃ 2(A″) States: Experiment and Theory. J. Phys. Chem. A 2000, 104, 9906−9913. (23) Williams, S.; Harding, L. B.; Stanton, J. F.; Weisshaar, J. C. Barrier to Methyl Internal Rotation of 1-Methylvinoxy Radical in the X̃ 2(A″) and B̃ 2(A″) States: Experiment and Theory. J. Phys. Chem. A 2000, 104, 10131−10138. (24) Zhang, L.; Callahan, K. M.; Derbyshire, D.; Dibble, T. S. LaserInduced Fluorescence Spectra of 4-Methylcyclohexoxy Radical and Perdeuterated Cyclohexoxy Radical and Direct Kinetic Studies of Their Reactions with O2. J. Phys. Chem. A 2005, 109, 9232−9240. (25) Welz, O.; Striebelz, F.; Olzmann, M. On the Thermal Unimolecular Decomposition of the Cyclohexoxy Radicalan Experimental and Theoretical Study. Phys. Chem. Chem. Phys. 2008, 10, 320−329. (26) Aschmann, S. M.; Chew, A. A.; Arey, J.; Atkinson, R. Products of the Gas-Phase Reaction of OH Radicals with Cyclohexane: Reactions of the Cyclohexoxy Radical. J. Phys. Chem. A 1997, 101, 8042−8048. (27) Orlando, J. J.; Iraci, L. T.; Tyndall, G. S. Chemistry of the Cyclopentoxy and Cyclohexoxy Radicals at Subambient Temperatures. J. Phys. Chem. A 2000, 104, 5072−5079. (28) Zhang, L.; Kitney, K. A.; Ferenac, M. A.; Deng, W.; Dibble, T. S. LIF Spectra of Cyclohexoxy Radical and Direct Kinetic Studies of Its Reaction with O2. J. Phys. Chem. A 2004, 108, 447−454. (29) Vereecken, L.; Peeters, J. A Decomposition of Substituted Alkoxy Radicals − Part I: A Generalized Structure-Activity Relationship for Reaction Barrier Heights. Phys. Chem. Chem. Phys. 2009, 11, 9062−9074. (30) Chhantyal-Pun, R.; Chen, M.; Sun, D.; Miller, T. A. Detection and Characterization of Products from Photodissociation of XCH2CH2ONO (X = F, Cl, Br, OH). J. Phys. Chem. A 2012, 116, 12032−12040. (31) Brock, L. R.; Rohlfing, E. A. Spectroscopic Studies of the B̃ 2A″ − X̃ 2A″ System of the Jet-Cooled Vinoxy Radical. J. Chem. Phys. 1997, 106, 10048−10065. (32) Washida, N.; Inomata, S.; Furubayashi, M. Laser-Induced Fluorescence of Methyl Substituted Vinoxy Radicals and Reactions of Oxygen Atoms with Olefins. J. Phys. Chem. A 1998, 102, 7924−7930. (33) Jin, J.; Sioutis, I.; Tarczay, G.; Gopalakrishnan, S.; Bezant, A.; Miller, T. A. Dispersed Fluorescence Spectroscopy of Primary and Secondary Alkoxy Radicals. J. Chem. Phys. 2004, 121, 11780−11797.
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