Vibrationally Resolved LIF Spectrum of Tertiary Methylcyclohexoxy

Article pubs.acs.org/JPCA

Vibrationally Resolved LIF Spectrum of Tertiary Methylcyclohexoxy Radical Qijun Wu, Gaiting Liang, Lily Zu,* and Weihai Fang Department of Chemistry, Beijing Normal University, Beijing 100875, P. R. China S Supporting Information *

ABSTRACT: Cyclohexoxy radical and its substitutes are intermediates of the combustion reaction in automobile engines, and hence play an important role in the atmospheric chemistry. Spectroscopic and conformational studies can provide convenient methods to monitor these species. In this work, we report the observation of the laser induced fluorescence (LIF) excitation spectrum of 1-methylcyclohexoxy, a tertiary ring alkoxy radical, in supersonic jet condition. The spectrum was assigned preliminarily to the chair-axial and chair-equatorial conformers of 1-methylcyclohexoxy. No C−O stretch progression was observed in 1-methylcyclohexoxy spectrum, which was different from t-butoxy. The short lifetime of excited state 1-methylcyclohexoxy and increased C−O bond length suggested a less stable excited state induced by the increased steric repulsion via methyl substitution on the alkoxy carbon. Dissociation rather than H transfer was suggested as the major nonradiative relaxation process, which competed with the fluorescence path.

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INTRODUCTION Cyclohexoxy and its methyl substituted variants are important intermediates of the combustion reaction in automobile engines.1−3 Direct spectral observation of these radicals can provide information about their structures and serve as convenient diagnostics for their monitoring. Recently, we reported the observation of LIF spectra of 2-, 3-, and 4methylcyclohexoxy radicals in supersonic jet-cooled condition.4 All these methyl substituted cyclohexoxy radicals fall into the secondary alkoxy category, i.e., the methyl group was located on a different carbon of the ring other than the alkoxy carbon. A systematic red shift of the origin band of 2-, 3-, and 4methylcyclohexoxy, decreasing with the separation between oxygen and methyl group was observed. The vibronic coarse structure in their excitation spectra revealed that the methyl location affected the spectral and chemical behavior of the secondary methylcyclohexoxy radicals. Different from 2-, 3-, and 4-methylcyclohexoxy radicals, 1-methylcyclohexoxy is a tertiary alkoxy radical, i.e., the methyl group is located on the alkoxy carbon of the cyclohexane ring. Fluorescence emission of tertiary alkoxy radicals was first reported by Bai et al.5 The LIF excitation spectrum of t-butoxy radical was then used as a tool for kinetic studies.6 In 1999, Wang et al.7 corrected the origin band frequency of the tbutoxy LIF excitation spectrum and reported a long vibrational progression of C−O stretch motion. The long progression (ν′ = 7) in the C−O stretch of t-butoxy compared with primary and secondary butoxy isomers (no ν′ > 1 in the C−O stretch was observed) was confirmed when jet-cooled spectra of these radicals were determined and high resolution (∼0.1 cm−1) was achieved.8−11 In contrast, the LIF spectra of secondary © 2012 American Chemical Society

methylcyclohexoxy radicals in the supersonic jet condition showed different phenomenon. The C−O stretch bands up to ν′ = 2 (4-methylcyclohexoxy) and ν′ = 3 (3-methylcyclohexoxy) were observed in the spectra.4 The spectroscopic results suggested that the secondary ring alkoxy radicals were more resistant to the nonradiative decay process than the chain secondary alkoxy radical 2-butoxy and 2-pentoxy9 although they all have multiple conformers. Studying the jet-cooled LIF spectrum of 1-methylcyclohexoxy radical is important to reveal the stereodynamic effect of the methyl group for the tertiary ring alkoxy radical. The results will contribute to the fundamental chemical and structure investigation of the ring compound. In this work, we obtained the jet-cooled LIF spectrum of 1methylcyclohexoxy radical and analyzed the spectrum assisted by theoretical calculations using B3LYP/6-31+G(d) and CASSCF/6-31+G(d) methods. The conformation identity of the spectrum was discussed, and the spectral and chemical property of 1-methylcyclohexoxy radical was compared with other tertiary alkoxy radicals and secondary methyl substituted cyclohexoxy radicals.

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EXPERIMENTAL SECTION The experimental setup was described previously in ref 12. Briefly, it consists of a dye laser (Narrowscan, Radiant Dyes) pumped by the second harmonic (532 nm) of a Nd:YAG laser (Continuum, Surelite III). The laser output was frequencyReceived: December 9, 2011 Revised: February 28, 2012 Published: March 1, 2012 3156

dx.doi.org/10.1021/jp211888c | J. Phys. Chem. A 2012, 116, 3156−3162

The Journal of Physical Chemistry A

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Figure 1. Potential energy curve of interconversion between chair conformers of 1-methylcyclohexoxy radical via boat-axial. (In the structures: gray, carbon atom; red, oxygen atom; blue, methyl group; hydrogen atoms on the ring carbons are not shown.)

Table 1. Relative Energya (To the Global Minimum, Chair-Axial, in kcal/mol) of Conformers and Energy Barriers of 1Methylcyclohexoxy Radical along the Three Conformational Interconversion Paths on the Ground State As Obtained at the B3LYP/6-31+G(d) Level of Theoryb conformational interconversion path

minimum-1

barrier

minimum-2

barrier

minimum-3

barrier

minimum-4

1

chair-e 1.60 chair-e 1.60 chair-e 1.60

long boat 10.22 half chair 11.46 long boat 10.22

twisted boat-a 1 5.65 twisted boat-e 1 6.83 twisted boat-i 1 5.26

boat-a 6.91 boat-e 9.76 boat-a 6.91

twisted boat-a 2 5.88 twisted boat-e 2 7.16 twisted boat-i 2 5.61

half chair 10.43 long chair 10.21 long chair 10.21

chair-a 0 chair-a 0 chair-a 0

2 3 a

Relative energy with zero-point energy (ZPE) corrections. bThe notation e, a, and i in the conformer name refers to the orientation of oxygen on the cyclohexane ring: e, equatorial; a, axial; i, inclinal.

doubled to generate the UV radiation required to probe the B̃ − X̃ electronic transition of 1-methylcyclohexoxy. Photolysis of the precursor molecules was achieved using the third harmonic (355 nm) of another Nd:YAG laser (Continuum, Surelite II), with power of typically 30 mJ per pulse. The 1-methylcyclohexyl nitrite precursor was synthesized by dropwise addition of sulfuric acid to a mixture of 1-methylcyclohexanol and sodium nitrite.13 The 1-methylcyclohexyl nitrite appeared as pale yellow liquid, which was verified by IR and UV spectra.14 A backing pressure of ∼0.1 atm argon passed over the nitrite sample, and the resulting gaseous mixture was expanded into the vacuum chamber using a standard pulse valve (General Valve) with a 0.5 mm orifice. Different sizes of orifice were also tried to examine the spectrum under different cooling conditions and to eliminate the hot bands (Figure S1 in Supporting Information). The photolysis beam was focused just above the throat of the nozzle, and the radicals produced were excited about 10 mm downstream by the counter-propagating probe beam. The total fluorescence was collected perpendicularly with a f = 80 mm lens and imaged onto a photomultiplier tube (Hamamasu, CR110). The valve was operated in a continuous mode. The vacuum chamber was evacuated by a molecular pump (600 L/s) backed by a mechanical pump (8 L/ s). The initial vacuum was ∼3 × 10−3 Pa and increased to ∼7 Pa when injecting sample.

Three dyes, Styryl8, Pyridine2, and Pyridine1 (Exciton, USA), were used to provide the tunable excitation laser source for 1-methylcyclohexoxy LIF spectra. Lasers were operated at 10 Hz rate and sequentially controlled by a Digital Pulse Generator (Stanford Research, DG535). The output of the photomultiplier was digitized on an oscilloscope (Tektronics, TBS3032B) and the gated signal was integrated by a LabView program. LIF spectra were obtained by recording the integrated fluorescence signal while continuously scanning the excitation laser wavelength. The dye laser wavelength was scanned at 0.01 nm per step, and all spectra were corrected by subtracting the background obtained with the photolysis laser off.

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COMPUTATIONAL SECTION

All calculations were performed using the Gaussian03 program.15 The geometry optimization at the ground state of all conformers of 1-methylcyclohexoxy radicals was performed using the B3LYP/6-31+G(d) method. The CASSCF/631+G(d) method was used to optimize both ground and excited state geometry of 1-methylcyclohexoxy chair conformers. Vibrational frequency analysis was conducted for the ground and excited states to assist the assignment of experimental spectra. The adiabatic excitation energy between 3157



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Table 2. Calculated Frequencies of Lowest 20 Vibrational Modes of Chair Conformers of 1-Methylcyclohexoxya chair-axial CASSCF(9,7)

chair-axial B3LYP

chair-equatorial CASSCF(9,7)

chair-equatorial B3LYP

vibration mode

symmetry

B̃

X̃

X̃

B̃

X̃

X̃

ν32 ν31 ν30 ν29 ν28 ν27 ν26 ν25 ν24 ν23 ν22 ν57 ν56 ν55 ν54 ν53 ν52 ν51 ν50 ν49

a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a′ a″ a″ a″ a″ a″ a″ a″ a″ a″

137 283 333 409 448 468 572 747 850 873 971 202 228 285 401 474 770 898 933 1016

135 297 339 420 462 566 718 831 866 900 980 193 220 282 386 456 791 887 947 1007

132 290 330 410 453 548 708 824 845 887 926 191 212 271 386 453 760 858 919 968

142 293 337 411 436 504 634 742 833 873 975 200 215 288 413 437 811 891 971 1025

148 304 350 431 452 560 708 836 873 892 990 178 211 299 384 442 806 891 962 1009

142 295 342 418 443 550 680 811 842 884 927 160 212 302 358 431 771 854 925 959

a

All calculations employed the 6-31+G(d) basis set. A uniform scale factor of 0.94 was used for CASSCF frequencies. The calculated B3LYP frequencies were scaled by 0.98.

the X̃ and B̃ states was obtained by the CASSCF/6-31+G(d) method.

the chair conformers. The barriers from the twisted-boat to the chair conformers are about 3.05−4.96 kcal/mol, while the barriers from chair conformers to twisted-boat conformers are 8.62−10.43 kcal/mol. The energy barriers between twistedboat conformers are lower than 3 kcal/mol. Thus, twisted-boat conformers are easy to convert to chair conformers, while the high barriers make the interconversion from one chair conformer to another chair conformer difficult. Preliminary calculation of the interconversion barriers between conformers in the B̃ excited state were performed using the CASSCF/631+G(d) method, and similar barrier heights as in the X̃ state were observed (see Supporting Information, Figure S2). These calculation results suggest that only chair conformers are likely present under the jet-cooled experimental condition.16,19 Therefore, we will analyze the LIF spectrum using calculated results of chair conformers. Both chair-axial and chair-equatorial conformers of 1methylcyclohexoxy radical have Cs symmetry. The Cs symmetry of the radical allows the excitation of a′ vibrational modes in its B̃ −X̃ electronic transition. The a″ vibrational modes are forbidden but might gain intensity via the pseudo-Jahn−Teller effect due to the low-lying Ã state of cyclohexoxy radicals.16 The CASSCF(9,7)/6-31+G(d) calculations show that the vibrational modes and their frequencies are very similar for chair-axial and chair-equatorial conformers. Table 2 lists the calculated lowest 20 vibrational frequencies of the two chair conformers. The B3LYP/6-31+G(d) and CASSCF(9,7)/631+G(d) calculations both show C−O stretch vibrations (ν23) at ∼900 cm−1 for chair-axial and chair-equatorial conformers on the ground state (Table 2). This is comparable with other methyl substituted cyclohexoxy radicals4 and cyclohexoxy radical itself.16 In the B̃ excited state, significantly low C−O stretch frequencies, 468 cm−1 (ν27) for the chair-axial conformer and 411 cm−1 (ν29) for the chair-equatorial conformer, were predicted. This is different from cyclohexoxy,

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RESULTS AND DISCUSSION Calculation Results. The conformational analysis of cyclohexoxy was well addressed in the literature.16−18 It is known that there are two types of conformers for cyclohexoxy, chair conformers and twisted-boat conformers. For 1methylcyclohexoxy radicals, chair conformers and twisted-boat conformers are still minima on the potential energy curve (Figure 1). Relative energies of conformers and barriers along the three different interconversion paths between conformers of 1-methylcyclohexoxy on the ground state were calculated at the B3LYP/6-31+G(d) level, and the results are listed in Table 1. For calculation, the initial structures were generated by replacing the hydrogen atom on the alkoxy carbon of cyclohexoxy with a methyl group. Then, geometry optimizations and frequency analysis were performed to identify the minima and transition states. The transition structures were further optimized by the synchronous transit-guided quasiNewton (STQN) method, and intrinsic reaction coordinate (IRC) calculations were conducted to find the respective two minima on the potential energy surface to which the transition state connected. Generally, the conformational status and the interconversion paths of 1-methylcyclohexoxy are similar like that of cyclohexoxy radical.16 The lowest energy conformer of 1-methylcyclohexoxy is a chair-axial conformer in which the oxygen atom is located at the axial orientation, while the methyl group is at the equatorial orientation. The chair-equatorial conformer (oxygen at equatorial and methyl group at axial) is 1.60 kcal/mol higher in energy than the chair-axial conformer. This is consistent with the analysis of steric interactions on the cyclohexane ring, i.e., the conformer with the bulkier group on the equatorial position is energetically favored.18 The twistedboat conformers are 3.66−7.16 kcal/mol higher in energy than 3158



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dense spectral contents observed in the high frequency region of the LIF spectra of cyclohexoxy 16 and 3- and 4methylcyclohexoxy radicals,4 the intensity of the 1-methylcyclohexoxy spectrum died out gradually in ∼2500 cm−1 beyond the origin band (Figure 2). Assigning the vibrational bands of the 1-methylcyclohexoxy spectrum is difficult because of the similarity of the two chair conformers and the rich vibrational modes of the radicals. However, previous studies of chain and cyclic alkoxy radicals can provide good starting ground. First, the conformer that was calculated to be the lowest in energy had always been observed for all alkoxy radicals reported.19 Second, the C−O stretch mode is expected to have high intensity in the LIF spectrum since the B̃ −X̃ transition of alkoxy radicals corresponds to the promotion of one electron from the σ orbital of the C−O bond to the half-filled nonbonding orbital that mainly localized on the O atom.20,21 Therefore, we can start the assignment using the calculated results of the chair-axial conformer, which is the lowest energy conformer of 1-methylcyclohexoxy, and look for the C−O stretch band in the spectrum according to the calculation results. The adiabatic excitation energy between the X̃ and B̃ states of the 1-methylcyclohexoxy chair-axial conformer was calculated as 27 631 cm−1 by the CASSCF(9, 7) method. It was calculated as 30 801 cm−1 by the CIS method and became 27 105 cm−1 when a scale factor of 0.88 was applied.22 The calculated results (Table 4) are consistent with the experimental origin frequency 27 277.7 cm−1. Band F, 481 cm−1 above the origin band in the experimental spectrum, fits well with the predicted C−O stretch vibration frequency (ν27 = 468 cm−1) of the chair-axial conformer. Bands C, D, E, and G can be assigned as ν31 (ring-bending mode), ν30 (ring-breathing mode), ν28 (ring-stretching mode), and ν26 (C−C−H bending mode), respectively (Table 5). The differences between the experimental frequencies and the predicted vibrational levels are within 3%. Calculated results of the chair-equatorial conformer were also compared to the experimental bands in this region, the differences between experimental and calculated frequencies are generally larger than assigning by chair-axial conformer parameters, especially no experimental band was observed at the predicted C−O stretch vibration frequency (411 cm−1) of the chair-equatorial conformer.

in which the C−O stretch mode has a much higher frequency (688 cm −1 as experimentally observed, 703 cm −1 as predicted16) in the B̃ state. Clear C−O stretch progressions of ∼680 cm−1 and ∼690 cm−1 were also observed in the LIF spectra of 3- and 4-methylcyclohexoxy radicals.4 The calculated structure parameters showed that the C−O bond stretched more (Table 3, from 1.4365 Å on X̃ state to 1.7075 Å on B̃ Table 3. Calculated Structure Parameters (in Å) of the Lowest Energy Conformer of 1-Methylcyclohexoxy, 2Methylcyclohexoxy, and 4-Methylcyclohexoxy Radicals at the CASSCF(9, 7)/6-31+G(d) Level 1methylcyclohexoxy chair-axial

2methylcyclohexoxy trans (e,e)

4methylcyclohexoxy trans (e,e)

bond

X̃

B̃

X̃

B̃

X̃

B̃

C−O C−CH3 O−H(CH3)

1.4365 1.5321 2.5795

1.7075 1.5004 2.6648

1.4222 1.5294 2.6674

1.6583 1.5255 2.6256

1.4229 1.5289 6.0015

1.6520 1.5291 6.1266

state) in the B̃ − X̃ transition of 1-methylcyclohexoxy. Simultaneously, the C−CH3 bond compressed significantly in 1-methylcyclohexoxy radical (1.5321 Å for X̃ and 1.5004 Å for B̃ ) than in secondary methylcyclohexoxy radicals. The increased steric repulsive effect introduced by methyl group on the alkoxy carbon seems to weaken the C−O bond of 1-methylcyclohexoxy, especially on the B̃ excited state. Spectroscopy Study. The LIF spectrum of 1-methylcyclohexoxy taken in supersonic jet condition is shown in Figure 2. The spectrum appears in the wavelength region of 27 000 to 30 000 cm−1. Unlike the LIF spectrum of t-butoxy, no distinguishable vibrational progression was observed in the spectrum. The low frequency region of the spectrum is enlarged and labeled in Figure 3. The origin band (band A) of 1-methylcyclohexoxy spectrum is at 27 277.7 cm−1, which is blue-shifted compared to the origin band of cyclohexoxy (26 754.3 cm−1)16 and secondary methylcyclohexoxy radicals.4 The strongest band (band H) in the 1-methylcyclohexoxy spectrum is at 27 865.2 cm−1, which is 588 cm−1 above the origin band. In the spectral region where the calculated C−O stretch frequency is located, one band (band F, 481 cm−1 above the origin) was observed. There are some additional bands in the high frequency side of the strongest band. However, not like the

Figure 2. Jet-cooled LIF excitation spectrum of 1-methylcyclohexoxy: (a) probe following photolysis of 1-methylcyclohexyl nitrite sample; (b) probe following photolysis of 1-methylcyclohexnol; (c) 1-methylcyclohexyl nitrite sample with photolysis laser off. 3159



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Figure 3. Assignment of experimental spectrum with calculated vibrational frequencies of chair-axial (red) and chair-equatorial (blue) conformers. Unified intensity was used for all calculated vibration modes except the C−O stretch vibrations.

Table 4. Calculated Adiabatic Excitation Energies (in cm−1) of the Two Chair Conformers of 1-Methylcyclohexoxy Radical CASSCF(9, 7)/631+G(d) B̃ - X̃

CIS/6-31+G(d) B̃ - X̃

CIS/6-31+G(d) Ã - X̃

conformer

ΔE + ΔZPE

ΔE + ΔZPE

ΔE +ΔZPE

chair-axial chairequatorial

27 631 28 336

30 801 31 228

1783 1787

Band B could be assigned as ν32 of the chair-axial conformer. However, the discrepancy between the experimental and the predicted frequency would be 23%, which is abnormal compared to other vibrational assignments of the experimental bands. The close spacing between band H (588 cm−1) and band I (597 cm−1) was not predicted by chair-axial calculations either. In addition, the bands that have medium intensity in the 600−1000 cm−1 range above the origin band cannot be explained by calculated vibrational modes of chair-axial conformer or possible combinations of strong bands below 600 cm−1. The spectral observation leads to the possibility of coexistence of two chair conformers in the experimental condition. If band B is assigned as the origin of the chairequatorial conformer, the high intensity doublet peak (band H and I) in the spectrum can be assigned as ν29 (C−O stretch) and ν28 (ring-stretching mode) of the chair-equatorial conformer with the discrepancy ∼4%. Bands K and N can be assigned as ν27 (C−C−H bending with part C−O stretch) and ν26 (C−H bending with part C−O stretch) of the chairequatorial conformer, respectively. In this way, the majority of the bands with reasonable intensity within 1000 cm−1 above the origin of the spectrum can be assigned by the prediction of a′ transitions of chair-axial and chair-equatorial conformers, and the observed vibrational modes are similar for chair-axial and chair-equatorial conformers. Calculated vibrational frequencies are shown in Figure 3 for comparing with the experimental data. Detail assignments are listed in Table 5. The CASSCF(9, 7) adiabatic excitation energy between the X̃ and B̃ states of chair-equatorial conformer was 28 336 cm−1, which was 705 cm−1 in high frequency direction of chair-axial conformer. This difference of the two chair conformers was 427 cm−1 as calculated by the CIS method. Although the uncertainty of the calculated adiabatic excitation energy is probably the magnitude of several hundred wavenumbers,22 the general conclusion we can draw from the calculation results is that the origin band of the chair-equatorial conformer should appear to the high frequency side of the chair-axial origin if the chair-equatorial conformer is present in the jet. This calculation result supports our assignment. In the previous studies of cyclohexoxy and secondary methylcyclohexoxy radicals, one single conformer (the lowest energy conformer) was assigned

Table 5. Assignment of the Bands (in cm−1) Observed in the Low-Frequency Region of the Jet-Cooled LIF Spectrum of 1Methylcyclohexoxy Radical Δexptl band

exptl freq

Δexptla

A B C D E F G H I J K L M N O P Q R

27 277.7 27 455.2 27 555.4 27 614.4 27 731.9 27 758.5 27 847.4 27 865.2 27 875.0 27 914.2 27 959.5 27 997.5 28 050.5 28 071.4 28 196.0 28 265.0 28 311.4 28 430.1

0 178 278 337 454 481 570 588 597 637 682 720 773 794 918 987 1034 1152

Δexptlb

predicted, chairac

predicted, chaire c,

freq

freq

0

assign

(0)

(276) (303)

283 333 448 468 572

ν31 ν30 ν28 ν27 C−O ν26

(410) (419) (504) 747

971

0

B−X ν00

293 337

ν31 ν30

411 436

ν29 C−O ν28

504

ν27

634 742

ν26 ν25

873 975

ν23 ν22

ν25

(616) (740) (856) (974)

assign

B−X ν00

ν22

B−X Observed band maxima (cm−1) relative to chair-axial ν00 at 27 277.7 B−X −1 b −1 cm . Observed band maxima (cm ) relative to chair-equatorial ν00 at 27 455.2 cm−1. cScaled CASSCF(9,7)/6-31+G(d) frequencies (scale factor 0.94). a

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Figure 4. Lifetime measurement of 1-methylcyclohexoxy: (a) time-resolved fluorescence decay of 1-methylcyclohexoxy; (b) signal obtained without sample injection.

as the spectral carrier.4,16 However, there is sufficient energy available in the photolysis to populate all conformers, and it is not surprising that both chair conformers were observed in the nonequilibrium supersonic jet environment.10,19 Coexistence of multiple conformers was observed previously by rotationally resolved spectroscopy studies in the nonequilibrium jet-cooled experimental condition of chain alkoxy radicals, for example, gauche (G) and trans (T) conformers of 1-propoxy,23 G1T2, T1T2, and T1G2 conformers of 1-butoxy.10 At the current stage, the two-conformer assignment of the 1-methylcyclohexoxy spectrum has the best agreement between experimental and calculation results. An unambiguous assignment requires further studies of rotationally resolved spectroscopy. According to the experimental spectrum and the assignment above, we can conclude that no ν′ > 1 of the C−O stretch mode was observed in the jet-cooled spectrum of 1methylcyclohexoxy radical. This is different from the typical tertiary alkoxy radical, t-butoxy. There is more than one conformer for 1-methylcyclohexoxy radical due to the ring structure, while t-butoxy has only one conformer. In the spectroscopic studies of 1- and 2-butoxy radicals, which have multiple conformers, the interconversion between conformers was suggested as an important mechanism for nonradiative decay of these species.9,21 Could the interconversion between the conformers be the reason of no observation of long C−O stretch progression in the 1-methylcyclohexoxy spectrum? The energy barrier (∼10 kcal/mol) of interconversion between the two chair conformers of 1-methylcyclohexoxy is much higher than the barriers (2.1−3.5 kcal/mol)21 between conformers of chain alkoxy radicals. The interconversion between chair conformers of the 1-methylcyclohexoxy radical should be difficult compared to the interconversion between chain alkoxy conformers. In addition, up to ν′ = 3 C−O stretch band was observed in the spectrum of 3- and 4-methylcyclohexoxy radicals, which had comparable interconversion energy barrier between their chair conformers. Therefore, the interconversion mechanism does not look like the major nonradiative process of 1-methylcyclohexoxy. There are two other possible nonradiative paths that the B̃ excited state tertiary methylcyclohexoxy radical can take, i.e., the dissociation of the radical and the

H transfer from the CH3 group to the alkoxy oxygen. For 2methylcyclohexoxy, the only secondary methylcyclohexoxy radical for the which C−O stretch band was not observed, the H transfer from ortho-methyl group to the alkoxy oxygen go through a five-membered ring transition state, which may lower the barrier to facilitate a nonradiative decay path.19,24,25 The H transfer process of 1-methylcyclohexoxy will have a fourmembered ring transition state. The high ring strain of the fourmembered ring will not favor this pathway.25 Ring-opening decomposition via β C−C bond fission of alkoxy radicals has been extensively studied by experimental and theoretical methods.3,26−29 The energy barrier of 4-methylcyclohexoxy ring-opening was reported as ∼12.2 kcal/mol.30 It is comparable with the energy required for chair conformer interconversion. Thus, the ring-opening process could not be the major contributor of the nonradiative decay on the B̃ excited state of 1-methylcyclohexoxy. A nonradiative mechanism of the internal conversion from the B̃ state to the X̃ state was promoted for large chain alkoxy radicals (propoxy, butoxy, and pentoxy) based on the fact that their B̃ states sank well below the dissociation limit of the X̃ state.9 For 1methylcyclohexoxy, the experimental value of B̃ to X̃ transition (T00, 78.0 kcal/mol) placed the vibrationless level of the B̃ state higher than the calculated X̃ state C−O dissociation limit (76.5 kcal/mol) (Supporting Information, Figure S3). In addition, the weak C−O bond in B̃ excited state (Table 3) induced by the methyl group on the alkoxy carbon of 1-methylcyclohexoxy suggested the C−O dissociation path in B̃ state. Low C−O dissociation energy in the B̃ state of 1-methylcyclohexoxy obtained in our preliminary calculation supported this suggestion (Figure S3 in Supporting Information). Therefore, we consider that the C−O dissociation is the major nonradiative decay route of 1-methylcyclohexoxy in the B̃ state. Short lifetime observed (Figure 4, ∼110 ns, compared to 2.02 μs for cyclohexoxy12) for origin bands (bands A and B) in the 1-methylcyclohexoxy spectrum agreed with this hypothesis, and it is consistent with the spectral observation that the intensity of the 1-methylcyclohexoxy spectrum eliminated ∼2500 cm−1 above the origin. Future studies of the fragments by photodissociation spectroscopy might hold the key to fully 3161



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understand the nonradiative mechanism of 1-methylcyclohexoxy on the B̃ excited state.31

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CONCLUSIONS In this work, we reported the LIF excitation spectrum of 1methylcyclohexoxy, a tertiary ring alkoxy radical. The spectrum was assigned preliminarily to the chair-axial and chair-equatorial conformers of 1-methylcyclohexoxy. Summarized with the previous work of cyclohexoxy16 and secondary methylcyclohexoxy,4 methyl substitution on the meta- and para-position of the cyclohexoxy ring stabilized the radicals, and C−O stretch progressions were observed in the jet-cooled LIF spectra. Short lifetime of excited state tertiary 1-methylcyclohexoxy and increased C−O bond length suggested a less stable B̃ excited state induced by the increased steric repulsion via methyl substitution on the alkoxy carbon. Therefore, no C−O stretch progression was observed in the 1-methylcyclohexoxy spectrum, which was different from t-butoxy. The spectral observation also suggested different nonradiative paths for 1methylcyclohexoxy and 2-methylcyclohexoxy radicals. For 2methylcyclohexoxy, methyl substitution was on the orthoposition, and the isomerization induced by H transfer via a fivemember ring configuration might play an important role in the relaxation process of 2-methylcyclohexoxy on the B̃ excited state. However, further experimental and theoretical studies are required to fully understand the relaxation mechanism of the B̃ excited state and verify the spectral assignment of methylcyclohexoxy radicals.

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ASSOCIATED CONTENT

S Supporting Information *

LIF spectra of 1-methylcyclohexoxy radical with different jet orifices (Figure S1), potential energy curve of the interconversion between conformers of 1-methylcyclohexoxy radical via boat-axial (Figure S2), schematic potential energy surfaces for the chair-axial conformer of 1-methylcyclohexoxy radical as a function of C−O bond length (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-10-58802075. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are pleased to acknowledge the financial support of this research by the National Natural Science Foundation of China (Grant No. 20673013) and support from Beijing Municipal Commission of Education. We also acknowledge the grant from the Major State Basic Research Development Program (Grant No. 2007CB815206).

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REFERENCES

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Vibrationally Resolved LIF Spectrum of Tertiary Methylcyclohexoxy

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