Laser-Induced Fluorescence of Isobutoxy in Competition with Ground

Apr 25, 2013 - Spectroscopic detection is an important method to monitor alkoxy radicals in atmospheric photochemistry studies. In this work, we repor...
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Laser-Induced Fluorescence of Isobutoxy in Competition with Ground State Decomposition Gaiting Liang, Chengxuan Liu, Haiyan Hao, Lily Zu,* and Weihai Fang College of Chemistry, Beijing Normal University, Beijing, 100875, P. R. China S Supporting Information *

ABSTRACT: Spectroscopic detection is an important method to monitor alkoxy radicals in atmospheric photochemistry studies. In this work, we report the first observation of the laser-induced fluorescence (LIF) spectra of isobutoxy, 2methyl-1-butoxy, and 3-methyl-1-butoxy in supersonic jetcooled condition. Ground state unimolecular decomposition and isomerization as well as excited state relaxation dynamics of isobutoxy were discussed in combination with the theoretical calculations. Analysis of the experimental and theoretical results showed that methyl substitution on the β carbon of the alkoxy radicals changed the LIF spectra of alkoxy radicals significantly. The competition between the ground state reactions and the photoexcitation process depended strongly on the radical structure and hence affected the involvement of alkoxy radicals in the photochemical reactions in the upper troposphere. This study will help to understand the dynamic role of alkoxy radicals in the atmosphere.

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INTRODUCTION Alkoxy radicals are key intermediates in the photooxidation of hydrocarbons on the earth surface and in the upper troposphere.1,2 The competition between various potential alkoxy radical reaction pathways determines the products of a particular oxidation scheme and thus the impact of a particular hydrocarbon. The most common reactions of the alkoxy radicals are reaction with O2, decomposition by β C−C bond fission, or isomerization via 1,5 H-shift.3 Numerous studies were conducted on the unimolecular decomposition and isomerization reactions in the ground state of alkoxy radicals,4−8 in which the rate constants were highly sensitive to molecular structure. The reactions in the ground state affect the population of molecules that can be photoexcited and hence the fluorescence spectra of alkoxy radicals. Recently, Miller’s group extended their alkoxy spectroscopic study in a supersonic jet to halogen substituted alkoxy radical XCH2CH2O and observed products HCHO from β C−C bond dissociation and CH2CHO via eliminating HX by photodissociation of XCH2CH2ONO.9 It will be interesting to study the correlation between the alkoxy radical structure and its reactivity in the ground and excited states simultaneously. The result will be very valuable to understand the behavior of alkoxy radical in the photochemical processes in the troposphere. Generally, decomposition of alkoxy radicals in the ground state has been recognized as mostly by β C−C bond fission, and its reaction kinetics is predominantly affected by the substitution around the breaking bond.3,4,10−13 Isomerization © 2013 American Chemical Society

by H-migration might become the dominant pathway for larger alkoxy radicals containing four carbon atoms or a longer backbone.5,6 Butoxy radicals are ideal candidates for studying the unimolecular reactions as 1-butoxy contains a linear chain of four carbons that fulfill the requirement of a 1,5 H-shift, while its isomers and methyl substituents provide different structural conditions for studying β C−C bond fission. In addition, the LIF spectra can provide information of butoxy radicals in the excited state competing with the ground state reactions. In this work, we studied vibrationally resolved LIF spectra of isobutoxy, neopentoxy, 2-methyl-1-butoxy, and 3-methyl-1butoxy radicals in a supersonic jet. Different spectral characters were observed for these radicals. The correlation between spectra and the radical structures were discussed with the aid of quantum chemical calculations using B3LYP/6-311++G(d,p) and CASSCF/6-311++G(d,p) methods. The structural effect of the competition balance between ground state decomposition and the excited state fluorescence of substituted butoxy radicals was concluded.

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EXPERIMENTAL SECTION The experimental setup was described previously in ref 14. Briefly, it consists of a dye laser (Narrowscan, Radiant Dyes) Special Issue: Terry A. Miller Festschrift Received: February 15, 2013 Revised: April 24, 2013 Published: April 25, 2013 13229

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Figure 1. Jet-cooled LIF spectrum of isobutoxy.

ground state and B̃ excited state of all isobutoxy conformers, and the most stable conformer of 2-methyl-1-butoxy and 3methyl-1-butoxy. An active space of 9 electrons distributed in 7 orbitals was chosen in the CASSCF method, involving the two nonbonding orbitals of O atom (HOMO and HOMO−1, in which the HOMO is half-filled), the σ (HOMO−2) and σ* (LUMO) orbitals of the C−O bond, and other active orbitals and electrons automatically chosen by the program (i.e., HOMO−3, HOMO−4, and LUMO+1 orbitals). Vibrational frequency analyses were conducted for the ground and excited states to assist the assignment of experimental spectra. The adiabatic excitation energy between the X̃ and B̃ states was obtained by the CASSCF/6-311++G(d,p) method. The potential energy curve for β C−C decomposition of isobutoxy in the ground state was calculated starting from the B3LYP/6-311++G(d,p) geometry. By increasing the length of the breaking C−C bond to ∼2.8 Å with a step size of 0.1 Å and performing a constrained optimization, the transition state for decomposition of β C−C bond was obtained. The transition state structure was further optimized by the synchronous transit-guided quasi-Newton (STQN) method, and intrinsic reaction coordinate (IRC) calculations were conducted to find the respective two minima on the potential energy surface that the transition state connects. The energy barrier was calculated for the most stable conformer. Similar calculation was performed to obtain the potential energy profile of the C−O bond dissociation of the isobutoxy radical in the ground state. Vertical excitation energies were then calculated to obtain the information of the three lowest excited states by TDDFT/6311++G(d,p) method.

pumped by the second harmonic (532 nm) of a Nd:YAG laser (Surelite III, Continuum). The laser output was frequencydoubled to generate the UV radiation required to probe the B̃ − X̃ transition of butoxy radicals. Photolysis of the precursor molecules was achieved using the third harmonic (355 nm) of another Nd:YAG laser (Surelite II, Continuum), with a power of typically 30 mJ per pulse. The alkyl nitrite precursor was synthesized by dropwise addition of sulfuric acid to a mixture of the appropriate alcohol and sodium nitrite.15 Synthesized precursors were verified by IR and UV spectra.16 All alcohols for synthesis were purchased from Sigma-Aldrich. Pure nitrite samples 1-butyl nitrite (95%), isobutyl nitrite (95%), and 3methyl-1-butyl nitrite (96%) from Sigma-Aldrich were also used for comparing experiments. A backing pressure of ∼1 atm argon passed over the nitrite sample and expanded into a vacuum chamber using a standard pulsed valve (General Valve) with a 0.5 mm orifice. 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 perpendicular with a f = 80 mm lens and imaged onto a photomultiplier tube (Hamamasu, CR110). Continuous (with the backing pressure reduced to ∼0.1 atm) and pulsed (10 Hz) modes of the valve were both operated. The chamber vacuum was maintained using a molecular pump (600 L/s) backed with a mechanical pump (8 L/s). The initial vacuum was ∼3 × 10−3 Pa and increased to ∼7 Pa when injecting sample. Two dyes, Pyridine1 and Pyridine2 (Exciton), were used to provide the tunable excitation laser source for the LIF spectra. Lasers were operated at 10 Hz rate and sequentially controlled by a Digital Pulse Generator (DG535, SRS). 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 the spectra were corrected by subtracting the background obtained with the photolysis laser off.

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RESULTS AND DISCUSSION According to the location of oxygen bond, alkoxy radicals can be divided into three groups, i.e. primary, secondary, and tertiary isomers. Detail vibrationally and rotationally resolved spectroscopic studies from Miller’s group revealed that the LIF spectra of chain alkoxy radicals fell into several families.18,19 The origin bands of the secondary alkoxy radicals lie in the frequency ∼1900 cm−1 red to the origin bands of the primary alkoxy radicals.18 The spectra of the tertiary alkoxy radicals expanded over the whole spectral region of primary and secondary alkoxy radicals.19 For butoxy radicals, 1-butoxy, 2butoxy, and tert-butoxy correspond to the three isomer groups, respectively, and the LIF spectra of these three isomers were well studied.20−22

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COMPUTATIONAL All calculations were performed using the Gaussian 03 program.17 The geometry optimization in the ground state of all conformers of the isobutoxy radical was performed using the B3LYP/6-311++G(d,p) method. The CASSCF/6-311++G(d,p) method was used to optimize the geometries of the 13230

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two conformers for the isobutoxy radical (Table 1), which had C1 and Cs symmetries, respectively. The C1 conformer was

From the structural point of view, there is another geometric isomer of butoxy that should be named as isobutoxy. It fits into the primary alkoxy group as it can be seen as that one hydrogen atom on β C of 1-propoxy was substituted by one methyl group. Therefore, the LIF spectrum of isobutoxy was expected to be similar to the spectrum of 1-propoxy and 1-butoxy. Interestingly, the LIF spectrum of isobutoxy (Figure 1) observed in the supersonic jet condition showed different character compared to 1-butoxy. The origin band (band A) of isobutoxy spectrum appeared at 28 090.6 cm−1, which was shifted 559 cm−1 to the red compared to the origin band of 1butoxy (28 649.5 cm−1).20 The red shift was less significant in isobutoxy when the methyl substitution occurred on the β carbon, compared to the red shift (1911 cm−1) observed in 2butoxy when the methyl substitution occurred on the α carbon (the origin band of 2-butoxy was at 26 738.4 cm−1).21 A set of closely spaced peaks was recorded in the 28 290−28 350 cm−1 region. The enlarged figure of this band set revealed that it belonged to the rotationally resolved 410 band of formaldehyde (HCHO).23,24 Strong HCHO fluorescence was also recorded in the LIF spectrum of 2-methyl-1-butoxy (Figure 2) when methyl substitution was on the β C of 1-butoxy. Two

Table 1. Relative Energies (Erel, kcal/mol) of C1 and Cs Conformers of Isobutoxy Radical in the Ground State and the Interconversion Transition State between the Two Conformers Calculated at the B3LYP/6-311++G(d,p) Level

slightly lower in energy (0.11 kcal/mol) than the Cs conformer. The energy barrier for interconversion from C1 to Cs was 3.42 kcal/mol in the ground state. The calculated values of adiabatic excitation energy for B̃ −X̃ transition of the two conformers were comparable (Table 2). Similar vibrational modes and Table 2. Calculated Adiabatic B̃ −X̃ Excitation Energies (cm−1) and B̃ State C−O Stretch Frequencies (cm−1) of the Two Conformers of Isobutoxy and the Lowest Energy Conformer (G1T2) of 2-Methyl-1-butoxy CASSCF(9,7)/6-311++G(d,p)a

Figure 2. LIF excitation spectra of isobutoxy, 2-methyl-1-butoxy, neopentoxy, and 3-methyl-1-butoxy. a

bands were observed for 2-methyl-1-butoxy at 28 096.9 and 28 720.8 cm−1, respectively. The origin band was 553 cm−1 redshifted compared to the origin band of 1-butoxy. When two hydrogen atoms on β C were both substituted by methyl groups as in neopentoxy (2,2-dimethyl-1-propoxy), only the LIF spectrum of formaldehyde was observed and no fluorescence of the alkoxy radical was detected. In contrast, the fluorescence signal from HCHO was absent in the LIF spectrum of the 3-methyl-1-butoxy radical (Figure 2) when the methyl substitution occurred on γ C. Instead, multiple bands from the alkoxy radical itself were recorded. HCHO was a decomposition product of alkoxy radicals via β C−C bond fission.3,4 The experimental observations indicated that the decomposition via β C−C bond fission competed to a different extent with the fluorescence process of alkoxy radicals as the structure of the radical varied. To understand the LIF spectra, calculations were performed for isobutoxy and substituted species of 1-butoxy. There were

radical

ΔEB−X ad

νCO

isobutoxy C1 isobutoxy Cs 2-methyl-1-butoxy G1T2

29556 29471 29782

674 519, 656 663

A uniform scale factor of 0.95 was used.

frequencies were obtained in the vibrational frequency analysis of the two conformers (Table S1 and S2 in Supporting Information). However, only one distinct C−O stretch vibration mode (674 cm−1) with high intensity was obtained in the vibrational analysis of C1 conformer, while two vibrational modes (519 and 656 cm−1) with typical C−O stretch character were obtained for the Cs conformer (Table 2). The B̃ −X̃ transition of the alkoxy radical corresponded to the excitation of one C−O σ-bonding electron to the nonbonding p−π orbital localized on the oxygen atom.25 Geometric calculations of the isobutoxy radical showed that the C−O bond length stretched from 1.413 Å in the ground state to 1.632 Å in the B̃ excited state. Therefore, relatively strong transitions were expected for the C−O stretch vibration mode. Previous experimental studies showed that the vibrational bands with a significant component along the C−O stretch constructed the coarse structure of the LIF spectra of linear and 13231

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cyclic alkoxy radicals.19,26 Experimentally, band B in the LIF spectrum of isobutoxy was 696.7 cm−1 above the origin band (band A), and no other band in the C−O stretch frequency region was observed (Figure 1). Therefore, we tentatively assigned the C1 conformer as the spectral carrier of isobutoxy. This assignment agreed with the experimental observation in the previous studies of alkoxy radicals that the lowest energy conformer was always observed in the jet-cooled LIF spectrum of alkoxy radicals.20,21 Bands C, D, and E in the LIF spectrum of isobutoxy can be assigned as the C−C symmetric stretch mode (ν28), the C−C asymmetric stretch mode (ν26), and the C−C bonds twisting mode (ν22) of the C1 conformer, respectively (Table 3). One might argue that band A and

internal energy can be imparted to alkoxy radicals by chemically activated ROONO complexes.1 The excitation laser beam in the supersonic jet for isobutoxy also acted as the excitation source of HCHO which was generated via β C−C fission of isobutoxy in the ground state. Table 4 lists the calculated energy barriers for β C−C bond fission for isobutoxy and some substituents of 1-butoxy. The results show that the β C−C dissociation barrier for isobutoxy (9.6 kcal/mol) is much lower than 1-butoxy (13.4 kcal/mol). This is consistent with the experimental observation that strong HCHO fluorescence was detected with isobutoxy signal while it was very weak in the 1butoxy LIF spectrum.28 The β C−C bond dissociation barrier of neopentoxy is even lower (7.0 kcal/mol) that only HCHO fluorescence was detected. The LIF intensity of HCHO suggested that the decomposition was more significant in isobutoxy, in which another decomposition product (CH3)2CH via β C−C fission was more stable than the decomposition product CH3CH2CH2 of 1butoxy. This observation agreed with the structure activity relationships (SAR) for the barrier height to decomposition reaction proposed by Atkinson3,29 and updated by Vereecken.4 The SAR decomposition barriers of alkoxy radicals were derived considering the available experimental values and theoretical calculations. It suggested that the reaction kinetics of β C−C bond cleavage of alkoxy radicals in the ground state was predominantly affected by the substitution around the breaking bond. Our calculated barriers are generally lower than the cited SAR values, but the general agreement and the relative trends are predicted well (Table 4). The difference came from the large basis set used in this work. Various methods were used for computing the energy barriers of decomposition and isomerization of alkoxy radicals.7,8,30,31 Calculations with large basis sets usually resulted in lower energy barriers for the decomposition via C−C bond fission compared to those with a smaller basis set but yield more accurate geometry and reaction energies.7,8 In this work, a large basis set was used because calculations of unimolecular isomerization via hydrogen shift and the transitions between electronic states were also required. On the other hand, the observation of the C−O stretch band in the LIF spectrum of isobutoxy implied that the B̃ excited state dynamics of isobutoxy was similar to that of 1-butoxy.20 Theoretical calculations showed that the energy barrier of β C− C bond fission in B̃ excited state of isobutoxy was 25.5 kcal/ mol, which was comparable with the energy for the C−O dissociation (26.2 kcal/mol, Figure S3 in the Supporting Information). The energy required for β C−C bond fission and the C−O dissociation in the B̃ excited state was well above the interconversion energy (5.36 kcal/mol, CASSCF method) between the two conformers. Therefore, the interconversion between conformers would be the dominant nonradiative pathway of isobutoxy in the B̃ excited state. Complete absence of the neopentoxy fluorescence signal and the strong HCHO band set indicated that the decomposition reaction in the ground state dominated. The β C−C decomposition products of neopentoxy were HCHO and (CH3)3C radicals. The second methyl substitution on the β carbon lowered the dissociation energy barrier further to 7.0 kcal/mol, which was 2.6 kcal/mol lower than the barrier of isobutoxy. This energy difference changed the competition balance between the ground state decomposition and the excitation of the radical to the B̃ excited state, thus changing the dynamic role of alkoxy radicals in the photochemistry of

Table 3. Assignment of the Bands (cm−1) Observed in the LIF Spectra of Isobutoxy and 2-Methyl-1-butoxy radical isobutoxy

2-methyl-1butoxy

experimental band

vibrational interval

28090.6(A) 28787.3(B) 28917.1(C) 29025.0(D) 29215.2(E) 28096.9

0 696.7 826.5 934.4 1124.6 0

0 674 830 942 1140 0

νB−X 00 ν29 C−O ν28 ν26 ν22 νB−X 00

623.9

663

ν36 C−O

28720.8 a

predicteda assignment

Scaled CASSCF(9,7)/6-311++g(d,p) frequencies (scale factor 0.95).

band B in the LIF spectrum of isobutoxy could be assigned as the origin bands of two different conformers as was seen in the LIF spectrum of 1-propoxy27 and 1-butoxy.20 This possibility was less likely considering the small difference (85 cm−1) between the calculated adiabatic transition energies of the two conformers of isobutoxy. However, an unambiguous assignment cannot be made until a rotationally resolved spectrum can be obtained in the future. The LIF spectrum of the 2-methyl-1butoxy radical supported our assignment as its second band was 623.9 cm−1 above its origin band, which agreed well with the calculated C−O stretch frequency (663 cm−1) in the B̃ excited state of the lowest energy conformer of 2-methyl-1-butoxy (Figure S1, Table S3 and S4 in Supporting Information). Six bands in the region of 28 862−29 228 cm−1 were observed in the LIF spectrum of the 3-methyl-1-butoxy radical. The experimental frequencies of these bands agreed with the calculated B̃ −X̃ adiabatic excitation energy of the conformers of 3-methyl-1-butoxy (Figure S2, Table S5 in Supporting Information). Vibrational frequencies of the lowest energy conformer were also calculated (Table S6 in Supporting Information). However, no assignment can be made at this stage without the information of the rotational contour of these bands. The fluorescence of HCHO was observed simultaneously with the LIF spectrum of isobutoxy radicals. Fluorescence of HCHO was well studied and reported by dye laser excitation between 350 and 355 nm.23,24 It was observed previously when alkyl nitrites, the precursors of alkoxy radical, were photolyzed.28 The decomposition product HCHO was suggested to be generated via β C−C bond fission from the “hot” alkoxy radical formed in the photolysis step.28 Chhantyal-Pun et al. confirmed that the photolysis beam for generating the alkoxy radical from the nitrite precursor could provide the extra energy for the C−C bond cleavage.9 In the atmospheric reactions, 13232

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Table 4. Zero-Point Energy Corrected Barrier Heights Eb (kcal/mol) for Decomposition of Alkoxy Radicals by β C−C Bond Fission in the Ground State at the B3LYP/6-311++G(d,p) Level of Theorya

a

SAR-predicted barrier heights from ref 4 are cited for comparison.

This situation is just like the competition between β C−C bond fission and the LIF process in isobutoxy. In 2-methyl-1-butoxy, where the structure of the radical fulfills the condition to have single methyl substitution on β carbon for C−C bond fission and 1,5-H shift isomerization, the enhanced ground state processes resulted in even weaker fluorescence signal of the alkoxy radical (Figure 2). When methyl substitution occurred on γ C of alkoxy radicals, no decomposition product and red shift of origin band was observed as in 3-methyl-1-butoxy. The fluorescence spectrum mainly depended on the excited state dynamics of the radical. In our study of methyl substituted cyclohexoxy radicals,26 methyl substitution on β carbon (2-methylcyclohexoxy) of the cyclic secondary alkoxy radical also induced a red shift of the origin band (26 274.6 cm−1) compared to cyclohexoxy (26 754.3 cm−1). The methyl substitution on γ (3-methylcyclohexoxy) and δ carbons (4-methylcyclohexoxy) of the cyclic secondary alkoxy radical had no significant effect on the LIF spectra. These results, combined with the structural effects on the fluorescence spectra of isobutoxy and substituted butoxy radicals, indicate that the substitution on the β carbon of the alkoxy radical will significantly change the LIF spectra of alkoxy radicals and affect the monitoring of alkoxy radicals by a spectroscopic method. The methyl substitution on the β carbon

atmosphere. The β C−C bond fission barrier for 2-methyl-1butoxy (9.4 kcal/mol) was close to the barrier for isobutoxy. Fluorescence from both the dissociation product HCHO and 2methyl-1-butoxy radical itself were detected in the experiment. No HCHO signal was detected with the 3-methyl-1-butoxy spectrum, for which the β C−C decomposition barrier was calculated as 12.9 kcal/mol. All these experimental data and calculated results suggested that an internal energy of 7−13 kcal/mol in the alkoxy radical molecules was critical to initiate the ground state decomposition of alkoxy radicals, hence decided the possibility to observe the fluorescence of alkoxy radicals. Isomerization via H-migration is another reaction pathway in the ground state that competes with the alkoxy radical photoexcitation. Calculated energy barriers of isomerization via 1,4 and 1,5 H-shift in the ground state for isobutoxy and 1butoxy are shown in Figure 3. It shows that β C−C bond fission is the dominant unimolecular reaction pathway in the ground state for isobutoxy, while the isomerization via the 1,5-H shift is the preferred reaction pathway for 1-butoxy in the ground state. The isomerization process coexisted with the photoexcitation process to the B̃ excited state as LIF spectrum was detected for 1-butoxy. That is, there is a population distribution balance between the ground state reaction and the excitation process. 13233

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potential energy curve for β C−C decomposition of isobutoxy in the ground state (Figure S4) are also provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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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 the National Natural Science Foundation of China (Grant Nos. 20673013 and 21173024). We also acknowledge the grant from the Major State Basic Research Development Program (Grant No. 2007CB815206) and the Fundamental Research Funds for the Central Universities.

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(1) Orlando, J. J.; Tyndall, G. S.; Wallington, T. J. The Atmospheric Chemistry of Alkoxy Radicals. Chem. Rev. 2003, 103, 4657−4689. (2) Atkinson, R.; Arey, J. Atmospheric Degradation of Volatile Organic Compounds. Chem. Rev. 2003, 103, 4605−4638. (3) Atkinson, R. Atmospheric Reactions of Alkoxy and βHydroxyalkoxy Radicals. Int. J. Chem. Kinet. 1997, 29, 99−111. (4) Vereecken, L.; Peeters, J. Decomposition of Substituted Alkoxy Radicals  Part I: A Generalized Structure−Activity Relationship for Reaction Barrier Heights. Phys. Chem. Chem. Phys. 2009, 11, 9062− 9074. (5) Vereecken, L.; Peeters, J. A Structure−Activity Relationship for the Rate Coefficient of H-Migration in Substituted Alkoxy Radicals. Phys. Chem. Chem. Phys. 2010, 12, 12608−12620. (6) Kwok, E. S. C.; Arey, J.; Atkinson, R. Alkoxy Radical Isomerization in the OH Radical-Initiated Reactions of C4−C8 nAlkanes. J. Phys. Chem. 1996, 100, 214−219. (7) Ferenac, M. A.; Davis, A. J.; Holloway, A. S.; Dibble, T. S. Isomerization and Decomposition Reactions of Primary Alkoxy Radicals Derived from Oxygenated Solvents. J. Phys. Chem. A 2003, 107, 63−72. (8) Méreau, R.; Rayez, M.-T.; Caralp, F.; Rayez, J.-C. Isomerisation Reactions of Alkoxy Radicals: Theoretical Study and Structure− Activity Relationships. Phys. Chem. Chem. Phys. 2003, 5, 4828−4833. (9) 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. (10) Wood, G. P. F.; Rauk, A. R.; Radom, L. Modeling β-Scission Reactions of Peptide Backbone Alkoxy Radicals: Backbone C-C Bond Fission. J. Chem. Theory Comput. 2005, 1, 889−899. (11) Jungkamp, T. P. W.; Smith, J. N.; Seinfeld, J. H. Atmospheric Oxidation Mechanism of n-Butane: The Fate of Alkoxy Radicals. J. Phys. Chem. A 1997, 101, 4392−4401. (12) Fittschen, C.; Hippler, H.; Viskolcz, B. The β C-C Bond Scission in Alkoxy Radicals: Thermal Unimolecular Decomposition of t-Butoxy Radicals. Phys. Chem. Chem. Phys. 2002, 2, 1677−1683. (13) Choi, H.; Bise, R. T.; Neumark, D. M. Photodissociation Dynamics of the Ethoxy Radical (C2H5O). J. Phys. Chem. A 2000, 104, 10112−10118. (14) Lin, J.; Zu, L.; Fang, W. Conformation and Spectroscopy Study of Cycloheptoxy Radical. J. Phys. Chem. A 2011, 115, 274−279. (15) Blatt, A. H. Organic Syntheses; Wiley: New York, 1966. (16) 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. (17) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.;

Figure 3. Potential energy profiles (kcal/mol) for the ground state decomposition and isomerization reactions of isobutoxy (a) and 1butoxy (b) at the B3LYP/6-311++G(d,p) level.

will also change the reaction involvement of alkoxy radicals in the atmospheric photochemistry.

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CONCLUSION In this work, we obtained vibrationally resolved LIF spectra of isobutoxy, 2-methyl-1-butoxy, and 3-methyl-1-butoxy radicals under the supersonic jet-cooled condition. A red shift of the origin band was observed when methyl substitution occurred on the β carbon of 1-propoxy and 1-butoxy. The energy barrier of β C−C bond fission and H-shift isomerization determined the dominant reaction pathway in the ground state of alkoxy radicals and further affected the population distribution for the photoexcitation process. The observation of alkoxy radical fluorescence is structurally dependent when the energy barrier of unimolecular reactions in the ground state is in the range 7− 13 kcal/mol. This study will help to understand the importance of different alkoxy radicals in the photochemistry of atmosphere.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

Calculated vibrational frequencies of C1 and Cs conformers of isobutoxy radical in the ground state and B̃ excited state are provided in Table S1 and S2. Structures and relative energies of different conformers are given in Figure S1 and Table S3 for 2methyl-1-butoxy, and in Figure S2 and Table S5 for 3-methyl-1butoxy. Table S4 and Table S6 list the calculated vibrational frequencies of the lowest energy conformer of 2-methyl-1butoxy and 3-methyl-1-butoxy, respectively. Schematic potential energy curves for the Cs conformer of isobutoxy as a function of the C−O bond length (Figure S3), and the 13234

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dx.doi.org/10.1021/jp4016239 | J. Phys. Chem. A 2013, 117, 13229−13235