ARTICLE pubs.acs.org/JPCA
Computational Study of the Reaction Mechanism of the Methylperoxy Self-Reaction Yan-Ni Liang,† Jun Li,‡ Quan-De Wang,† Fan Wang,*,† and Xiang-Yuan Li‡ †
College of Chemistry and ‡College of Chemical Engineering, Sichuan University, Chengdu 610065, People’s Republic of China
bS Supporting Information ABSTRACT: To provide insight on the reaction mechanism of the methyperoxy (CH3O2•) self-reaction, stationary points on both the spin-singlet and the spin-triplet potential energy surfaces of 2(CH3O2•) have been searched at the B3LYP/6-311++G(2df,2p) level. The relative energies, enthalpies, and free energies of these stationary points are calculated using CCSD(T)/ cc-pVTZ. Our theoretical results indicate that reactions on a spin-triplet potential energy surface are kinetically unfavorable due to high free energy barriers, while they are more complicated on the spin-singlet surface. CH3OOCH3 + O21 can be produced directly from 2(CH3O2•), while in other channels, three spin-singlet chain-structure intermediates are first formed and subsequently dissociated to produce different products. Besides the dominant channels producing 2CH3O• + O2 and CH3OH + CH2O + O2 as determined before, the channels leading to CH3OOOH + CH2O and CH3O• + CH2O + HO2• are also energetically favorable in the self-reaction of CH3O2• especially at low temperature according to our results.
1. INTRODUCTION Peroxyl radicals (RO2•, R = H or any organic group) have been the subject of intensive research during the last decades, both experimentally and theoretically,13 since RO2• are important trace tropospheric constituents that are involved in the oxidation and production of many other atmospheric species.4 For example, RO2• are responsible for ozone production in the troposphere as well as for the formation of hydrogen peroxide (H2O2) and hydroperoxides (ROOH).5 In addition, they are key reactive intermediates in the oxidation and combustion of hydrocarbons,6,7 especially at low temperature and moderate pressure conditions, which is initiated by the following addition of molecular oxygen to the major alkyl radicals:8
channels have been suggested for this reaction:11,12 2CH3 O2 • f 2CH3 O• þ O2
ð7aÞ
2CH3 O2 • f CH2 O þ CH3 OH þ O2
ð7bÞ
2CH3 O2 • f CH3 OOCH3 þ O2
ð7cÞ
2CH3 O2 • f CH3 OOH þ •CH2 O2 •
ð7dÞ
Some experimental studies have been conducted to investigate the kinetics of the self-reaction of CH3O2• previously. Niki and co-workers13 studied the self-reaction of CH3O2• by means of Fourier transform infrared spectroscopy (FTIR) at 297 K. Besides the above primary reactions 7a7d, the following secondary reactions were also proposed:
R• þ O2 f RO2 •
ð1Þ
RO2 • þ HO2 • f ROOH þ O2
ð2Þ
CH3 O• þ O2 f CH2 O þ HO2 •
ð8Þ
RO2 • þ H2 O2 f ROOH þ HO2 •
ð3Þ
CH3 O2 • þ HO2 • f CH3 OOH þ O2
ð9Þ
RO2 • þ CH3 O2 • f RO• þ CH3 O• þ O2
ð4Þ
RO2 • þ R 0 O2 • f R 0 O• þ RO• þ O2
ð5Þ
ROOH f RO• þ •OH
ð6Þ
The methylperoxy radical (CH3O2•) is an important peroxy radical in the oxidation of hydrocarbons and other organic compounds.9,10 The self-reaction of CH3O2• becomes significant in the atmospheric oxidation when nitrogen oxides are present in low concentration6 or in low temperature combustion of methane as shown in reaction 4.8 Four different r 2011 American Chemical Society
The relative rate constants for reactions 7a7c were determined to be 0.32:0.60:0.08, respectively, which indicated that path 7c was not important among all reactions. A similar conclusion has also been drawn for the self-reaction of C2H5O2 radicals.14 Lightfoot et al.7 performed flash photolysis studies of the CH3O2• self-reaction from 248 to 573 K. Their results showed paths 7a and 7b to be highly competitive with a strong temperature dependence in their branching ratios, and the contribution from pathway 7c was insignificant at all temperatures. According Received: May 24, 2011 Revised: October 15, 2011 Published: October 17, 2011 13534
dx.doi.org/10.1021/jp2048508 | J. Phys. Chem. A 2011, 115, 13534–13541
The Journal of Physical Chemistry A Scheme 1
to their results, the existence of reaction 7d up to 573 K is not supported by their data, and the radical-producing channel 7a is by far the most important, which would account for nearly 90% of all reactions at 700 K.7 They found that although reaction 7a acts as a termination reaction under atmospheric conditions, it largely serves to convert CH3O2• to HO2• under combustion conditions via reaction 8. Tyndall et al.15 determined the products of the self-reaction of CH3O2• at 296 K using continuous ultraviolet (UV) photolysis with FTIR detection. Their experiments were carried out through the photolysis of AZM (azomethane)-O2 or Cl2 CH4 O2 mixtures to determine the product yields from the CH3O2• self-reaction. The major products observed were CH2O, CH3OH, and CH3OOH with minor amounts of CO and HCOOH. Wallington and co-workers1618 have shown that CH3OOH is the major product of reaction 9, while CO and HCOOH are formed from secondary reactions involving Cl atoms.19 There was no evidence for the production of CH3OOCH3 (yield
