PCCP
Degradation of three oxygenated alkoxy radicals of atmospheric interest: HOCH2O , CH3OCH2O , CH3OCH2OCH2O . RRKM theoretical study of the b-C–H bond scission and the 1,6-isomerisation kinetics E. Henon,*a F. Bohr,a N. Sokolowski-Gomeza and F. Caralpb a
b
GSMA, UMR CNRS 6089, Equipe de Chimie The´orique, UFR Sciences Exactes et Naturelles, Universite´ de Reims Champagne-Ardenne, Moulin de la Housse BP 1039, 51687 Reims Cedex 2, France Laboratoire de Physico-Chimie Mole´culaire, UMR CNRS 5803, Universite´ Bordeaux I, 33405 Talence cedex, France
Received 10th July 2003, Accepted 29th October 2003 First published as an Advance Article on the web 18th November 2003
A detailed theoretical study on the pressure and temperature dependence of the rate constants k1 , k2 , k3 for the thermal b-C–H dissociation of the three radicals: HOCH2O , CH3OCH2O , CH3OCH2OCH2O is presented. This investigation is extended to the rate constant k4 for the 1,6-H-shift isomerisation of CH3OCH2OCH2O . High-level ab initio computations (CCSD(T)//MP2) have been performed and combined with RRKM theory to obtain rate constants. The b-C–H scission pathway is predicted to occur with an activation energy of 10–13 kcal mol1. Estimation of the competition between the b-C–H and b-C–O decompositions, the isomerisation process, and the reaction with oxygen has been done. At 760 Torr and 298 K, k1 , k2 , k3 , k4 are 4.4 104 s1, 5.2 104 s1, 4.2 103 s1 and 5.6 103 s1 respectively. An interesting result is that the isomerisation through a seven-membered transition state may compete with the H-atom elimination from the CH3OCH2OCH2O radical.
I.
Introduction
In this paper, we present a theoretical study of the three following b-C–H dissociations of oxygenated alkoxy radicals:
HOCH2 O þ M ! HCOOH þ H þ M
ð1Þ
CH3 OCH2 O þ M ! CH3 OCHO þ H þ M
CH3 OCH2 OCH2 O þ M ! CH3 OCH2 OCHO þ H þ M
ð2Þ ð3Þ
and of the isomerisation process:
CH3 OCH2 OCH2 O þ M ! CH2 OCH2 OCH2 OH þ M ð4Þ for which one pressure data were available at room temperature from experimental measurements.1–3 Actually, because of the increasing use as industrial solvents and fuel additives of oxygenated compounds such as ethers, their tropospheric oxidation has received significant attention. Dimethyl ether (DME) CH3OCH3 , and dimethoxymethane (DMM) CH3OCH2OCH3 , have been proposed as possible alternative diesel fuels. Atmospheric degradation of these compounds is initiated by OH radical attack which abstracts an alkyl hydrogen, and the alkyl radical formed will rapidly add oxygen to give peroxy radicals RO2 . In polluted atmosphere, peroxy radicals react with NO to give the corresponding alkoxy radicals RO . In the case of DME, these successive reactions
CH3 OCH3 þ OH ! CH3 OCH2 þ H2 O
CH3 OCH2 þ O2 þ M ! CH3 OCH2 O2 þ M
CH3 OCH2 O2 þ NO ! CH3 OCH2 O þ NO2 DOI: 10.1039/b313251j
ð5Þ ð6Þ ð7Þ
produce the simplest a-alkoxyalkoxy radical CH3OCH2O . By similar reactions, OH oxidation of DMM gives two different alkoxy radicals, CH3OCH2OCH2O and CH3OCH(O )OCH3 depending on the group (–CH3 or –CH2) from which there is H-abstraction. The most common reactions of alkoxy radicals are reaction with O2 (if an a-H is present), isomerisation via 1,5 H-shift through a 6-membered transition state and decomposition by b-C–C or b-C–O bond fission (if an ether oxygen atom is present).4 However, a kinetic study of the self-reaction of the peroxy radical CH3OCH2O2 1 complemented by a detailed product analysis has provided evidence for a mechanism involving the rapid thermal decomposition of the alkoxy radical CH3OCH2O by H-atom elimination occurring in competition with the reaction with O2 .
CH3 OCH2 O þ M ! CH3 OCHO Methylformate ðMFÞ þ H þ M ð2Þ
CH3 OCH2 O þ O2 ! CH3 OCHO Methylformate ðMFÞ þ HO2
ð8Þ
1
The authors have obtained a value of the decomposition rate constant of about 3 103 s1 (assuming kO2 1014 cm3 molecule1 s1) at 25 Torr total pressure. Moreover, it is shown in this work that the rate constant k2 for H-atom elimination exhibits a pressure dependence in the 25–800 Torr pressure range. In the same way, Wallington et al.,2 in a FTIR-smog chamber study of the atmospheric chemistry of DMM, analysed the fate of CH3OCH2OCH2O by measuring the yield of methoxymethylformate (MMF) and suggested the occurence of the Phys. Chem. Chem. Phys., 2003, 5, 5431–5437
This journal is # The Owner Societies 2003
5431
