Kinetics and mechanism of the reaction of OH with the trimethylbenzenes – experimental evidence for the formation of adduct isomers

Abstract

The reversible gas-phase addition of OH radicals to the trimethylbenzenes was investigated in pulsed experiments utilizing VUV flash-photolysis resonance-fluorescence of H(2)O in the temperature range of 275-340 K. Triexponential OH decays were observed in the presence of the trimethylbenzenes, indicating the participation of more than one adduct species. Analytical solutions for the system of differential equations with two adduct isomers were derived, and the OH decay curves were evaluated based on this reaction model. This led to significant improvements of fit qualities and notable changes in OH rate constants compared to a previous model with a single adduct species. The detailed analysis was confined to 1,3,5-trimethylbenzene where reversible formation of two OH-aromatic ortho- and ipso-adduct isomers is feasible in accordance with the extended reaction model. Only after inclusion of additional isomerization reactions, consistent thermochemical data were obtained from the fitted rate constants. Reaction enthalpies of -83 ± 7 kJ mol(-1) and -35 ± 22 kJ mol(-1) were derived for the formation of one adduct isomer and the isomerization into the other, respectively. Based on literature data, the more and less stable adducts were assigned to ipso- and ortho-adduct isomers, respectively. The potential isomerization precluded the determination of primary yields of adduct isomers but formation of the ipso-adduct in any case is a minor process. For the rate constants of the OH + 1,3,5-trimethylbenzene reaction an Arrhenius expression k(OH) = 1.32 × 10(-11) cm(3) s(-1) exp(450 ± 50 K/T) was obtained. Based on the same approach, the rate constants of the OH reactions with 1,2,3-trimethylbenzene and 1,2,4-trimethylbenzene were derived as k(OH) = 3.61 × 10(-12) cm(3) s(-1) exp(620 ± 80 K/T) and k(OH) = 2.73 × 10(-12) cm(3) s(-1) exp(730 ± 70 K/T), respectively.

Kinetic and mechanistic study of the reaction of OH radicals with methylated benzenes: 1,4-dimethyl-, 1,3,5-trimethyl-, 1,2,4,5-, 1,2,3,5- and 1,2,3,4-tetramethyl-, pentamethyl-, and hexamethylbenzene

“…The k OH of 135-TMB led to a slightly weaker temperature dependence than in previous work of Aschmann et al, 31 but in agreement with the work of Bohn and Zetzsch. 11 However, the k OH of this work are greater by about 25%. Room temperature k OH for 1245-TeMB and PMB are in good agreement with recent relative rate measurements by Aschmann et al 32 at 298 K while for 1234-TeMB and 1235-TeMB no literature data are available.…”

“…k L1 = [k À11a + k 31 ] and 4,11,20 but they may be affected by the use of the wrong reaction model. 11 In contrast, the activation energy for the OH-HMB adduct is greater (89 kJ mol À1 ).…”

“…In addition, the following relationships (that generally apply for model-2, model-3 and any intermediate case) have been established previously: 11 [k 11a + k 12a + k 1b ] = k OH = const (1) [k 11a k À11a ] + [k 12a k À12a ] = const (2) [k À11a + k 12 …”

“…11 It can be summarised by the following list of reactions with the corresponding rate constants given in parentheses:…”

Gas-Phase Reaction of Hydroxyl Radical with Hexamethylbenzene

“…Very recently, a study of OH + trimethylbenzenes has also reported evidence for ipso-adduct formation, but this channel was in any case a minor process. 8 Among the various oxidation products arising from the OHinitiated oxidation of monocyclic aromatic compounds, ringretaining species such as benzaldehyde, cresols, as well as highly…”