Rate constants for the gas-phase reaction of hexamethylbenzene with OH radicals and H atoms and of 1,3,5-trimethylbenzene with H atoms

2012

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.

Section: Model-3 With Pure Isomerization and Intermediate Casesmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 95%

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

2012

“…This indicates formation of an ipso adduct where OH is bound at an already occupied position. Berndt and Bo ¨ge 8 found that the rate constant of the OH reaction with HMB was about 30 times greater than the expected rate constant for H-atom abstraction from a total of six methyl groups, 2 demonstrating the importance of the ipso addition in the case of HMB. Further measurements by von Buttlar et al, 9 and recently by Loison et al, 10 confirmed the reversibility of the OH + HMB reaction, where formation of an ipso adduct is the only plausible reaction channel in the temperature range of these investigations.…”

Section: Introductionmentioning

confidence: 97%

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

Alarcón

2015

The reaction of OH radicals with a series of methylated benzenes was studied in a temperature range 300-350 K using a flash-photolysis resonance fluorescence technique. Reversible OH additions led to complex OH decays dependent on the number of distinguishable adducts. Except for hexamethylbenzene, triexponential OH decay curves were obtained, consistent with formation of at least two adduct species. For three compounds that can strictly form two adduct isomers for symmetry reasons (1,4-dimethyl-, 1,3,5-trimethyl-, and 1,2,4,5-tetramethylbenzene) with OH bound ortho or ipso with respect to the methyl groups, OH decay curves were analysed in terms of a reaction mechanism in which the two adducts can be formed directly by OH addition or indirectly by isomerization. In all cases one adduct (add1) is dominating the decomposition back to OH. The other (add2) is more elusive and only detectable at elevated temperatures, similar to the single OH adduct of hexamethylbenzene. Two limiting cases of the general reaction mechanism could be examined quantitatively: reversible formation of add2 exclusively in the OH reaction or by isomerization of add1. Total OH rate constants, adduct loss rate constants and products of forward and reverse rate constants of reversible reactions were determined. From these quantities, adduct yields, equilibrium constants, as well as reaction enthalpies and entropies were derived for the three aromatics. Adduct yields strongly depend on the selected reaction model but generally formation of add1 predominates. For both models equilibrium constants of OH reactions lie between those of OH + benzene from the literature and those obtained for OH + hexamethylbenzene. The corresponding reaction enthalpies of add1 and add2 formations are in a range -87 ± 20 kJ mol(-1), less exothermic than for hexamethylbenzene (-101 kJ mol(-1)). Reaction enthalpies of possible add1 → add2 isomerizations are comparatively small. Because results for 1,3,5-trimethylbenzene are partly inconsistent with a direct formation of add2, we promote the existence of isomerization reactions. Moreover, based on available theoretical work in the literature, add1 and add2 are tentatively identified as ortho and ipso adducts, respectively. Total OH rate constants were obtained for all title compounds. They can be described by Arrhenius equations: kOH = A × exp(-B/T). The parameters ln(A/(cm(3) s(-1))) = -25.6 ± 0.3, -25.3 ± 0.6, -27.3 ± 0.3, -24.6 ± 0.3, -26.2 ± 0.4, -26.2 ± 0.4 and -24.5 ± 0.2, and B/K = -160 ± 90, -550 ± 180, -1120 ± 90, -330 ± 100, -820 ± 100, -980 ± 130, and -570 ± 40 were determined for 1,4-dimethyl-, 1,3,5-trimethyl-, 1,2,4,5-, 1,2,3,5- and 1,2,3,4-tetramethyl-, pentamethyl-, and hexamethylbenzene.

“…5 The observation of hexamethyl-2,4-cyclohexadienone by GC-MS as the only product from the reaction of OH with hexamethylbenzene in the presence of NO 2 implies that an addition of OH is a major first step of this reaction. 6 Biexponential FP-RF-decays of OH in the presence of hexamethylbenzene as a prototype molecule, where ipso positions alone are available for addition, demonstrated a reversible reaction, 7 and the molecular ion of the adduct has been observed by single-photon VUV photoionization as an intermediate in a flow reactor very recently. 8 Ipso-type adducts may react by dealkylation and corresponding products from the reaction of toluene, o-, m-, and p-xylene with OH radicals have been observed by chemical ionization mass spectrometry in a flowtube study (5.4% phenol from toluene and similar yields of the cresols expected from dealkylation of the xylenes).…”

Section: Introductionmentioning

confidence: 99%

Reversible addition of the OH radical top-cymene in the gas phase: multiple adduct formation. Part 2

Alarcón

et al. 2014

A flash photolysis-resonance fluorescence (FP-RF) system was used to study the p-cymene (PC) + OH reaction at temperatures between 299 and 349 K in helium. Triexponential functions were fitted to groups of observed OH decay curves according to a model considering a reversible addition to form two adducts as thermolabile reservoirs of OH. Compared to Part 1 of this paper, consideration of a second adduct strongly improved the fits to our measurements, and the rate constants for the major pathways were optimized between 299 and 349 K. The Arrhenius expression for the rate constant of the sum of OH addition and H-atom abstraction pathways was found to be kOH = 1.9 × 10(-12) exp[(610 ± 210) K/T] cm(3) s(-1). Rate constants of unimolecular decomposition reactions of the adducts were similar to other aromatic compounds with the following Arrhenius expressions: 1 × 10(12) exp[(-7600 ± 800) K/T] s(-1) for adduct 1 and 4 × 10(11) exp[(-8000 ± 300) K/T] s(-1) for adduct 2. Adduct yields increased and decreased with temperature for adduct 1 and 2, respectively, but were similar (∼0.4) around room temperature. Equilibrium constants yielded values for reaction enthalpies and entropies of adduct formations. While for one adduct reasonable agreement was obtained with theoretical predictions, there were significant deviations for the other adduct. This indicates the presence of more than two adduct isomers that were not accounted for in the reaction model. Quantum chemical calculations (DFT M06-2X/6-31G(d,p)) and RRKM kinetics were employed with the aim of clarifying the mechanism of the OH addition to PC. These calculations show that formation of adducts with OH in ortho positions to the isopropyl and methyl substituents is predominant (55% and 24%) to those with OH in ipso positions (21% and 3%). A large fraction (>90%) of the ipso-C3H7 adduct is predicted to react by dealkylation forming p-cresol (in the absence of oxygen) and isopropyl radicals. These theoretical results agree well with the interpretation of the experimental results showing that the two ortho adducts (which appeared as OH reservoirs in the experiment) have been observed.