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

Abstract

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.

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

“…This is confirmed qualitatively in work by Huang et al on 12-DMB 45 and 13-DMB, 46 and in recent work by Pan and Wang on 13-DMB, 47 as well as on p-cymene (except for the ipso adduct with respect to the C 3 H 7 substituent). 12 These theoretical studies also imply that generally ortho adducts are formed with greater yields and typically decompose more quickly than ipso adducts, despite more exothermic formation enthalpies, i.e. for the compounds studied here add 1 = ortho and add 2 = ipso is implicated.…”

“…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 …”

In situ secondary organic aerosol formation from ambient pine forest air using an oxidation flow reactor

“…Signals from isotopes, internal standards, and possible artifacts (e.g., saturated hydrocarbons that correlate with O 3 concentration in the reactor) were removed from the analysis. When calculating pre-dicted depletion for α-pinene, β-pinene, 3-carene, toluene, p-cymene, methanol, and sesquiterpenes (SQT; using longifolene as a representative compound) in the subsequent analysis, the following rate constants used were: k OH = 5.3 × 10 −11 , 7.7 × 10 −11 , 8.7 × 10 −11 , 5.5 × 10 −12 , 1.5 × 10 −11 , 9.1 × 10 −13 , and 4.8 × 10 −11 cm 3 molec −1 s −1 , respectively (Calvert et al, 2002;Atkinson and Arey, 2003;Alarcón et al, 2014). As an approximation of previous measurements at this site, MT are assumed to be an equal mix of α-pinene, βpinene, and 3-carene for this analysis (Kim et al, 2010;Ortega et al, 2014).…”

In situ secondary organic aerosol formation from ambient pine forest air using an oxidation flow reactor

“…Signals from isotopes, internal standards, and possible artifacts (e.g., saturated hydrocarbons that correlate with O 3 concentration in the reactor) were removed from the analysis. When calculating pre-dicted depletion for α-pinene, β-pinene, 3-carene, toluene, p-cymene, methanol, and sesquiterpenes (SQT; using longifolene as a representative compound) in the subsequent analysis, the following rate constants used were: k OH = 5.3 × 10 −11 , 7.7 × 10 −11 , 8.7 × 10 −11 , 5.5 × 10 −12 , 1.5 × 10 −11 , 9.1 × 10 −13 , and 4.8 × 10 −11 cm 3 molec −1 s −1 , respectively (Calvert et al, 2002;Atkinson and Arey, 2003;Alarcón et al, 2014). As an approximation of previous measurements at this site, MT are assumed to be an equal mix of α-pinene, β-pinene, and 3-carene for this analysis (Kim et al, 2010;Ortega et al, 2014).…”