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.
Highlights
The biogenic aromatic compound p-cymene (1-methyl-4isopropylbenzene or 4-isopropyltoluene) has four distinguishable positions where the OH radicals may add to the aromatic ring, as illustrated in Scheme 1: two equivalent ones ortho to the isopropyl group (PCortho–C3H7), two equivalent ones ortho to the methyl group (PCortho–CH3), and two non-equivalent ipso positions at the site of the methyl and the isopropyl group (PCipso–CH3 and PCipso–C3H7)
Addition to an already occupied position is in general considered to be less important, but triexponential decays of OH have been observed by VUV flash photolysis/resonance fluorescence (FP-RF) in the presence of 1,3,5-trimethylbenzene, indicating that two different thermolabile adducts can regenerate OH.[5]
The initial fast decay corresponds to the consumption of OH radicals by H-atom abstraction and OH addition, while the subsequent slower OH decays are caused by regeneration of OH by a unimolecular decomposition of unstable adducts
Summary
The biogenic aromatic compound p-cymene (1-methyl-4isopropylbenzene or 4-isopropyltoluene) has four distinguishable positions where the OH radicals may add to the aromatic ring, as illustrated in Scheme 1: two equivalent ones ortho to the isopropyl group (PCortho–C3H7), two equivalent ones ortho to the methyl group (PCortho–CH3), and two non-equivalent ipso positions at the site of the methyl and the isopropyl group (PCipso–CH3 and PCipso–C3H7). Diffusion of OH radicals from the observation zone due to concentration gradient between the irradiated volume and the cell walls as well as transport due to the constant gas flow may contribute to the OH radical losses described by the rate constant k2. This rate constant comprised several processes, and its complete understanding is complicated and related to the experimental conditions applied.
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