Thermochemical Properties of Hydroxycyclohexadienyl Peroxy Isomers from Reaction of O₂ with the Benzene-OH adduct

Abstract

Abstract:The addition of OH radical to benzene and other aromatic species to form a cyclohexadienyl radical (C•HD-OH), is the first step for conversion (oxidation) of aromatic species in atmospheric chemistry. Reaction of this C•HD-OH intermediate, with ground-state molecular oxygen forms a number of hydroxylcyclohexadienyl peroxy (CHD-OH-OO•) isomers, which further react to form aerosols. Ab initio and density functional calculations were used to study the structures and thermochemical properties (Δ o 298 , o ( ) and ( ) of the hydroxyl-cyclohexadienyl peroxy isomers (CHD-OH-OO•) ((i) para-cis-(Z) and para-trans-(E) (OO• relative to the OH group); (ii) ortho-cis-(Z) and ortho-trans-(E) at pseudo-equatorial (e') and pseudo-axial (a') positions). The thermochemical properties are important for modeling studies on the kinetics of the aerosol formation. Molecular structures and vibration frequencies were determined at the B3LYP/6-31G(d,p) level. Energy calculations were performed at the CBS-lq, CBS-4, G3(MP2)//B3LYP/6-31G(d,p), and BLYP/6-311++G(2d,p)//BLYP/6-31G(d,p) levels. Enthalpies of formation (Δ o 298 ) were calculated at each level using group balance isodesmic reactions. An analysis is performed for entropy from inclusion of internal rotor analysis versus use of torsion frequencies and conformer statistics. Standard entropy, o ( ), and heat capacity, ( ), from vibrational, translational, and external rotation contributions were calculated using statistical mechanics based on the vibration frequencies and structures. Hindered rotational contributions to o ( ) and ( ) were calculated from the energy levels, where the internal rotation potential was calculated at the B3LYP/6-31G(d) level. This analysis of the hindered internal rotor contributions yields higher entropy than the values calculated when the rigid-rotor-harmonic-oscillator approximation (RRHO) applied for the calculated torsion frequencies. Differences be-

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“…The difference in IDT predictions at 900 K is about an order of magnitude. Using the higher entropy value from the QC calculations, the equilibrium shifts to form this radical which promotes reactivity, resulting in shorter predicted IDTs and Sebbar et al 20 10 hydroxycyclohexadienyl peroxy isomers Chen et al 21 11 C 1 −C 4 hydroperoxides and peroxy radicals Wang et al 22 12 alcohols, hydroperoxides, and vinylic, alkoxy, and alkylperoxy radicals Yommee et al 23 13 hydroxyl-and hydroperoxide-substituted furan, methylfuran, and methoxyfuran Hudzik et al 24 14 isooctane and carbon radicals Snitsiriwat et al 25 better agreement with the experimental data. Further investigation found that, using the GA method, there are two different ways in which the thermochemistry of 2-propenyl-1methyl radical can be calculated, as shown in Figure 4 and Table 2.…”

“…Ritter and Bozzelli developed THERM (THermo Estimation for Radicals and Molecules) based on Benson’s GA method, which can be used as illustrated in Figure . Bozzelli and co-workers − have also studied the thermochemistry of different classes of species including sulfurated, nitridated, fluorinated, oxygenated, and saturated hydrocarbons, Table . Enthalpies of formation, entropies, and heat capacities have been compiled for certain classes of species using density functional theory (DFT) and composite compound methods, from which bond dissociation energies (BDEs) of similar type of bonds and GAVs were systematically reported and compared.…”

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