Thermochemical properties of hydroxycyclohexadienyl peroxy isomers from reaction of O₂ with the benzene-OH adduct

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_f H° ₂₉₈, S° (T) and C_p (T) 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 frequencieswere 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_fH°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, S° (T), and heat capacity, C_p (T), from vibrational, translational, and external rotation contributions were calculated using statistical mechanics based on the vibration frequencies and structures. Hindered rotational contributions to S° (T) and C_p (T) 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 calculatedwhen the rigid-rotor-harmonic-oscillator approximation (RRHO) applied for the calculated torsion frequencies. Differences between the uses of RRHO for torsions with correction for statistical mix of conformations, versus hindered internal rotor analysis are as high as 3.5 cal mol^-1 K^-1 in entropy and 3.6 cal mol^-1 K^-1 in heat capacity at 50 ≤ T/K ≤ 5000. Anumber of intramolecular interactions are found to stabilize the CHD-OH-OO· isomers: (i) intramolecular hydrogen bonding (between the OH and the peroxy OO· groups, OO-H-O), (ii) a negative hyperconjugation where the lone pair on oxygen atom in OH group is hyperconjugated with the antibonding σ∗ orbital of adjacent C=C bond, (iii) an anomeric interaction inwhich antibonding σ∗ orbital of C-O bond in a pseudo-axial position with a neighboring double bond π C-C. The eclipsing steric repulsion between OH and OO· groups counteract the stabilizing hydrogen bonding interaction. The importance of these effects are evaluated in determining the thermodynamic stability of the CHD-OH-OO· isomers. Evaluations of data from the isodesmic reaction at G3(MP2)//B3LYP/6-31G(d,p) level, results in the enthalpy of formation of the most stable isomers of CHD-OH-OO· in a range of -0.47 to -3.62 kcalmol^-1 at 298 K(for (Z)-o-II, -0.47 kcalmol^-1 and (E)-o-II, -3.62 kcalmol^-1). The reaction energies (Δ_rH° 298) of hydroxycyclohexadienyl radical (C·HD-OH)+O₂ →CHD-OH-OO· isomers were determined to be -10.63 ∼-13.67 (-10.56 ∼-13.34) kcalmol^-1. Entropy (S° 298) of CHD-OH-OO· was in range of 93.17 ∼96.18 (90.82∼95.55) calmol^-1 K^-1. Data in parentheses are based on RRHO/torsion frequency approximation with correction for the mix of conformers. The literature values of reaction energy (Δ_rH° 298 K) and entropy (Δ_rH° 298 K) were reported -10.5±1.3 kcalmol^-1 and -33.9±1.4 cal mol^-1 K^-1, respectively, based on second-law analysis at temperature between 265 and 345 K.