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-