Structure, intramolecular rotation barrier, and thermochemical properties of hydroxycyclohexadienyl radical

Ab initio and density functional calculations are used to study the structures and thermochemical properties, DeltaH(f)(0)(T), S-0(T), and C-p(T) (100 less than or equal to T/K less than or equal to 5000), of the benzene-OH adduct (hydroxycyclohexadienyl, CHD-OH). Molecular structures and vibrational frequencies were determined at the B3LYP/6-31G(d,p) and MP2(full)/6-31G(d) levels. Energy calculations were performed at the G3, CBS-Q, CBS-QB3, G3(MP2)H B3LYP/6-31G(d,p), CBS-Q//B3LYP/6-31G(d,p), G3(MP2)H MP2(full)/6-31G(d), and CBS-Q//MP2(full)/ 6-31G(d) levels. Enthalpies of formation (DeltaH(f298)(0)) were determined at each calculation level using group balance isodesmic reactions. The new value of DeltaH(f298)(0)(0H) = 8.96 kcal mol(-1) was used in this study. Standard entropy, S-0(T), and heat capacity, Cp(T), from vibrational, translational, and external rotation contributions were calculated using statistical mechanics based on the vibration frequencies and structures. Hindered rotational contributions to SO(T) and Cp(T) were calculated from the energy levels of the internal rotational potential calculated at the B3LYP/6-31G(d,p) level. The anomeric effect where the lone pair on oxygen is hyperconjugated with the antibonding orbital of adjacent C-C bonds is found to stabilize the CHD-OH in the lowest energy conformation. This hyperconjugation increases the barrier of the OH rotor and leads to a lower entropy value of CHD-OH from our previous estimation. Evaluations of data from the isodesmic reaction at each calculation level results in the enthalpy of formation of CHD-OH in a range of 10.76 [G3MP2//MP2(full)/6-31G(d)] to 9.92 [CBS-Q//B3LYP/6-31G(d,p)] kcal mol(-1) at 298 K. The reaction energy (DeltaH(r298)(0)) of C6H6 + OH <----> CHD-OH (1) was determined to be -18.38 kcal mol(-1) in good agreement with literature values that range from -19.9 +/- 1.2 to -16.5 kcal mol(-1). Entropy (S-298(0)) of CHD-OH was 2 estimated as 79.24 cal mol(-1) K-1, 3.5 cal mol(-1) K-1 lower than our previous value of 82.7 cal mol(-1) K-1. The reaction entropy DeltaS(r298)(0) was calculated as -29.01 cal mol(-1) K-1 about 4.6 cal mol(-1) K-1 higher than literature value of -33.6 +/- 2.6 cal mol(-1) K-1. The rate constant for CHD-OH adduct dissociation to C6H6 + OH was calculated and compared to literature data.