Structure, Intramolecular Rotation Barrier, and Thermochemical Properties of Hydroxycyclohexadienyl Radical

摘要

Ab initio and density functional calculations are used to study the structures and thermochemical properties, △H_f~0(T), C_p(T) (100 ≤ T/K ≤ 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)//B3LYP/6-31G(d,p), CBS-Q//B3LYP/6-31G(d,p), G3(MP2)// MP2(full)/6-31G(d), and CBS-Q//MP2(full)/6-31G(d) levels. Enthalpies of formation (△H_(f298)~0) were determined at each calculation level using group balance isodesmic reactions. The new value of △H_(f298)~0(OH) = 8.96 kcal mol~(-1) was used in this study. Standard entropy, S~0(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~0(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 (△H_(r298)~0) of C_6H_6 + 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 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 △S_(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 C_6H_6 + OH was calculated and compared to literature data.