Study of hydrogen abstraction reactions by density-functional methods

Four popular functionals, BLYP, BP86, B3LYP and B3P86 were utilized for studying hydrogen abstraction reactions R1 + RH → R1H + R (R1 = H, CH3, NH2, and OH radicals; R = H and CH3). Hybrid functionals, B3P86 and B3LYP, provide significantly more accurate transition state energies and vibrational frequencies than BLYP or BP86. In particular, B3LYP/6–31G(d) activation energies for six reactions studied here are found to be on average within 1.6 kcal/mol of experimental activation energies. It is shown that increasing basis set beyond 6–31G(d) does not provide better transition state geometries or barriers.