Calculation of the Effects of Branching and Conjugation on Intrinsic Barriers for H Atom Transfer Reactions Involving Hydrocarbons

This paper describes the use of the semiempirical molecular orbital (MO) theoretical methods (AM1 and MNDO-PM3) to calculate barriers for a series of H atom transfer identity reactions involving alkyl, alkenyl, arylalkyl, and hydroaryl systems. Transition state (TS) energies were calculated for a series of known H abstractions and show to correlate linearly with experimental TS energies. Barriers for H abstraction reactions decrease with the degree of alkyl substitution at the radical site, and increase with the degree of conjugation. Barriers for transfer of a β-hydrogen from a radical to an unsaturated hydrocarbon (radical hydrogen transfer or RHT) were also calculated. The results show that methyl group substitutions at the radical site lower the barrier while substitutions at the site β to the radical, the position from which the H originates, raise the barrier. The barriers for RHT reactions involving conjugated systems increase with increasing radical delocalization and correlate linearly with the strength of the donor radical β-C−H bond. RHT barriers are estimated to range from E a ≈ 17−20 kcal/mol for benzene-plus-cyclohexadiene to E a ≈ 26−29 kcal/mol for anthracene-plus-9-hydroanthracene.