Theoretical Studies of Proton Transfer Reactions - Energy Barriers and the Marcus Equation

The reactants, ion-dipole complexes, transition states, and products for the proton transfer reactions HBCOH⁺ + OCXH → HBCO + ⁺HOCXH optimized at the SCF/4-31G* level of theory for B, X = F, Cl, H, CH₃, CH₂Cl, CHCl₂ and CCl₃ are studied. The intrinsic barrier ΔE^#_BX correlates with the degree of the O-O bond contraction in the transition structure. Both intrinsic and overall barriers can be predicted with the aid of Marcus theory. Progressive degrees of chlorination of the alkyl group in B produce decreases in the barrier to proton transfer from HBCOH⁺ to OCXH and increases in the reverse transfer barriers. These changes can be quantitatively reproduced by the Marcus equation for all systems.