Isotope effects and concerted two-hydron-transfer between formamidine molecules

With the intention to assist mechanistic studies of two-hydron-transfer reactions using 1H/2H isotope effects we have carried out theoretical studies of the two-hydron-transfer reaction between formamidine molecules. The ab initio STO-3 G potential energy surface has been calculated and all stationary points characterized by their force constant matrices. With this basis set the two hydrons are transferred concertedly and synchronously. The transition state (TS) has D2h symmetry. 1H/2H isotope effects were calculated using the Bebovib-IV program. Our results deviated from predictions made from common simple vibrational models. The isotope effects are dominated by the zero-point-energy contribution. The effect due to the stretching vibrations of the initial state is to a small extent counteracted by stretching of the TS. The bending vibrations appear to play a dominant role in reducing the magnitude of the isotope effect for the two-hydron-transfer reaction to a value expected for a one-hydron-transfer reaction. (orig.)