AB-INITIO STUDY OF THE POTENTIAL-ENERGY FUNCTIONS RELEVANT FOR HYDROGEN-TRANSFER IN FORMAMIDE, ITS DIMER AND ITS COMPLEX WITH WATER

The potential energy (PE) functions of the electronic ground state and the lowest n pi* and pi pi* excited singlet states of formamide, its dimer and its complex with water have been characterized theoretically along the proton transfer (PT) reaction coordinate. The calculations were performed using the ab initio complete-active-space self-consistent-field (CASSCF) method and second-order perturbation theory employing the CASSCF wavefunction as the reference state. The results suggest that the photoinduced PT reaction in an isolated molecule occurs via successive dissociation and association of the hydrogen atom, whereas in hydrogen-bonded complexes, the PE functions along the PT reaction coordinate are qualitatively the same as for the PT reaction along the intramolecular hydrogen bond. Some implications concerning biological molecules containing peptide linkages are discussed.