Ab initio multireference configuration interaction and coupled cluster studies of potential surfaces for proton transfer in (H3N-H-OH2)

Proton-transfer reactions are important in chemical and biological processes, including photosynthesis and vision. The multiple reference double-excitation configuration interaction method (MRD-CI) and the coupled cluster method (CCM) were applied for the studies of the ground-state and low-lying excited states for the proton-transfer system (H{sub 3}N---H---OH{sub 2}){sup +}. The geometry optimization at the SCF level indicates the rapid change in geometry of subunits while the proton moves between N and O atoms. The significant difference was found between the structure of potential curves for the short N-O distances (2.707, 2.95, 3.2 {Angstrom}) and the long N-O distance (5.0 {Angstrom}). The complicated multireference structure of potential curves results from the strong interactions between them. The ground state is described by a single determinant wave function for short N-O distances; however, for a distance of 5.0 {Angstrom} the multireference structure becomes significant for intermediate regions of the hydrogen bond. The correlation between the protonation potential surfaces for NH{sub 3} and H{sub 2}O and the structure of surfaces for the proton-transfer system of the complex can be recognized. The simple interpretation of the gross atomic population on the transferred proton indicates that the reaction proceeds as a {open_quotes}proton transfer{close_quotes} in the ground electronic state and a {open_quotes}hydrogen transfer{close_quotes} in low-lying excited states. 33 refs., 13 figs., 3 tabs.