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

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 (H₃N-H-OH₂)⁺. 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 Å) and the long N-O distance (5.0 Å). 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 Å the multireference structure becomes significant for intermediate regions of the hydrogen bond. The correlation between the protonation potential surfaces for NH₃ and H₂O 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 "proton transfer" in the ground electronic state and a "hydrogen transfer" in low-lying excited states.