ABINITIO STUDY OF PROTON-TRANSFER IN [H3N-H-NH3]+ AND [H3N-H-OH2]+

Quantum mechanical ab initio calculations at the MP2/6-31G* level are performed on two proton bound dimer systems, [H3N-H-NH3]+ and [H3N-H-OH2]+. Several calculations using a medium-size polarized basis set were performed as a check of the 6-31G* results. Energies are calculated at heavy-atom separations of 2.25-3.25 angstrom. At fixed monomer separations, H is moved along the intermonomer axis, thus mapping out the proton transfer potential energy surface. For the ammonia dimer, the energy for displacements of H perpendicular to the N-N axis are also calculated. For the ammonia-water dimer, two different binding geometries for the water molecule are considered. All data are fit to analytical functions. We discuss the effects of squeezing and stretching the donor-acceptor distance on proton transfer.