THEORETICAL-STUDIES OF PROTON TRANSFERS .1. THE POTENTIAL-ENERGY SURFACES OF THE IDENTITY REACTIONS OF THE 1ST-ROW AND 2ND-ROW NONMETAL HYDRIDES WITH THEIR CONJUGATE BASES

High-level ab initio calculations are used to investigate the potentia l energy surfaces of the identity proton transfers between CH4, NH3, O H2, FH, SiH4, PH3, SH2, and ClH and their conjugate bases. Energies ar e reported at the MP4/6-311+G(d,p)//MP2/6-31+G(d,p) and G2+ levels. At the highest level, there is a good correlation between the calculated and experimental proton affinities of the conjugate bases (average er ror, +/-0.5 kcal/mol). The proton-transfer potential energy-surfaces v ary from single wells with stable, symmetric intermediates (FH) to dou ble wells with significant central barriers (CH4, NH3, SiH4, and PH3). In some systems, a barrier exists on the electronic potential energy surface but disappears when vibrational energy corrections are applied (OH2, SH2, and ClH). Considering the full series, it is clear that fo r a given acidity, the second-row systems face much larger barriers to proton transfer than the first-row analogs. The surfaces are also inv estigated with Bader's electron density analysis approach. Integrated populations and critical point densities are reported for the complexe s and transition structures at the MP2/6-31+G(d,p) level. In addition, Laplacian representations of the density are shown for the transition structures. The energetics of the potential energy surfaces are discu ssed in terms of the electron density distributions.