MOLECULAR-SURFACE ELECTROSTATIC POTENTIALS AND LOCAL IONIZATION ENERGIES OF GROUP-V-VII HYDRIDES AND THEIR ANIONS - RELATIONSHIPS FOR AQUEOUS AND GAS-PHASE ACIDITIES

We have computed ab initio HF/6-31 + G* electrostatic potentials and average local ionization energies on the molecular surfaces of the Group V-VII hydrides and corresponding anions of the first three rows of the periodic table. The surfaces were defined to be specified contours (0.002 or 0.001 au) of the molecular electronic density. The most negative potentials, V(S,min), and lowest ionization energies I(S,min)BAR, were located and determined. Their magnitudes separately satisfy limited correlations with gas-phase protonation enthalpies and aqueous pK(a) values. Our results indicate that V(S,min) and I(S,min)BAR are complementary, the former reflecting electrostatic factors and the latter being related to charge transfer/polarization. More general relationships for protonation enthalpies are obtained when both V(S,min) and I(S,min)BAR are explicitly included. Solution-phase and gas-phase acidities are shown to correlate very well if electrostatic effects are explicitly taken into account. (C) 1993 John Wiley & Sons, Inc.