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 a verage local ionization energies on the molecular surfaces of the Grou p V-VII hydrides and corresponding anions of the first three rows of t he periodic table. The surfaces were defined to be specified contours (0.002 or 0.001 au) of the molecular electronic density. The most nega tive potentials, V(S,min), and lowest ionization energies I(S,min)BAR, were located and determined. Their magnitudes separately satisfy limi ted correlations with gas-phase protonation enthalpies and aqueous pK( a) values. Our results indicate that V(S,min) and I(S,min)BAR are comp lementary, the former reflecting electrostatic factors and the latter being related to charge transfer/polarization. More general relationsh ips for protonation enthalpies are obtained when both V(S,min) and I(S ,min)BAR are explicitly included. Solution-phase and gas-phase aciditi es are shown to correlate very well if electrostatic effects are expli citly taken into account. (C) 1993 John Wiley & Sons, Inc.