Basis set and correlation effects on computed negative ion hydrogen bond energies of the complexes AHn � AHn?1?1: AHn ? NH3, OH2, and FH

Abstract

Basis set and correlation effects on computed hydrogen bond energies of the negative ion complexes AHn · AHn−1−1, for AHn NH3, OH2, and FH, have been evaluated. The addition of diffuse functions on nonhydrogen atoms to valence double- and triple-split plus polarization basis sets [6-31G(d, p) and 6-311G(d, p)] significantly decreases binding energies by 9–19 kcal/mol, depending on the particular complex and the level of theory. Adding diffuse functions to hydrogens has a negligible effect, while replacing the single set of polarization functions on each atom by two sets alters energies by 1 kcal/mol or less. Electron correlation increases the hydrogen bond energies of these complexes and has a greater effect for basis sets without diffuse functions. Since the Hartee-Fock energies computed with these basis sets are already too large, correlation energy calculations should not be performed in these cases. For basis sets including diffuse functions, the correlation energy contribution to the binding energies of these complexes is significant, with the Moller-Plesset second-order term being the largest term and having a stabilizing effect of from 3–6 kcal/mol. The third and fourth order terms are smaller, and may be of opposite sign. As a result, the MP2 and MP4 energies differ by no more than 1 kcal/mol, with the MP2 stabilization energy being greater except for N2H5−1. The computed standard solvation enthalpy of OH−1 by H2O based on either MP4/6–311 + G(2d, 2p) or MP2/6–31 + G(d, p) electronic energies is –26.8 kcal/mol, in excellent agreement with a recent gas-phase experimental measurement.