| Popis: |
Ab initio calculations have been performed at correlated levels of theory to investigate the convergence of the binding energies of hydrogen-bonded neutral, positive ion and negative ion complexes with systematic expansion of the basis set. For this study, the Dunning correlation-consistent polarized split-valence basis sets augmented with diffuse functions only on nonhydrogen atoms (aug'-cc-pVXZ, where X = D for double-split, T for triplesplit, and Q for quadruple-split) have been used. Computed hydrogen bond energies of all complexes converge with increasing basis set size, with satisfactory convergence occurring at aug'-cc-pVTZ. Binding energies are not sensitive to the presence of diffuse f functions in the aug'-cc-pVTZ basis set, and these f functions may be omitted. The presence or absence of diffuse d functions has little effect on the computed binding energies of neutral and positive ion complexes. However, these functions are important for describing the binding energies of the negative ion complexes. In contrast, quantitatively accurate proton affinities for corresponding neutral and anionic monomeric bases require the presence of both diffuse f and diffuse d functions in the aug'-cc-pVTZ basis set. For neutral and positive ion hydrogen-bonded complexes, MP4/aug'-cc-pVTZ binding energies are within 0.1 kcal mol −1 of CCSD+T(CCSD) binding energies, but may be as much as 1 kcal mol −1 smaller for the negative ion complexes. |
