STRUCTURE, ENERGETICS, AND VIBRATIONAL-SPECTRUM OF H2O-X- (X=F, CL) COMPLEXES

The structure and energetics of the complexes formed between H2O and F- or Cl- were investigated using an ab initio method at the self-consistent-field (SCF) and electron correlation levels with large basis sets. The energy of the linear hydrogen bond was found to be lower than the energy of the bifurcated hydrogen bond. For the linear configurations the calculated DELTA-H(298 K) values are -25.40 and -13.20 kcal mol-1 for the H2O-F- and H2O-Cl- complexes, respectively, and are in good agreement with the experimental values. The electron-correlation contributions to the interaction energy were calculated and compared using Moller-Plesset perturbation theory at second (MP2), third (MP3) and full-fourth (MP4) order and the coupled cluster method. The energy decomposition analysis at the SCF and MP2 levels shows the important role of the electrostatic interaction in the H2O-Cl- complex. Vibrational frequencies, IR and Raman intensities were calculated at the SCF and MP2 levels. A very large red shift and an intensification of the O-H stretching vibration associated with the bond involved in the hydrogen bonding was observed for the H2O-F- complex.