Structure, energetics, and vibrational spectrum of H 2O-X − (X = F, Cl) complexes

Abstract

The structure and energetics of the complexes formed between H 2O 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 Δ H(298 K) values are −25.40 and −13.20 kcal mol −1 for the H 2O-F − and H 2O-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 Møller-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 H 2O-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 H 2O-F − complex.