Ab Initio Studies of Hydrogen Bihalide Anions: Anharmonic Frequencies and Hydrogen-Bond Energies

Hydrogen bihalide anions, $XHX^-$ (X = F, Cl, and Br) have been studied by high level ab initio methods to determine the molecular structure, vibrational frequencies, and energetics of the anions. All bihalide anions are found to be of linear and symmetric structures, and the calculated bond lengths are consistent with experimental data. The harmonic frequencies exhibit large deviations from the experimental frequencies, suggesting the vibrations of these anions are very anharmonic. Two different approaches, the VSCF and VPT2 methods, are employed to calculate the anharmonic frequencies, and the results are compared with the experimental frequencies. While the ${\nu}_1$ and ${\nu}_2$ frequencies are in reasonable agreement with the experimental values, the ${\nu}_3$ and ${\nu}_1+{\nu}_3$ frequencies still exhibit large deviations. The hydrogen-bond energies and enthalpies are calculated at various levels including the W1BD and G4 composite methods. The hydrogen-bond enthalpies calculated are in good agreement with the experimental values.