A comparative study of vibrational anharmonicity in the bihalide anions XHX-, X = F, Cl, Br

MP2/6-31 + G(d, p), MP2/aug'-cc-p-VTZ, CCSD/aug'-cc-pVTZ and CCSD(T)/aug'-c c-pVTZ calculations were performed for the bihalide ions XHX-, for X = F, C l and Br. Equilibrium geometries were determined and harmonic vibrational f requencies computed. At each level of theory potential surfaces were genera ted in the two X-H bondlengths and a model two-dimensional vibrational prob lem was-solved to obtain anharmonic symmetric (v(1)) and asymmetric (v(3)) stretching frequencies. The computed harmonic frequencies are very dependen t on the level of theory used, a consequence of the difficulties in describ ing the various XHX- potential energy surfaces in the vicinity of the globa l minimum. Vibrational averaging in the anharmonic calculations results in anharmonic frequencies which exhibit much less variation with level of theo ry. Improved results are obtained even at the lowest level of theory, MP2/6 -31 + G(d, p). Anharmonic calculations on the CCSD(T)/aug'-cc-pVTZ potentia l energy surfaces reproduce the experimentally observed frequencies for bot h the XHX- complexes and their deuterated analogs, XDX-. Anharmonic effects become increasingly important along the series F, Cl and Br and anharmonic frequencies for the fundamental vibrations and combination bands are neces sary for understanding and assigning the experimental spectra of these comp lexes. (C) 1999 Elsevier Science B.V. All rights reserved.