COORDINATION EFFECTS ON THE STRETCHING VIBRATION OF THE OH- ION

IR and Raman measurements on crystalline hydroxides reported in the literature have shown that both frequency upshifts and downshifts with respect to the free-ion frequency occur. Here the fundamental stretching vibrational frequency of a bound OH- ion in different point charge environments has been examined by ab initio calculations at the MP2 level. For a given geometry of a q+ . OH- . q- complex, the ab initio frequency is found to vary in a systematic way as the electric field is increased: the frequency increases, passes through a maximum and then decreases. Both the value of the maximum frequency and the field strength at which it occurs are highly dependent on the geometry of the complex. Only the field component parallel to the OH- axis is effective in changing the OH frequency. The arc-like shape of the frequency versus field correlation curves ''explains'' the large degree of non-additivity found for the environment-induced frequency shifts. H-bonds donated by the OH- ion may, or may not, lead to a frequency downshift, depending on the other neighbours present. It is also shown that a model which explicitly takes the field inhomogeneity into account (by a simple polynomial in E(parallel-to), (E(parallel-to))2, E(parallel-to)', (E(parallel-to)')2 and cross-terms, evaluated at two different ''probing sites'' in the ion), manages to represent the OH- frequency shift for an ''arbitrary'' point-charge environment.