Frequency shifts in the hydrogen-bonded OH stretch in halide-water clusters. The importance of charge transfer

We investigate the nature of the hydrogen bonding in the gas-phase halide-water clusters (X-... H2O), with special emphasis on how the hydrogen bonding affects the frequency of the hydrogen-bended OFT stretch. We present two models for describing the electronic structure of the hydrogen bond. The first model (non-charge-transfer, or non-CT) includes only electrostatic interactions between the halide ion and the water molecule. The second is a two-valence-bond (VB) state model in which the first VB state has the charge character X-... H2O and the second is a charge-transfer VB state with electronic structure XH ... OH-. We find that the non-CT model is inadequate for describing the frequency shifts in the hydrogen-bended OH stretch for the halide-water clusters as compared with both experimental and ab initio results. Further, analysis of the charge distributions of the clusters obtained from ab initio calculations indicates significant contribution of charge transfer in the electronic structure. This analysis also allows the distinction to be made between polarization and charge-transfer effects. The two-VB state model is used to provide an estimate of the charge-transfer contribution, which increases in the order I < Br < Cl < F, a result in contrast with the order one would predict solely on the basis of the electron affinities. The ordering is due to the more dominant effects of the homolytic bond dissociation energies in the HX series and the smaller O ... X distances for the smaller ions.