Surface solvation of halogen anions in water clusters: An ab initio molecular dynamics study of the Cl-(H2O)(6) complex

The structure and dynamics of Cl-(H2O)(6) has been studied by ab initio mol ecular dynamics using the Car-Parrinello approach, and compared to results of ab initio quantum chemical calculations, molecular dynamics based on bot h polarizable and nonpolarizable empirical potentials, and vibrational spec troscopy. The electronic structure methodology (density functional theory w ith the gradient-corrected BLYP exchange-correlation functional) used in th e Car-Parrinello dynamics has been shown to give good agreement with second -order Moller-Plesset results for the structures and energies of Cl-(H2O)(n ), n=1-4, clusters. The configurational sampling during the 5 ps ab initio molecular dynamics simulation at 250 K was sufficient to demonstrate that t he chloride anion preferred a location on the surface of the cluster which was significantly extended compared to the minimum energy geometry. The str ucture of the cluster predicted by the polarizable force field simulation i s in agreement with the ab initio simulation, while the nonpolarizable forc e field calculation was in qualitative disagreement, predicting an interior location for the anion. The time evolution of the electronic structure dur ing the ab initio simulation was analyzed in terms of maximally localized o rbitals (Wannier functions). Calculation of the dipole moments from the cen ters of the Wannier orbitals revealed that the chloride anion is significan tly polarized, and that the extent of water polarization depends on locatio n in the cluster, thus underscoring the importance of electronic polarizati on in halogen ion solvation. The infrared absorption spectrum was computed from the dipole-dipole correlation function, including both nuclear and ele ctronic contributions. Aside from a systematic redshift by 3%-5% in the fre quencies, the computed spectrum was in quantitative agreement with vibratio nal predissociation data on Cl-(H2O)(5). Our analysis suggests that account ing for anharmonicity and couplings between modes is more important than th e fine tuning of the electronic structure method for the quantitative predi ction of hydrogen bond dynamics in aqueous clusters at elevated temperature s. (C) 2001 American Institute of Physics.