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

Abstract

The structure and dynamics of Cl−(H2O)6 has been studied by ab initio molecular dynamics using the Car–Parrinello approach, and compared to results of ab initio quantum chemical calculations, molecular dynamics based on both polarizable and nonpolarizable empirical potentials, and vibrational spectroscopy. The electronic structure methodology (density functional theory with the gradient-corrected BLYP exchange-correlation functional) used in the Car–Parrinello dynamics has been shown to give good agreement with second-order Møller–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 the chloride anion preferred a location on the surface of the cluster which was significantly extended compared to the minimum energy geometry. The structure of the cluster predicted by the polarizable force field simulation is in agreement with the ab initio simulation, while the nonpolarizable force field calculation was in qualitative disagreement, predicting an interior location for the anion. The time evolution of the electronic structure during the ab initio simulation was analyzed in terms of maximally localized orbitals (Wannier functions). Calculation of the dipole moments from the centers of the Wannier orbitals revealed that the chloride anion is significantly polarized, and that the extent of water polarization depends on location in the cluster, thus underscoring the importance of electronic polarization in halogen ion solvation. The infrared absorption spectrum was computed from the dipole–dipole correlation function, including both nuclear and electronic contributions. Aside from a systematic redshift by 3%–5% in the frequencies, the computed spectrum was in quantitative agreement with vibrational predissociation data on Cl−(H2O)5. Our analysis suggests that accounting for anharmonicity and couplings between modes is more important than the fine tuning of the electronic structure method for the quantitative prediction of hydrogen bond dynamics in aqueous clusters at elevated temperatures.