The hydration structures of F− and Cl− investigated by ab initio QM/MM molecular dynamics simulations

Abstract

Combined ab initio quantum mechanical/molecular mechanical (QM/MM) molecular dynamics simulations have been performed to investigate the hydration shell properties of F⁻ and Cl⁻. The chemically most relevant region, the hydration sphere of the anions, was treated by Born–Oppenheimer ab initio quantum mechanics using D95V+, 6-31+G and D95V++ basis sets for F⁻, Cl⁻ and water, respectively, while the remaining part was described by classical pair potentials. The QM/MM simulations have predicted average coordination numbers of 4.6 ± 0.2 for F⁻ and 5.6 ± 0.1 for Cl⁻, in contrast to the corresponding values of 5.8 ± 0.1 and 5.9 ± 0.1 resulting from classical pair potential simulations. Within the first hydration shell of F⁻, the QM/MM results indicate more flexibility of the hydration complex in which the F⁻⋯H–O bond appears to be linear. For the case of Cl⁻, a combination of linear and bridged forms, together with a competition between the solvation of the ion and hydrogen bonding among water molecules, are observed.