Structures and Spectroscopic Properties of F-(H2O) n with n = 1-10 Clusters from a Global Search Based On Density Functional Theory.

Using a genetic algorithm incorporated in density functional theory, we explore the ground state structures of fluoride anion-water clusters F-(H2O) n with n = 1-10. The F-(H2O) n clusters prefer structures in which the F- anion remains at the surface of the structure and coordinates with four water molecules, as the F-(H2O) n clusters have strong F--H2O interactions as well as strong hydrogen bonds between H2O molecules. The strong interaction between the F- anion and adjacent H2O molecule leads to a longer O-H distance in the adjacent molecule than in an individual water molecule. The simulated infrared (IR) spectra of the F-(H2O)1-5 clusters obtained via second-order vibrational perturbation theory (VPT2) and including anharmonic effects reproduce the experimental results quite well. The strong interaction between the F- anion and water molecules results in a large redshift (600-2300 cm-1) of the adjacent O-H stretching mode. Natural bond orbital (NBO) analysis of the lowest-energy structures of the F-(H2O)1-10 clusters illustrates that charge transfer from the lone pair electron orbital of F- to the antibonding orbital of the adjacent O-H is mainly responsible for the strong interaction between the F- anion and water molecules, which leads to distinctly different geometric and vibrational properties compared with neutral water clusters.