Structures and Spectroscopic Properties of F^–(H₂O)_<i>n</i> with <i>n</i> = 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)_<i>n</i> with <i>n</i> = 1–10. The F^–(H₂O)_<i>n</i> 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)_<i>n</i> 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