An ab initio study of microsolvation of LiF in water: structures and properties of LiF-Wn, n = 1-9 complexes.

Geometries of clusters of water molecules (W(n)) and those of the LiF-W(n) (n = 1-9) complexes were optimized using the B3LYP/6-31+G** method. Geometries of the complexes up to n = 7 were also optimized using the MP2/6-31+G** approach. Only one structure of each of W(n), n = 1-5 was considered to generate the complexes with LiF while two structures, one of a cage type and the other of a prism type, were considered for n = 6-9. The LiF-W(2) complex is found to be most stable among the various complexes. The LiF-W(6) complex, where W(6) is of a cage type, is predicted to be substantially less stable than that where W(6) is of a prism type. Certain existing ambiguities regarding the most stable structures of the LiF-W(n) (n = 1-3) complexes have been resolved. The LiF molecule seems to divide the W(n) clusters in the LiF-W(n) (n = 3-6) complexes into different fragments where at least one W(2)-like fragment is present. In LiF-W(6) (cage), there is one W(2)-like fragment while in LiF-W(6) (prism), there are three W(2)-like fragments. The LiF bond length is substantially increased in going from the gas phase to the different complexes, this increase being most prominent in LiF-W(6), where W(6) is of the cage or prism type. The LiF molecule, however, does not acquire the ionic structure Li(+)F(-) in any of the complexes studied here. An appreciable amount of electronic charge is transferred from LiF to the water molecules involved in the different complexes. In this process, the Li atom gains electronic charge in some cases, while the F atom considered separately, as well as the Li and F atoms taken together, lose the same in most cases.