Ab initio study on the MBF 4 and MAlF 4 molecules

Abstract

The optimized geometries and vibrational frequencies of the T d symmetry anions BF 4 −, AlF 4 −, BH 4 − and AlH 4 − and of the D 3h, symmetry ions BeF 3 −, MgF 3 −, BeH 3 − and MgH 3 − are reported. The interaction of these anions with the Li + and Na + cations was studied at the HF-SCF and MP2 levels of theory. Particular attention was focused on the bidentate ( C 2v) and tridentate ( C 3v) structures of MBF 4 and MAlF 4 ion pairs (M = Li + and Na +). LiBF 4 and NaBF 4 prefer tridentate binding; however, the stability of the bidentate structure of LiBF 4 is comparable to that of the tridentate isomer. LiAlF 4 and NaAlF 4 prefer bidentate coordination; however, the stability of the tridentate isomers, and particularly that of the tridentate structure of NaAlF 4, is high. Tridentate and bidentate MBH 4 and MAlH 4 ion pairs are both stable species; tridentate LiBH 4 and NaBH 4 are more stable than the respective bidentate forms and the stability order is reversed for the MAlH 4 species. This work includes FT-IR matrix isolation measurements on some MBF 4 and MAlF 4 molecules.