Red versus blue shifts in trimethylammoiumhexafluorosilicate

Hartree–Fock and density functional methods have been used to obtain the molecular structure, electron density topography and the vibrational frequencies of trimethylammoiumhexafluorosilicate (TMAHFS). In the isolated molecule the N–H⋯F bond is predicted to be nonlinear with a deviation of 16° from linearity and the SiF62− orients with a dihedral angle of 27° relative to the trimethylammonium cation. Blue shifts of 66 and 45 cm−1 for the Si–F and C–N stretchings, respectively, compared to the respective vibrations in the free anion and cation, predicted from the density functional calculations have been explained from the difference electron density and bond critical points in electron density topography.