We have used ab initio (HF/3-21G(*) and HF/6-31+G * ) single point energy calculations on model structures to investigate the nature of methyl C-H to anion interactions in tetramethylammonium tetrahydroborate. The preferred structure has four cations about the anion in a D 2 d arrangement, each of which forms a CHHB dihydrogen bond to anion. This arrangement is in good accordance with previous infrared spectral studies. The alternative D 2 d arrangement with cation C-H directed to faces of the BH 4 - tetrahedron is 6kcalmol - 1 higher in energy than the dihydrogen bonded model.