Group IIIa Hydrides XH2 and XH2- (X = B, Al, Ga): Electron Affinities and Singlet-Triplet Splittings Revisited

Geometries, electron affinities (EA) and singlet-triplet (S-T) splittings of XH 2 /XH 2 - molecules (X = B, Al, Ga) are calculated by coupled-cluster methods, using the sequence of basis sets. The EA values and S-T splittings for aluminium and gallium dihydrides are an order of magnitude larger (in absolute values) than those for boron. For boron and aluminium dihydrides, two types of extrapolations towards complete basis set limit are applied, leading to EA = 0.24 eV, ST = -0.01 eV (BH 2 ), and EA = 1.10 eV, ST = -0.62 eV. The best calculated values for gallium dihydrides are EA = 1.13 eV and ST = -0.74 eV. All three S-T splittings favour singlet as the ground state, although the S-T splittings of BH 2 - is exceptionally small. In addition, vertical electron affinities and vertical electron detachments are reported for these molecules.