DIATOMIC-MOLECULES CONTAINING ELECTROPOSITIVE ATOMS FAVOR HIGH-SPIN STATES

The equilibrium geometries and harmonic vibrational frequencies of the low lying electronic states of LiSi, BeSi, BSi, NaSi, MgSi and AlSi were calculated at the SCF/6-31G*, MP2(full)/6-31G*, MP2(full)/6-311+G* levels of theory. For each of these species, the ground states were also studied at the QCISD(T)/6-311+G(2df) level to obtain more accurate data. The bonding in these species is found to involve high-spin ground electronic states (4SIGMA- for LiSi, 3SIGMA- for BeSi, 4SIGMA- for BSi, 4SIGMA- for NaSi, 3SIGMA- for MgSi and 4SIGMA- for AlSi) in which two electrons occupy bonding pi orbitals. In each case, the number of unpaired electrons is larger than that anticipated based on straightforward covalent Si-X sigma-bond formation with no further rearrangement of electronic configuration. We suggest that pi2 configurations are favored over pi1 because of the relatively low sigma-to-pi promotion energies particular to these species and the low Coulomb and favorable exchange energies of the pi2 situation, and over pi3 and pi4 configurations because the latter correlate with excited states of Si and have high Coulomb energies. The pi2 character is then the source of the high-spin nature of the ground states. Bond dissociation energies were also computed and found to be 41.5 kcal/mol (LiSi), 28.8 kcal/mol (BeSi), 71.5 kcal/mol (BSi), 33.3 kcal/mol (NaSi), 21.7 kcal/mol (MgSi) and 56.0 kcal/mol (AlSi), with all data referring to the QCISD-(T)/6-311+G(2df)+ZPE level, within which the calculated bond lengths are 2.383 angstrom (LiSi), 2.137 angstrom (Besi), 1.905 angstrom (BSi), 2.697 angstrom (NaSi), 2.561 angstrom (MgSi) and 2.430 angstrom (AlSi).