A b i n i t i o quantum chemical study of the dimerization of silicon monoxide

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Abstract
As part of a theoretical program to understand the fundamental molecular constituents of silicate glasses, we have carried out accurate quantum chemical computations to describe silicon monoxide (SiO), its dimer (Si2O2) and the process of dimerization. The dimer Si2O2 is found to be cyclic with alternating silicon and oxygen atoms in the ring. Each silicon atom in the dimer is bonded to two oxygen atoms; It is in a divalent state, in contrast to the tetravalent state of silicon in silicate glasses. The equilibrium SiO bond length in the dimer is computed to be 1.683 Å and OSiO bond angle to be 85.7° as compared with 1.59 Å bond lengths and 109.5° bond angles in typical silicate glasses. The heat of formation of the dimer is predicted to be −43.0 kcal/mole, in excellent agreement with the gas phase expeimental value of −44.6±3.0 kcal/mole. The normal vibrational modes and frequencies of the cyclic dimer have been computed, and are in qualitative agreement with the frequencies of infrared absorptions attributed to the dimer in matrix isolation experiments. We have computed potential energy surfaces for the reaction of two SiO monomers to form the cyclic dimer, and find no barrier to dimerization. The SiO bond length and OSiO bond angle of the dimer are close to those observed for amorphous silicon monoxide by x‐ray diffraction. This suggests that amorphous silicon monoxide also contains divalent silicon.