A theoretical study of the Si-O bond in disiloxane and related molecules

A comparison of semi-empirical (MNDO) and ab initio (GAUSSIAN) calculations for disiloxane and related molecules is given. The STO-3G* basis set well produced the observed geometries of disiloxane (less than SiOSi observed 144 degrees, calculated 140 degrees), dimethoxy-dimethylsilane (less than OSiO obsd tetrahedral, calc 102 degrees), methyl silyl ether (less than COSi obsd 121 degrees, calc 118 degrees) and correctly predicted the planar geometry found for cyclotrisiloxane. In contrast, more complex basis sets (3-21G(*), DZP, TZVP) gave much poorer agreement with the observed geometries. Comparison of the STO-3G* and the STO-3G basis sets demonstrates the necessity of including d-orbitals on the silicon. However, the semi-empirical MNDO program gave, despite the absence of d-orbitals, a better approximation to the molecular geometry than the complex ab initio basis sets. Force field parameters have been calculated for kSiOSi, kOSiO, 0.089 and 0.73 mdyneA/rad2, and the SiOSiO torsion which has a V1 potential of -0.68 kcal/mol. In addition, the HSiOH torsion is shown to have a three-fold potential of 0.78 kcal/mol. These are profoundly different from the analogous carbon-oxygen force constants, demonstrating that C-O parameters cannot be transferred to the corresponding Si-O systems.