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The effect of polarization functions for ab initio molecular orbital calculations at the 3-21G* level has been studied for disiloxane. Calculated molecular geometry, dipole moment, and the linearization barrier variation were analyzed for different uncontracted polarization functions. It was concluded that variation of the polarization function on oxygen has only a minor influence on the molecular properties of disiloxane, but its presence is required to obtain a bent geometry for the disiloxane bond. The calculated molecular properties of disiloxane are greatly influenced when the polarization function on silicon is varied. Two different values (0.3 and 0.9) for the exponent of the silicon polarization function provide results comparable to the experimental values for disiloxane. The only significant differences between the results obtained from ab initio calculations using the two polarization functions are in net atomic charges. The uncontracted polarization function of silicon with a value of 0.3 for its exponent is transferable to other organosilicon compounds. Calculated molecular geometries of flexible or rigid structures are in very good agreement with the experimental values.

Molecular structures of 26 organosilicon compounds have been optimized using ab initio calculations at the 3-21G* (modified) level. From these optimized structures, the internal coordinates have been deformed and the variation of the total molecular energy has been studied. Parameters for stretching and bending deformations are reported herein. The bending potential for the Si-0-Si bond which has an unusual flexibility is also included. Nonbonding interactions are described in terms of steric and electrostatic potentials. For systems which do not include bond resonance effects, torsional behavior is well described by steric potentials with van der Waals radii 20% larger than the previous values and simple electrostatic potential (monopole-monopole) with net atomic charges obtained from ab initio or Extended Huckel Theory calculations. The nonbonding potentials, as defined in this study, have an advantage in that they allow for the computation of torsional barriers without torsional potentials, in the case of single bonds where no additional electronic effects interfere. As an example, it is shown that no torsional potentials are necessary to estimate the torsional barriers in the case of ethane. The newly defined potentials are used to study the torsional barrier in hexamethyldisiloxane and the conformation of octamethylcyclotetrasiloxane (DJ. The most stable calculated conformation of D4, coincides with the experimentally determined structure. This study shows that the most stable conformation is determined by the steric repulsion of methyl groups.

Immobilized lipase B from Candida antarctica (Novozyme 435) catalyzed the regioselective formation of ester bonds between organosilicon carboxylic diacids and a C1-O-alkylated sugar under mild reaction conditions (i.e., low temperature, neutral pH, solventless). Specifically, the acid-functionalized organosilicones reacted with the primary hydroxyl group at the C6 position of alpha,beta-ethyl glucoside during the regioselective esterification. The pure organosilicon-sugar conjugates were prepared in a one-step reaction without protection-deprotection steps and without activation of the acid groups with the integrity of the siloxane bonds. [reaction: see text]