The anomeric effect at silicon

Abstract

The anomeric effect at silicon has been studied by ab initio calculations. The geometries and energies of H 2Si(XH) 2, X = O, S were optimized with the basis sets 3-21G, 3-21G (*), and 6-31G *. Single point MP3/6-31G * //6-31G * calculations were also carried out. H 2Si(XCH 3) 2, X = O, S, were studied with the 3-21G basis set. All compounds are most stable in the gauche, gauche conformation, pointing to the operation of an anomeric effect. The total anomeric interactions given by the energies of the equations: H 2Si(XH) 2 + SiH 4 → 2H 3SiXH are (MP3/6-31G *): 8.6 and 2.2 kcal/mol for X = O and X = S, respectively. Rotation barriers on going from the ( g, g), to the ( g, a) and to the ( a, a) conformations are (MP3/6-31G *, kcal/mol): 2.5 and 3.8 in H 2Si(OH) 2 and 2.1 and 3.2 in H 2Si(SH) 2. Thus, the anomeric effect in H 2Si(OH) 2 is large, although smaller than in H 2C(OH) 2. In H 2Si(SH) 2 and H 2C(SH) 2 the anomeric effects are comparable, and both relatively small. The anomeric effect is predicted to be important in determining the conformations of compounds with silicon bonded to 2 oxygens such as R 2Si(OR′) 2, disilaoxiranes, and related molecules.