Conformations of oligosilanes with ethyl and methyl substituents

Low-energy conformations of 2,3-diethyloctamrthyl-m-tetrasilane Me3SiSiMeEtSiMeEtSiMe3 (R,R = 1, R,S = 2) and 2,2,3,3-tetraethylhexamethyl-m-tetrasilane Me3SiSiEt2SiEt2SiMe3 (3) have been examined by geometry optimization at the MM3, HF/6-31G* and MP2/6-31G* (1 and 2) and the MM3 and HF/3-21G* (3) levels. Single-point MP2/6-31G*//HF/3-21G* energies of conformers of 3 have also been obtained. Six dans of favored backbone conformations of 3 have been identified, with SiSiSiSi dihedral angles omega = similar to +/- 40 degrees (C+/-, cisoid), similar to +/- 60 degrees (G(+/-) gauche), similar to +/- 90 degrees (O+/-, ortho), similar to +/- 150 degrees (D+/-, deviant), similar to +/- 165 degrees (T+/-, transoid), and similar to 180 degrees (A, anti). Each dan contains many conformers differing in the arrangement of substituent ethyl groups; each arrangement defines a family. A family generally contains live to eight members belonging to different clans. The ethyl group arrangements that produce the A, D+/- and C+/- backbone conformations, which do not occur in permethylated oligosilanes, have been investigated in greatest detail. Implications for the conformational analysis of general peralkylated polysilanes are noted. (C) 2000 Elsevier Science B.V. All rights reserved.