Conformations of oligosilanes with ethyl and methyl substituents

Low-energy conformations of 2,3-diethyloctamrthyl-m-tetrasilane Me3SiSiMeEt SiMeEtSiMe3 (R,R = 1, R,S = 2) and 2,2,3,3-tetraethylhexamethyl-m-tetrasila ne 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) l evels. 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 be en identified, with SiSiSiSi dihedral angles omega = similar to +/- 40 degr ees (C+/-, cisoid), similar to +/- 60 degrees (G(+/-) gauche), similar to /- 90 degrees (O+/-, ortho), similar to +/- 150 degrees (D+/-, deviant), si milar to +/- 165 degrees (T+/-, transoid), and similar to 180 degrees (A, a nti). Each dan contains many conformers differing in the arrangement of sub stituent ethyl groups; each arrangement defines a family. A family generall y contains live to eight members belonging to different clans. The ethyl gr oup 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 p eralkylated polysilanes are noted. (C) 2000 Elsevier Science B.V. All right s reserved.