The five favored backbone conformations of n-Si4Et10:: cisoid, gauche, ortho, deviant, and transoid

Low-energy conformations of n-Si4Et10 (1) and a few simpler medals (2-8) have been examined by geometry optimization at the HF/6-31G* level, and MP2/6-31G* single-point energies have been obtained. Five clans of favored backbone conformations of 1, with SiSiSiSi dihedral angles of similar to+/-40 degrees (C+/-, cisoid), similar to+/-60 degrees (G(+/-), gauche), similar to+/-90 degrees (O+/-, ortho), similar to+/-145 degrees (D+/- deviant), and similar to+/-165 degrees (T+/- transoid), have been identified. Each contains numerous families characterized by terminal group Et3Si conformations, with ag(+)g(+) and ag(+)g(-) particularly advantageous. Each family contains a large number of conformers differing by internal Et2Si conformations, with g(+/-)g(+/-)/g(+/-)g(+/-) and ag(+/-)/ag(+/-) usually energetically the lowest. A simple rationalization is provided for the existence of the C+/- and D+/- conformations which do not occur in the permethglated oligosilanes, and implications for the conformational analysis of peralkylated polysilanes are noted. (C) 2000 Elsevier Science B.V. All rights reserved.