Static electronic and vibrational polarizabilities of poly(dimethylsilane) chains

The static electronic and vibrational contributions to the polarizability of poly(dimethylsilane) chains have been computed ab initio within the coupled Hartree-Fock and double harmonic schemes, respectively. Both vibrational and electronic components present a supralinear increase with chain length that can be attributed to sigma-conjugation. With respect to the polysilane chains which adopt, in their equilibrium structure, an all-trans backbone conformation, the poly(dimethylsilane) chains present a helical conformation characterized by a 165 degrees torsion angle. On one hand, the electronic component is larger than in polysilane chains due to the intrinsic contribution of the methyl groups and their inductive effects which decrease the band gap, while, on the other hand, its vibrational counterpart, which is mainly due to wigging motions,remains nearly unchanged by the methyl substitution.