Conformational Energies and Unperturbed Chain Dimensions of Polysilane and Poly(dimethylsilylene)

摘要

Conformational energy calculations using molecular mechanics (MM) methods were carried out on segments of polysilane -SiH2- and poly(dimethylsilylene) -Si(CH3)2-, and the results were compared with the published full-relaxation calculations of Damewood and West. The three MM methods compared (designated NR for no relaxation, PR for partial relaxation, and FR for full relaxation) differ in the extent to which they permit molecular relaxation (deformation) in order to achieve energy minimization. All three MM methods show polysilane to prefer G+G+ states over the corresponding TT state by 0.5-0.7 kcal/mol in contrast to the analogous n-alkanes, which prefer TT over G+G+ by ca. 1.0 kcal/mol. Even G+G+ states, commonly found to be prohibitively repulsive for the n-alkanes and most other polymers, were preferred over the TT states by 0.4 kcal/mol. Nearly all regions of configurational space were within 2 kcal/mol of the minima, indicating considerable chain flexibility. For poly(dimethylsilylene) the three MM methods differ in terms predicted conformational preferences. While the NR calculations predict for TT and TG+ states over the corresponding G+G+ states by ca. 4 kcal/mol, the FR calculations in contrast indicate preferences for G + or - G + or - states over TT and TG + or - by ca.0.9 kcal/mol, with the PR calculations yielding results intermediate between these two.