SEMIEMPIRICAL SCF-MO MODELING OF POLYSILANES USING AM1 AND PM3 METHODS

AM1 and PM3 semiempirical SCF-MO methods have been used to calculate structures and properties of model polysilanes. Bond lengths and molecular orbital energies of poly(hydrosilanes) calculated by both methods are in good agreement with ab initio results, but bond angles deviate slightly. Our studies of molecular geometry and electronic properties of polysilane models show similar behavior for the AM1 and PM3 methods in most cases, but the PM3 method appears to be superior in predicting bandgaps. The PM3 method predicts that the anti conformer of Si4(CH3)10 lies almost-equal-to 1.0 kcal/mol lower in energy than the gauche conformer. Studies evaluating polysilanes bearing substituent groups with different steric requirements and electronic properties (Me, Et, F, Cl, and OCH3) were instructive. Compared to the unsubstituted model, all of the substituted polysilanes have larger Si-Si bond angles, in agreement with experiment. Our calculations suggest that substituents may have a significant effect on Si-Si-Si bond angles pushing them to well above the 120-degrees value estimated for the hexyl derivative from X-ray data. The effect of substitution on electronic properties is predicted to be complicated. For calculation of structure and properties of polysilane models overall, the AM1 and PM3 methods are judged to provide excellent agreement with experimental data and ab initio methods.