Calculated hyperpolarisabilities of substituted phenylsilanes

Abstract

Calculations are reported on the hyperpolarisabilities of aryltrimethylsilanes and diphenyldisilanes at the CNDOVSB level of approximation. A parameterisation scheme has been developed for the SiMe₃ group by relating a combination of experimental data and ab initio π-electron distributions at the aromatic ring to the values of the 4s, 4p and 3d bonding parameters for silicon. The SiMe₃ group is predicted to be a weak π-electron attractor, though overall the group is a net donor. The calculated results show that the presence of a donor in the 4-position of the aromatic ring gives rise to small hyperpolarisabilities, with the SiMe₃ group showing a similar though somewhat smaller effect to the CN group. Calculations on donor–acceptor diphenyldisilanes show that the Si—Si bond is a much less efficient conjugation path than the etheno bridge in related stilbenes. Reasonable correlations are found between the calculations and experimental data where available.