Application of electronic and ir spectroscopy to the determination of σp-, σ+p- and σoR-constants of some organosilicon and organogermanium substituents

The σ_p- and σ⁺_p- constants of some silicon- and germanium-containing substituents have been determined from electronic absorption spectroscopy data of charge-transfer (CT) complexes of phenylgermanes, benzylsilanes and benzylgermanes with tetracyanoethylene (TCNE). Resonance constants of corresponding substituents have been determined from the integrated intensity values of aromatic ring stretching modes (IR spectroscope data) of benzylsilanes and benzylgermanes. It is shown that the effect of dπpπ interaction in phenylgermanes remains practically unchanged in the course of the transition from the ground state to the CT state. The high negative values of σ⁺_p-constants for benzylorganosilicon and benzylorganogermanium substituents indicate the presence of considerable σ, π-conjugation in the CT state of benzylsilanes and benzylgermanes. This effect is also established for the ground state of the above mentioned compounds from their σ^o_R-values. The effect of σ,π-conjugation is displayed in greater degree in the ground state as well as in the CT state in benzylgermanes as compared with benzylsilanes.