Understanding substituent effects on 29Si chemical shifts and bonding in disilynes.: A quantum-chemical analysis

The Si-29 chemical shifts in substituted disilynes have been analyzed by quantum-chemical studies using a detailed breakdown of paramagnetic contributions into couplings of occupied and virtual canonical molecular orbitals. The results give indications of substituent effects on shielding and confirm the importance of energy denominators in the equation for sigma(P) in symmetrical substituted disilynes (H3SiSiSiSiH3 and H3CSiSiCH3) on their chemical shifts. In the unsymmetrical substituted disilyne H3CSiSiSiH3 the energy denominators are identical for both sites and thus cannot explain the widely different chemical shifts. It turns out that the asymmetric charge distribution (reflected in both occupied and virtual MOs and sampled by the PSO matrix elements) allows more efficient couplings at the methyl-substituted silicon side. Furthermore, shielding values in disilynes depend strongly on the size of the transbent angle phi, as indicated clearly by analyses of the Si-29 chemical shifts in the model H3SiSiSiSiH3 at selected values of phi. Small changes in phi influence the Si-29 shielding values appreciably. At lower values of T, couplings between bonding and virtual MOs exhibit stronger deshielding caused by significantly lower energy denominators.