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

The Si-29 chemical shift tensors in both symmetrically and unsymmetrically substituted disilenes have been analyzed by quantum chemical calculations. A detailed breakdown of the paramagnetic contributions to nuclear shielding into individual sum-over-states terms allows new insights into the interrelations between bonding and chemical shifts in these unsaturated compounds. While the substituent effects in symmetrically substituted systems may be understood to a large extent from the energy denominators of the perturbation expression, the very different shifts for the two silicon nuclei in unsymmetrically substituted systems are mainly due to the spatial extent and orientation of the occupied and unoccupied molecular orbitals, as sampled by the matrix elements of nuclear magnetic moments at both silicon sites. The unequal shifts may thus be understood and could serve as a diagnostic tool to distinguish between symmetrically and unsymmetrically substituted disilenes. Furthermore, unequal charge transfer and an alternating charge distribution can rationalize unexpectedly larger pyramidalization of the silyl-substituted silicon center in unsymmetrically substituted systems.