The chemical shift tensor of silylenes

The 29Si-NMR chemical shift tensors of 21 simple singlet silylenes, R2Si:, are studied at the GIAO/MP2/6-311+G(2df,p)//B3LYP/6-311G(d,p) level of theory. The NMR chemical shielding tensor for all silylenes is predicted to be highly anisotropic, with a very large paramagnetic eigenvalue δ11 in the plane of the central R2Si unit and perpendicular to its C2 axis. Very large substituent effects on δ29Si are found, i.e., while for H2Si: δ29Si=772 is predicted, the divalent silicon in F2Si is strongly shielded (δ29Si=−9.3). On the other hand for (H3Si)2Si: δ29Si=1223 is computed. This exceptional substituent effects are a direct consequence of large changes of the dominant eigenvalue δ11. The theoretical analysis reveals, that δ11 in silylenes is determined by the energy difference between its lowest excited singlet state, S1, and the S0 ground state. Therefore, a direct relation between the experimentally easy accessible λmax of silylenes and the isotropic 29Si-NMR chemical shift exists. This correlation can be used to facilitate the NMR characterization of highly substituted silylenes.