The chemical shift tensor of silylenes

Abstract

The 29Si-NMR chemical shift tensors of 21 simple singlet silylenes, R 2Si:, 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 δ 11 in the plane of the central R 2Si unit and perpendicular to its C 2 axis. Very large substituent effects on δ 29Si are found, i.e., while for H 2Si: δ 29Si=772 is predicted, the divalent silicon in F 2Si is strongly shielded ( δ 29Si=−9.3). On the other hand for (H 3Si) 2Si: δ 29Si=1223 is computed. This exceptional substituent effects are a direct consequence of large changes of the dominant eigenvalue δ 11. The theoretical analysis reveals, that δ 11 in silylenes is determined by the energy difference between its lowest excited singlet state, S 1, and the S 0 ground state. Therefore, a direct relation between the experimentally easy accessible λ max of silylenes and the isotropic 29Si-NMR chemical shift exists. This correlation can be used to facilitate the NMR characterization of highly substituted silylenes.