The chemical shift tensor of silylenes

The Si-29-NMR chemical shift tensors of 21 simple singlet silylenes, R2Si:, are studied at the GIAO/MP2/6-311 +G(2dfp)//B3LYP/ 6-311 G(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 delta(11), in the plane of the central R2Si unit and perpendicular to its C-2 axis. Very large substituent effects on delta(29)Si are found, i.e., while for H,Si: delta(29)Si = 772 is predicted, the divalent silicon in F,Si is strongly shielded (delta(29)Si = - 9.3). On the other hand for (H3Si)(2)Si: delta(29)Si = 1223 is computed. This exceptional substituent effects are a direct consequence of large changes of the dominant eigenvalue delta(11). The theoretical analysis reveals, that delta(11) in silylenes is determined by the energy difference between its lowest excited singlet state, SI, and the So ground state. Therefore, a direct relation between the experimentally easy accessible lambda(max) of silylenes and the isotropic Si-29-NMR chemical shift exists. This correlation can be used to facilitate the NMR characterization of highly substituted silylenes. (C) 2003 Elsevier B.V. All rights reserved.