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Origin of the 29Si NMR chemical shift in R3Si-X and relationship to the formation of silylium (R3Si+) ions.

The origin in deshielding of 29Si NMR chemical shifts in R3Si-X, where X = H, OMe, Cl, OTf, [CH6B11X6], toluene, and OX (OX = surface oxygen), as well as iPr3Si+ and Mes3Si+ were studied using DFT methods. At the M06-L/6-31G(d,p) level of theory the geometry optimized structures agree well with those obtained experimentally. The trends in 29Si NMR chemical shift also reproduce experimental trends; iPr3Si-H has the most shielded 29Si NMR chemical shift and free iPr3Si+ or isolable Mes3Si+ have the most deshielded 29Si NMR chemical shift. Natural localized molecular orbital (NLMO) analysis of the contributions to paramagnetic shielding (σp) in these compounds shows that Si-R (R = alkyl, H) bonding orbitals are the major contributors to deshielding in this series. The Si-R bonding orbitals are coupled to the empty p-orbital in iPr3Si+ or Mes3Si+, or to the orbital in R3Si-X. This trend also applies to surface bound R3Si-OX. This model also explains chemical shift trends in recently isolated tBu2SiH2+, tBuSiH2+, and SiH3+ that show more shielded 29Si NMR signals than R3Si+ species. There is no correlation between isotropic 29Si NMR chemical shift and charge at silicon.