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

Abstract

The origin in deshielding of ²⁹Si NMR chemical shifts in R₃Si–X, where X = H, OMe, Cl, OTf, [CH₆B₁₁X₆], toluene, and O_X (O_X = 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 (σ^p) 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–O_X. 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.