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The origin in deshielding of 29 Si NMR chemical shifts in R 3 Si–X, where X = H, OMe, Cl, OTf, [CH 6 B 11 X 6 ], toluene, and O X (O X = surface oxygen), as well as i Pr 3 Si + and Mes 3 Si + 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 29 Si NMR chemical shift also reproduce experimental trends; i Pr 3 Si–H has the most shielded 29 Si NMR chemical shift and free i Pr 3 Si + or isolable Mes 3 Si + have the most deshielded 29 Si 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 i Pr 3 Si + or Mes 3 Si + , or to the orbital in R 3 Si–X. This trend also applies to surface bound R 3 Si–O X . This model also explains chemical shift trends in recently isolated t Bu 2 SiH 2 + , t BuSiH 2 + , and SiH 3 + that show more shielded 29 Si NMR signals than R 3 Si + species. There is no correlation between isotropic 29 Si NMR chemical shift and charge at silicon.
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