Solid-state NMR spectroscopy is a powerful technique to determine structure and to assess bonding. In solution NMR, rapid tumbling results in an averaging of chemical shift tensors, and only the isotropic chemical shift can be measured. In rigid solids, chemical shift tensors can be measured directly. The magnitude of the chemical shift tensor relates near frontier filled and empty orbitals, which correlates electronic structure to this measurement. DFT methods model these interactions, and allows these orbitals to be “read out” to determine how various bonding and antibonding combinations contribute to the chemical shift tensor. In tetrahedral iPr3Si-X, the bonding σ(Si-C) couple to antibonding σ*(Si-X) orbitals, and as a consequence the energy gap between σ(Si-C) and σ*(Si-X) orbitals correlates to the deshielding of 29Si. In conjunction with the 29Si NMR chemical shift, the deprotonation energy calculations of HX and the fluoride ion affinity calculations of [iPr3Si][X] with tBu-F as a reference molecule provides a benchmark for characterizing the Si-X interaction in covalently bound iPr3Si-X and ion-paired [iPr 3Si][X]. This method is applicable to the interactions of the iPr 3Si+ fragment on solid oxides such as silica, forming covalently bound iPr3Si-OSi≡ and sulfated zirconia, forming ion-paired [iPr3Si][SZO].

Origin of the 29Si NMR chemical shift in iPr3Si-X and its relationship to the formation of silylium like ions on solid oxides

ACS Fall 2021

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