Correlation of 29Si nuclear magnetic resonance chemical shifts in silicates with orbital energy differences obtained from X-ray spectra

Abstract

Trends in 29Si nuclear magnetic resonance (NMR) chemical shifts in silicates are shown to arise primarily from changes in the paramagnetic contribution to the shift, σpara. Using the average excitation energy approximation, changes in σpara may be correlated with changes in the separation of high energy occupied and low energy unoccupied molecular orbitals. The required molecular orbital (MO) energy differences may be obtained from x-ray emission and absorption spectra. Shifts in σ to high field as silicate polymerization increases are associated with increased occupied to unoccupied MO energy differences. Substitution of Al for Si (and consequent introduction of electropositive cations, such as Na) reduces the occupied to unoccupied orbital energy gap causing shifts of σ to low field. The observed correlation indicates that 29Si NMR is capable of giving information on silicate electronic structure.