Oxygen-17 NMR Study of Bonding in Silicates: The d-Orbital Controversy

Bonding models for silicates are assessed in relation to the local environment of oxygen, as determined by analysis of the oxygen-17 nuclear quadrupole coupling constants (NQCC), using Townes-Dailey methods. The experimental NQCC of the silica polymorph low cristobalite is indicative of covalent charge transfer from the oxygen lone pairs to silicon and is consistent with Pauling’s (d-p) π-bonding model. Bonding models for both hybridized and unhybridized oxygen which exclude lone pair charge transfer give poor agreement with the experimental results. The oxygen-17 NQCC of the bridging oxygen of diopside is shown to be in agreement with McDonald’s (d-p) π-bonding hypothesis. Calcium coordination to the diopside bridging oxygen is consistent with calcium acting as a charge acceptor. The structural significance of the (d-p) π-bonding effect is discussed and related to the expected variation in the NQCC as a function of bridging bond angle. Trends in the NQCC of oxygen bonded to other elements are discussed and related to bond ionicities. The use of Pauling ionicities in conjunction with the Townes-Dailey model gives good agreement with the experimental NQCC results for a variety of well-defined oxide and silicate systems and further supports the (d-p) π-bonding hypothesis in silicates.