Relationships between bridging oxygen O17 quadrupolar coupling parameters and structure in alkali silicates

Abstract

We have performed ab initio calculations on the model cluster (OH)3Si–O–Si(OH)3 with lithium or sodium cations coordinated to the central oxygen in order to refine the relationships between O17 quadrupolar coupling parameters and the local structure around the bridging oxygen in the case of alkali silicates. We have also used a point charge model to derive approximate expressions to describe the dependence of the O17 electric field gradient tensor on the orientation of the alkali cation-bridging oxygen internuclear vector(s). From these calculations we predict that the previously established trend in O17 quadrupolar coupling constant, Cq, with Si–O–Si angle is systematically shifted to lower magnitudes with increasing number and field strength of coordinating alkali cations and that Cq will be relatively insensitive to variations in the alkali cation-bridging oxygen internuclear vector orientation. The previously established trend in O17 quadrupolar coupling asymmetry parameter, ηq, with Si–O–Si angle is systematically shifted to higher values by the presence of one coordinating alkali cation, and only slightly shifted to higher values by the presence of two coordinating alkali cations. As with the quadrupolar coupling constant, the magnitude of the shift in asymmetry parameter increases with increasing field strength of the coordinating alkali cation(s). In contrast to Cq, ηq is additionally dependent on variations in the alkali cation-bridging oxygen internuclear vector orientation. With two or more coordinating alkali cations, however, the magnitude of these variations are reduced, and for certain alkali cation configurations are eliminated. Finally, the parametric dependence of Cq on ηq via the Si–O–Si angle can be used to distinguish between bridging oxygen environments with differing number of coordinating alkali cations.