Relationships between bridging oxygen 17O quadrupolar coupling parameters and structure in alkali silicates

We have performed ab initio calculations on the model cluster (OH)(3)Si-O-Si(OH)(3) with lithium or sodium cations coordinated to the central oxygen in order to refine the relationships between O-17 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 O-17 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 O-17 quadrupolar coupling constant, C-q, with Si-O-Si angle is systematically shifted to lower magnitudes with increasing number and field strength of coordinating alkali cations and that C-q will be relatively insensitive to variations in the alkali cation-bridging oxygen internuclear vector orientation. The previously established trend in O-17 quadrupolar coupling asymmetry parameter, eta(q), 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 C-q, eta(q) 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 C-q on eta(q) via the Si-O-Si angle can be used to distinguish between bridging oxygen environments with differing number of coordinating alkali cations. (C) 1998 American Institute of Physics.