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

We have performed ab initio calculations on the model cluster (OH)(3)Ge-O-Ge(OH)(3) in order to refine the relationships between O-17 quadrupolar coupling parameters and the local structure around the bridging oxygen. From these calculations the trend in the bridging oxygen O-17 quadrupolar coupling constant, C-q, and asymmetry parameter, eta(q), with changing Ge-O-Ge angle was found to be similar to previously established trends in silicate clusters with the overall C-q values being systematically increased in magnitude by approximately 3 MHz. Such a shift can be attributed to differences in cation-oxygen distances as well as coordinating cation electronegativity. In addition, we also investigated the effect of changing intratetrahedron bond angles over a range that has been suggested to exist in germanate systems. While such variations in intratetrahedron bond angles lead to a small variation in C-q, eta(q) remains relatively unaffected. In such cases, eta(q), can still be a reliable probe of Ge-O-Ge angle while C-q could serve as a probe of tetrahedron distortions. (C) 2000 Elsevier Science B.V. All rights reserved.