O triclusters revisited:: Classical MD and quantum cluster results for glasses of composition (Al2O3)2(SiO2)

The O-17 NMR spectrum of CaAl2Si2O8 glass shows two types of O sites that are not present in the crystalline material. One of these, with O-17 NMR parameters C-Q = 2.3 MHz and delta = +20 ppm, has been assigned to a "tricluster" O, a local geometry in which the 0 is coordinated to three tetrahedrally coordinated atoms, either Al or Si. For crystalline CaAl4O7, a tricluster site (with three Al linkages to O, i.e., OAl3) has been characterized experimentally, with a C-Q of 2.5 MHz and a delta of about +40 ppm. Thus, a C-Q value of 2.5 MHz or less seems to be a characteristic of such sites, although they may show a range of 6 values. However, several different quantum chemical cluster calculations employing energy-optimized geometries for various tricluster species have given C-Q values considerably larger than that seen experimentally in the CaAl2Si2O8 glass (with minimum C-Q values of 3.0 MHz even for all Al species). We have recently shown that for edge-sharing geometries, in which the tricluster O atoms participate in "two-membered rings" of composition Al2O2, the calculated C-Q values are considerably lower, in the range identified in the glass. However, such two-membered ring geometries had been observed only in crystalline inorganic alumoxanes. Ab initio MD calculations on related compositions, such as the calcium aluminosilicate, CAS, (CaO)(0.21)(Al2O3)(0.12)(SiO2)(0.67), show a small percentage of O triclusters, but none in two-membered rings of the Al2O2 type, and the calculated C-Q values for the triclusters that do exist are higher than seen in the original experiments on CaAl2Si2O8 glass and not significantly different from those for two-coordinate O in Si-O-Al sites. However, a classical MD simulation of the structure of glassy aluminum silicate AS2, (Al2O3)2(SiO2), gave a predominance of O triclusters within two-membered rings, with structures much like those seen in the alumoxanes. We have now calculated O-17 nuclear quadrupole coupling constants and NMR shielding values for clusters extracted from these simulations, using standard quantum chemical methods. The calculated C-Q values for these O triclusters are now in the range observed experimentally in the CaAl2Si2O8 glass (around 2.3-2.6 MHz) when the tricluster O is surrounded by three Al, two of which are part of an Al2O2 ring. This supports the experimentalists' contention that such tricluster O species do exist and have been seen by O-17 NMR.