O Triclusters Revisited: Classical MD and Quantum Cluster Results for Glasses of Composition (Al2O3)2(SiO2)
- 13 January 2005
- journal article
- research article
- Published by American Chemical Society (ACS) in The Journal of Physical Chemistry B
- Vol. 109 (5), 1794-1797
- https://doi.org/10.1021/jp0454873
Abstract
The (17)O NMR spectrum of CaAl(2)Si(2)O(8) glass shows two types of O sites that are not present in the crystalline material. One of these, with (17)O NMR parameters C(Q) = 2.3 MHz and delta = +20 ppm, has been assigned to a "tricluster" O, a local geometry in which the O is coordinated to three tetrahedrally coordinated atoms, either Al or Si. For crystalline CaAl(4)O(7), a tricluster site (with three Al linkages to O, i.e., OAl(3)) 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 delta 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 CaAl(2)Si(2)O(8) 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 Al(2)O(2), 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)(Al(2)O(3))(0.12)(SiO(2))(0.67), show a small percentage of O triclusters, but none in two-membered rings of the Al(2)O(2) type, and the calculated C(Q) values for the triclusters that do exist are higher than seen in the original experiments on CaAl(2)Si(2)O(8) 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, (Al(2)O(3))2(SiO(2)), gave a predominance of O triclusters within two-membered rings, with structures much like those seen in the alumoxanes. We have now calculated (17)O 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 CaAl(2)Si(2)O(8) glass (around 2.3-2.6 MHz) when the tricluster O is surrounded by three Al, two of which are part of an Al(2)O(2) ring. This supports the experimentalists' contention that such tricluster O species do exist and have been seen by (17)O NMR.Keywords
This publication has 20 references indexed in Scilit:
- Ab Initio Calculations of NMR Parameters of Highly Coordinated Oxygen Sites in AluminosilicatesThe Journal of Physical Chemistry B, 2004
- NMR Characteristics of Possible Oxygen Sites in Aluminosilicate Glasses and Melts: An ab Initio StudyThe Journal of Physical Chemistry B, 1999
- Static and dynamic properties of a viscous silica meltPhysical Review B, 1999
- Zeolites versus aluminosilicate clusters: the validity of a local descriptionJournal of the American Chemical Society, 1991
- Efficient implementation of the gauge-independent atomic orbital method for NMR chemical shift calculationsJournal of the American Chemical Society, 1990
- Structure of Sodium Aluminosilicate GlassesJournal of the American Ceramic Society, 1990
- Cleavage of poly(diorganosiloxanes) by trimethylaluminumOrganometallics, 1990
- Force fields for silicas and aluminophosphates based onab initiocalculationsPhysical Review Letters, 1990
- Bond-valence parameters obtained from a systematic analysis of the Inorganic Crystal Structure DatabaseActa crystallographica Section B, Structural science, crystal engineering and materials, 1985
- The formation and structure of the novel aluminoxane anion [Me2AlO·AlMe3]22–Journal of the Chemical Society, Chemical Communications, 1983