ISSN:
1600-5724
Source:
Crystallography Journals Online : IUCR Backfile Archive 1948-2001
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
Notes:
CNDO/2 molecular-orbital calculations are shown to predict trends in bridging angle, ∠T-Y-T, with reasonable accuracy for gas-phase molecules of the type TX3YTX3, where Y is an anion (= O, OH, S, NH) bridging the tetrahedrally coordinated cations T(= C, Si, Al, Be, B) and where X are the nonbridging anions (= H, F). For example, calculated equilibrium ∠T-O-T are 100, 125 and 150° in CH3OCH3, SiH3OCH3 and SiH3OSiH3, respectively, compared to experimental values of 111, 121 and 144°. The equilibrium ∠Si-O-Si in SiH3OSiH3 is also in close agreement with the average value of this angle (148°) in the silica polymorphs. The SiH3YTH3 molecule is proposed as a model system for studying bridging bond angles in silicates and is found to give bridging-angle trends in reasonable agreement with those observed in Si-containing solids. In particular, calculations on SiH3YSiH3 predict LSi-Y-Si to decrease from 150 to 130 to 109° as Y is changed from O to NH to S, in agreement with observed average angles of 144, 120 and 110°, respectively, in silicates, silicon nitrides and silicon sulfides. For SiH3OSiH3 the equilibrium ∠Si-O-Si is calculated to increase as the Si-O bond distance decreases, at a rate within a factor of two of that observed for the silica polymorphs. Calculations on clusters with Be or B replacing Si invariably lead to predicted equilibrium ∠Si -O-T values from 125 to 105°, in accord with the small angles that tend to be observed for such bridge bonds in minerals. Since Si(3d) orbitals are excluded in these calculations, it is apparent that the observed angular trends can be explained without invoking d-p π bonding. Inspection of one and two-center energy components of the CNDO/2 total energy also shows that the form of the Si-T interaction is not that of a nonbonded repulsion.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1107/S056773947800090X
Permalink
