THE SILICA POLYMORPH COESITE: AN EXPLORATION OF THE ELECTRON DENSITY DISTRIBUTION

A multipole representation of the experimental electron density distribution for coesite, using Hirshfeld-type radial functions, has been generated with single crystal X-ray diffraction data recorded at 100 K. Deformation density maps display banana shaped isosurfaces in the lone pair regions of each the oxygen atoms as well as teardrop shaped ones along the SiO bond vectors. Laplacian maps display belt-shaped isosurfaces, centered near the apices of the bent SiOSi angles, that wrap about half way around the oxide anions, with a ring torus-shaped isosurface surrounding O1, the anion involved in the straight angle. An analysis of the Laplacian revealed that the (3,-3) critical point associated with the anions involved in the bent angles generally are associated with larger maxima than those involved in the straight angle, evidence that the electron density is more locally concentrated on the anions involved in the bent angles. As such, these anions are more susceptible to electrophilic attack by hydrogen, a feature that provides an experimental basis for why hydrogen in H-bearing coesite avoids O1 and docks in the vicinity of oxide anions involved in the bent angles. The bond critical point properties of the experimental multipole representation of the electron density distribution together with those for the very high pressure silica polymorph, stishovite, conform with those calculated for a relatively large number of silicate crystals. Not only are they similar in value with the theoretical properties but each correlates with the observed SiO bond lengths as predicted by the calculations.