An Exploration of Theoretical and Experimental Electron Density Distributions and SiO Bonded Interactions for the Silica Polymorph Coesite

A multipole representation of the experimental electron density distribution for the high-pressure silica polymorph coesite, using Hirshfeld-type radial functions, has been generated with single-crystal X-ray diffraction data recorded to a sin θmax/λ value of 1.21 Å-1 at 100 K. Unlike an earlier modeling of the distribution, where a more limited data set was analyzed, deformation electron density maps display bananashaped isosurfaces in the lone-pair regions of each of the oxide anions involved in the bent SiOSi angles as well as teardrop-shaped ones along each of the SiO bond vectors. They also display a ring torus isosurface about O1, the oxide anion involved in the straight angle. Laplacian -∇2F maps display belt-shaped isosurfaces, centered near the apexes of the bent angles, that wrap about halfway around the oxide anions, with a ring torus-shaped isosurface surrounding O1. An analysis of -∇2F revealed that the (3,-3) critical point associated with the anions involved in the bent angles are associated in general with larger maxima than that associated with the straight angle, evidence that the electron density is more locally concentrated on the oxide anions involved in the bent angles. As such, these anions are asserted to be more susceptible to electrophilic attack by hydrogen, a feature that provides an experimental basis for why hydrogen in Hand Al-bearing coesite avoids O1 and is observed to dock in the vicinity of the oxide anions involved in the bent angles. The bond critical point properties of the experimental multipole representation of the electron density distribution for coesite together with those for the very high-pressure silica polymorph, stishovite, conform with those calculated for the SiO bonded interactions for a relatively large number of silicate crystals. Not only are they similar in value with the theoretical properties, but together they correlate with the observed SiO bond lengths as predicted by the calculations. The observed SiO bonds display a relatively wide range of ∇2F(rc) values between ∼10 e Å-5 for stishovite and ∼20 e Å-5 for coesite. The larger ∇2F(rc) values recorded for coesite, considered typical of first row closed-shell ionically bonded atoms, may not be typical for a closed-shell bonded interaction involving second row atoms such as the four-coordinate Si in coesite. The maxima along the bond vectors and in the lone-pair regions displayed by the experimental ∆F and -∇2F maps are indicative of shared covalent bonded interactions. The evidence suggests that the value of the electron density at the bond critical point for a given bonded interaction is a reliable measure of bond type: the larger the value F(rc), the greater the shared covalent interaction.