Bonded interactions and the crystal chemistry of minerals: A review


Connections established during last century between bond length, radii, bond strength, bond valence and crystal and molecular chemistry are briefly reviewed followed by a survey of the physical properties of the electron density distributions for a variety of minerals and representative molecules, recently generated with first-principles local energy density quantum mechanical methods. The structures for several minerals, geometry-optimized at zero pressure and at a variety of pressures were found to agree with the experimental structures within a few percent. The experimental Si–O bond lengths and the Si–O–Si angle, the Si–O bond energy and the bond critical point properties for crystal quartz are comparable with those calculated for the H6Si2O7 disilicic acid molecule, an indication that the bonded interactions in silica are largely short ranged and local in nature. The topology of model experimental electron density distributions for first and second row metal M atoms bonded to O, determined with high resolution and high energy synchrotron single crystal X-ray diffraction data are compared with the topology of theoretical distributions calculated with first principles methods. As the electron density is progressively accumulated between pairs of bonded atoms, the distributions show that the nuclei are progressively shielded as the bond lengths and the bonded radii of the atoms decrease. Concomitant with the decrease in the M–O bond lengths, the local kinetic energy, G(rc), the local potential energy, V(rc), and the electronic energy density, H(rc) = G(rc) + V(rc), evaluated at the bond critical points, rc, each increases in magnitude with the local potential energy dominating the kinetic energy density in the internuclear region for intermediate and shared interactions. The shorter the bonds, the more negative the local electronic energy density, the greater the stabilization and the greater the shared character of the intermediate and shared bonded interactions. In contrast, the local kinetic energy density increases with decreasing bond length for closed shell interactions with G(rc) dominating V(rc) in the internuclear region, typical of an ionic bond.
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