Application of Quantum-Mechanical Methods to Simple Inorganic “Molecules” of Relevance to Mineralogy, and to Oxide Minerals

As noted in the introduction to this text, much can be learned through the application of both quantum-mechanical calculations and experimental techniques to simple molecules that contain bonds of the type found in the important groups of minerals. One reason for this approach is that calculations at a higher level of quantum-mechanical rigor can be applied to such simple systems. This approach will be illustrated with reference to the SiO, SiO2, Si2O2, Si3O3, and SiF4 molecules. Attention will then be turned to the major oxide minerals MgO, Al2O3, and SiO2 and the binary transition-metal oxides of Ti, Mn, and Fe, with some brief discussion of the series of transition-metal monoxides (MnO, FeO, CoO, NiO) and complex oxides (FeCr2O4, FeTiO3, etc.), and of the problem of the calculation of Mössbauer parameters in iron oxides (and other compounds). Although silicon monoxide, SiO, is not an important component of minerals, it is an important chemical constituent in interstellar and circumstellar space and an important starting material for the gas-phase synthesis of silicates from components of the nebula (Day and Donn, 1978). The structure, energetics, and spectral properties of SiO have been calculated by a number of different methods. The Si-O bond distance calculated using ab initio Hartree-Fock-Roothaan SCF methods at the 6-31G* basis- set level is 1.487 Å (Snyder and Raghavachari, 1984), slightly smaller than the experimental value of 1.5097 Å (Field et al., 1976). A near Hartree- Fock limit basis set and limited configuration-interaction calculation has given the slightly better value of 1.496 Å (Langhoff and Arnold, 1979). This study also gave a bond dissociation energy of 8.10 eV, compared to an experimental value of 8.26 ± 0.13 eV (Hildenbrand, 1972), and a bond stretching frequency of 1248 cm- 1 , compared to an experimental value of 1242 cm-1 (Anderson and Ogden, 1969). Even more highly correlated calculations give a bond distance of 1.515 Å and a stretching frequency of 1242 cm-1 (Werner et al., 1982). The 6-31G* basis-set Hartree-Fock-Roothaan calculation also gives an almost exactly correct bond-stretching frequency after the standard correction factor describing correlation effects is applied (Hehre et al., 1986).