A molecular orbital study of bonding in sulfate molecules: Implications for sulfate crystal structures |
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| Authors: | C G Lindsay G V Gibbs |
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| Institution: | 1. Department of Geological Sciences, Virginia Polytechnic Institute and State University, 24061, Blacksburg, VA, USA
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| Abstract: | Molecular orbital calculations have been completed on sulfate monomers and a dimer in a determination of minimum-energy geometries and electron density distributions. SO bond lengths calculated for the monomer and dimer correlate linearly with the fractional s-characters of the bonds, as observed for sulfate groups in crystals. With increasing coordination number of S, the bonded radii of S and O, as determined from electron density maps, increase at the same rate. This is at variance with the assumption that the radius of the oxide ion is nearly constant and that bond length variations arise primarily from changes in cation radii. The dimer shows a relatively large change in energy as its SOS angle is deformed from its minimum-energy value (125.6°) to 180°, in conformity with the small variation among observed angles. This is in contrast to the wide variation of bridging angles observed for silicate and phosphate dimers in crystals and molecules, and may imply that significant differences should be expected in the behavior of sulfates with respect to polymorphism and glass formation. The reaction energy of SO3 + H2O → H2SO4, calculated with second-order Møller-Plesset perturbation theory, agrees with the experimental value. Other properties of H2SO4 are also calculated and compared with experimental observations and previous calculations. |
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