Authors : Anselm I. Igbafe , Linda L. Jewell and Stuart J. Piketh
Abstract: A theoretical study of the thermochemical properties of some common sulphur species present in the atmosphere including the intermediates and end-product of their transformation is reported here. These properties were obtained from the approximation of the Schrödinger equation as applied in Gaussian 03 (G03) model chemistry package. Analyses of the chemical reaction equilibrium for a variety of atmospheric sulphur transformations were investigated with a view to establish the thermodynamically favourable reaction pathways over ambient tropospheric temperature range of between -50 and +50�C. Seven high-energy accuracy model chemistries methods integrated in G03 comprising several ab initio methods and density functional theory (DFT) methods were applied for the computation. The computational methods were tested with a number of basis sets to yield values approximating those of experimental observations. Of all chosen methods, the complete basis set (CBS-Q) method was observed to closely approximate the experimentally determined thermodynamic enthalpies and Gibbs free energies of reactions. The CBS-Q method produced a mean absolute deviation (MAD) of 1.08% as against experimental data. Of the gas-phase and aqueous-phase reactions about 80 and 59.6%, respectively will most likely attain equilibrium over a temperature range of between -100 and +100�C. Whilst about 15 and 23.4% will not like reach equilibrium the specified temperature range and about 5 and 17% may attain equilibrium at higher temperatures above 20�C. The 4 most important oxidising species involved in the gas-phase reactions of atmospheric SO2 have been characterized in an order of increasing oxidant�s potentials in the form CH3O2* < HO2* < OH* < O*.
Anselm I. Igbafe , Linda L. Jewell and Stuart J. Piketh , 2008. Thermochemical Analyses of Sulphur Compounds: Implications for Atmospheric Sulphur Oxidation. Research Journal of Applied Sciences, 3: 393-406.