A study is performed of the mechanisms of the formation of H3SO4+, HSO3+, and H3S2O7+ cations. The cations can act as sulphurizing agents, due to dissociation and rearrangement in oleum. Modeling is done at the level of the density functional theory and the theory of coupled clusters with implicit (COSMO) and combined explicit and implicit allowance for the solvent (sulfuric acid). The thermodynamic characteristics of the formation of sulphurizing agents are determined. The mechanism of the formation of HSO3+ cations via interaction between H3SO4+ cations and SO3 molecules is studied along with the decomposition of H3S2O7+ cations. The constant of the autoprotolysis of pyrosulfuric acid in sulfuric acid (5.37 x 10(7)) is calculated. It is more likely to form from H3S2O7+ cations than from H3SO4+ cations, as was thought earlier. The equilibrium constant of HSO4+ forming from H3S2O7+ (K = 5.8 x 10(-8), Delta G = 41.28 kJ/mol) is calculated.
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