Dissociation of sulfur oxoacids by two water molecules studied using ab initio and density functional theory calculations |
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| Authors: | You Kyoung Chung Seong Kyu Kim |
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| Affiliation: | Department of Chemistry and Basic Science Research Institute, Sungkyunkwan University, Suwon, Korea |
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| Abstract: | Using ab initio SCS‐MP2 and CCSD(T)] and density functional theory (M062X) calculations, we have studied the geometries and energies of sulfur oxoacids H2SmO6 (m = 2–4) and their monohydrated and dihydrated clusters. When including the results from previously reported disulfuric acid (H2S2O7) cases, the gas phase acidity is ordered as H2S2O6 < H2S3O6 < H2S2O7 < H2S4O6. The intramolecular H‐bonding, which may indicate the degree of structural flexibility in this molecular series, is an important factor for the order of the gas phase acidity. All these sulfur oxoacids show dissociated (or deprotonated) geometries with only two water molecules, although the energies of the dissociated conformers are ranked differently. All of the dissociated conformers form a unique H‐bonding network structure in which the protonated first water (H3O+) is triply H‐bonded to each oxygen atom of two SO3 moieties as well as the second water, which in turn is H‐bonded to a SO3 moiety. H2S3O6 has the best molecular flexibility for adopting such an H‐bonding network structure, and thereby all the low‐lying conformers of H2S3O6(H2O)2 are dissociated. In contrast, the least flexible H2S2O6 forms such a structure with a high strain, and dissociation of H2S2O6(H2O)2 is found from the third lowest conformer. Although the gas phase acidity of H2S4O6 is the highest in this series, the lowest dissociated conformer and the lowest undissociated conformer of H2S4O6(H2O)2 are very close in energy. This is because forming the H‐bonding network structure is somewhat difficult due to the large distance between the two SO3 moieties. |
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| Keywords: | ab initio H2S2O6 H2S3O6 H2S4O6 sulfur oxoacid |
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