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Dissociation of sulfur oxoacids by two water molecules studied using ab initio and density functional theory calculations
Author(s) -
Chung You Kyoung,
Kim Seong Kyu
Publication year - 2017
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.25419
Subject(s) - conformational isomerism , chemistry , dissociation (chemistry) , density functional theory , protonation , ab initio , intramolecular force , deprotonation , molecule , computational chemistry , ab initio quantum chemistry methods , crystallography , moiety , bond dissociation energy , stereochemistry , organic chemistry , ion
Using ab initio [SCS‐MP2 and CCSD(T)] and density functional theory (M062X) calculations, we have studied the geometries and energies of sulfur oxoacids H 2 S m O 6 ( m = 2–4) and their monohydrated and dihydrated clusters. When including the results from previously reported disulfuric acid (H 2 S 2 O 7 ) cases, the gas phase acidity is ordered as H 2 S 2 O 6 < H 2 S 3 O 6 < H 2 S 2 O 7 < H 2 S 4 O 6 . 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 (H 3 O + ) is triply H‐bonded to each oxygen atom of two SO 3 moieties as well as the second water, which in turn is H‐bonded to a SO 3 moiety. H 2 S 3 O 6 has the best molecular flexibility for adopting such an H‐bonding network structure, and thereby all the low‐lying conformers of H 2 S 3 O 6 (H 2 O) 2 are dissociated. In contrast, the least flexible H 2 S 2 O 6 forms such a structure with a high strain, and dissociation of H 2 S 2 O 6 (H 2 O) 2 is found from the third lowest conformer. Although the gas phase acidity of H 2 S 4 O 6 is the highest in this series, the lowest dissociated conformer and the lowest undissociated conformer of H 2 S 4 O 6 (H 2 O) 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 SO 3 moieties.