Abstract
The structures of thiosulfate (S2O3 2−) and four other isovalent sulfur oxyanions, S m O n 2− have been investigated at the MP2, MP4, and B3LYP levels of theory, with 6−31+G* as the basis set. By comparing the bond lengths in the dianions with those in the doubly protonated molecules, H2S m O n , the sulfur–oxygen and sulfur–sulfur bond orders have been estimated, based on a valence bond theory model. The conventional Lewis structures, which represent the central S atom as forming six covalent bonds with the four ligands, consistently give accurate descriptions of the structures for all the dianions. Natural bond order analyses, however, show that the 3d orbitals do not participate in valence bonding to any significant degree. Chemically, the peripheral sulfur atoms in all the dianions are predicted to be stronger nucleophiles as well as stronger bases than the oxygen atoms in the gas phase.
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Acknowledgement
The authors thank Professor William Jenks (Iowa State) for useful comments pertaining to the preparation of this manuscript.
Notes
1Because the B3LYP method produces adequately accurate results and, at the same time, uses much less CPU time in comparison to the MP2 and MP4 methods, the diprotonated molecules were investigated by this DFT method only.
2The transition state for each reaction has been located at the B3LYP/6−31+G** level of theory. The ΔG ‡ values show the same trends as the ΔG, such that both values decrease down the column as shown in . Therefore, the kinetic and thermodynamic data agree, in accord with Hammonds postulate. Unfortunately, due to the basis-set superimposition error (BSSE), as well as zero-point corrections, some of the ΔG ‡ values turned out to be negative. Therefore, we feel that these ΔG ‡ values are not themselves meaningful enough to be tabulated and presented in the text, but only their values relative to the others are used in the discussion.