Beyond benzene sulfides and thiepins: Tautomerizations and thiepins inversions at theoretical levels

Geometries, enthalpies and energy barriers for tautomerization of X-benzene sulfides (X-B) to their corresponding X-thiepins (X-T), as well as thiepins ring inversions, are calculated at HF, MP2, and B3LYP levels of theory, using 6-311G∗ basis set (X = F, Cl, Br, CH3, OCH3, CF3, CN and NH2). The gas phase change of enthalpy of activation for tautomerization of H-B to H-T is 18.90 kcal mol−1, while that for the reversed reaction (H-T to H-B) is 12.04 kcal mol−1, at B3LYP level. These calculations appear in clear contrast to those calculated by MNDO [R. Gleiter, G. Krennrich, D. Cremer, K. Yamamoto, I. Muratas, J. Am. Chem. Soc. 107 (1985) 6874], although successfully reproduce the previous estimations of Pye et al. [C.C. Pye, J.D. Xidos, R.A. Poirier, D.J. Burnell, J. Phys. Chem. A, 101 (1997) 3371] concerning the prevailed thermodynamic data in favor of benzene sulfides. Substituents are found to have small effects on (X-B)-(X-T) equilibria as well as conformational inversions of X-T. In any case, locking H-T or H-B may not be the real challenge, in the gas phase, compared to the intrinsic tendency of either compound for losing sulfur and producing benzene. The aromatic characters of X-T and planar transition states for their inversions are estimated using magnetic (NICS) and structural criteria (bond length localizations) indicating the moderate aromaticity of the former and extreme antiaromaticity of the latter. A rather good consistency is found between results of B3LYP calculations and those of the more time consuming MP2 method.