In order to investiaate the molecular geometry, structural analysis of the title compound (4a) was carried out by single crystal X-ray diffraction. Tautomeric properties and conformations were investigated by semi-empirical (AM1) and ab initio quantum mechanical (RHF/6-31G) calculations. The compound prefers thione tautomer in the solid state. On the other hand, semiempirical calculations show that in the gas phase the thiol tautomer is energetically favored in most cases. (C) 2006 Elsevier B.V. All rights reserved.