Thiacyclobutadiene and thiabenzene. A comparative theoretical analysis

The two title compounds have been investigated theoretically using an ab initio SCF-MO treatment at the minimal STO-3G level. It has been found that in the most stable conformation thiacyclobutadiene has all atoms approximately in the same plane except for the H atom bonded to sulphur (i.e. a pyramidal sulphur). On the other hand, in the most stable conformation of thiabenzene, the sulphur atom not only is pyramidal, but also ~10° out of the plane containing the carbon system. The barriers to pyramidal inversion at the sulphur centres are ~48 and ~56 kcal mol for thiacyclobutadiene and thiabenzene, respectively. The properties of these molecules are rationalized by means of Perturbation Molecular Orbital (PMO) theory.