λ4σ2-Sulfur Heterocycles. An ab Initio and Density Functional Study

In order to better understand the structure and properties of 2λ-thioallyl (alternatively named 2-thiapropanediyl or thioformaldehyde S-methide) and the heterocyclic derivatives 3,4-dimethylenethiophene, naphtho[1,8-c,d]thiopyran, thieno[3,4-c]thiophene, and thieno[3,4-f]benzo[c]thionaphthene ab initio quantum chemical and density functional theory were utilized. The general conclusions drawn from the distinctly different approaches are the same. The low d-orbital occupation is not compatible with any hypervalency of sulfur including d-valence orbitals. Rather, the structures may be understood as ylidic ones with Coulombic CS-bond contraction, and as indicated by low singlet/triplet (S 0/T 1) splitting energies, the structures are more or less diradicaloid as well. The molecular geometries calculated at the MP2 and DFT/Becke3LYP level of theory are very similar with CS bond lengths between 1.64 and 1.70 Å for the nonclassical structures. The ionization energies are relatively low. For the sake of comparison the quinoid structures benzo[c]thiophene, acenaphtheno[5,6-cd]thiopyran and naphtho[1,8-c,d;5,4-c‘d‘]dithiopyran were included in the study. They proved to be borderline cases between compounds of nonclassical and classical structure. According to the calculated molecular geometry and the charge distribution acenaphtheno[5,6-cd]thiopyran has a nonclassical structure.