lambda(4)sigma(2)-Sulfur heterocycles. An ab initio and density functional study

in order to better understand the structure and properties of 2 lambda-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 Angstrom 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.