STABILITY AND CHEMICAL-PROPERTIES OF THIIRANIMINE

The stabilty and the molecular properties of thiiranimine are studied by using ab initio MO methods. Geometries are optimized at the HF/6-31G* and MP2/6-31G** levels while relative energies are estimated at the MP4/6-31G** level together with the zero-point energies. Our analysis points out that the interaction between the C = N moiety and the three-membered ring is responsible for several unusual properties of thiiranimine such as the high C = N stretching frequency, the relatively small ring strain (as compared with thiirane), and the large proton affinity at nitrogen. Our calculations also suggest that the first photoelectron band is due to an S ionization and that the first UV band arises essentially from an intraatomic sulfur transition. The fragmentation reaction is endothermic, but consideration of entropy terms modifies the picture significantly. The resulting free energies show that thiiranimine is quite stable towards fragmentation into hydrogen isocyanide plus thioformaldehyde in agreement with experiments on substituted thiiranimines. Finally, the regiochemistry of thiiranimine in cycloadditions is also discussed on the basis of the frontier orbitals.