Abinitiomolecular orbital calculations have been carried out on the lowest closed shell singlet states of six isomeric forms of C2H2S at the SCF and CI levels using STO-3G and 6-31G* basis sets. Molecular geometries of the isomers and the activated complexes for their isomerization reactions were optimized by the energy gradient method at the SCF level and the CI calculations were performed at these geometries. In addition, the lowest singlet and triplet states of the •SCHĊH diradical were also studied and their geometries optimized by the Restricted Hartree Fock method.The computed stabilities follow the orderThe pseudo antiaromatic character of the 4n-π electron thiirene is manifested by its drastically reduced C—S bond energy, ~15 kcal mol−1as compared to the value of > 40 kcal mol−1estimated for its 2n-π electron analog, thiirane. This explains why thiirenes can be stabilized in inert matrices at 8 K but decompose readily at higher temperatures. In agreement with experimental results the lowest energy path for the unimolecular decomposition of thiirene leads to thioketene via the lowest singlet (or triplet) state of thioformylmethylene.The ir frequencies calculated for thiirene fromabinitiomo theory using 6-31G + polarization functions on the sulfur show reasonably good agreement with the experimental frequencies, confirming the earlier assignments of the ir spectrum of thiirene