Ab initio studies on 1,2-oxathietane, Z and E 3,4-dimethyl-1,2-oxathietane, and their products of formal [σ2s + σ2a] cycloreversion

Ab initio calculations at the level of 3-21G* SCF and MP3/3-21G* have been performed on the novel heterocycle 1,2-oxathietane and the formal [σ2s + σ2a] cycloreversion products. Hartree–Fock calculations have been performed for the E and Z 3,4-dimethyl-1,2-oxathietanes and their corresponding cycloreversion products. The calculations afford fully geometry optimized conformations, energies, net atomic charges, and HOMO values for the heterocycles. 1,2-Oxathietane is found to be close in energy to the sum of energies of either pair of its cycloreversion products. The most plausible mechanism of cycloreversion features the formation of ground state formaldehyde and thioformaldehyde via a biradical intermediate. This result is in accord with the lack of detection of chemiluminescence with the 1,2-oxathietanes, in contrast to the 1,2-dioxetanes.