Abstract: The two isomerization reactions of formyl(thioformyl)ketene forming two four membered unsaturated heterocycle products thietone or oxetone have been studied by using ab initio method at HF/6 31G * level.All the geometries of the stationary points on the reaction path have been optimized with the Berny energy gradient technique.The transition states were characterized by vibration frequency analysis.The results show that two reactions both proceed via one elementary step,The barrier is 65.42 or 111.17 kJ·...