Abstract: The cycloaddition reactions of isothiocyanic acid and methylenimine forming four-or six-mem- bered ring products were studied theoretically. Bader type electron density topological analysis was also used to study the electronic structure of stationary points for the equimolar reaction. All the geometries of the stationary points on the reaction path have been optimized by RHF/6-31G^*, MP2/6-31G^* and B3LYP/6-31G^* with energy gradient technique, and the single point energy calculations of the stationary points of equimolar reaction have been performed by MP4. 6-31G^* method at MP2/6-31G^* optimized geometries. All the transition states were characterized by vibration frequency analysis. The equimolar reaction is stepwise, proceeding via a zwitterionic intermediate; the second step is the rate controlling one and the activation barrier is 107.86 kJ/mol. In addition, the above intermediate can react further with another methylenlmine or isothiocyanic acid molecule respectively to form two six-membered ring(1: 2 and 2: 1) products. The 1: 2 reaction is concerted but asynchronous, the 2: 1 reaction is a synchronous concerted process. The barriers of these two reactions are 15. 88 and 21. 82 kJ/mol respectively and they are lower than that of reaction forming four-membered ring product. This is consistment with experimental fact.

Key words: Isothiocyanic acid, Methyleniminel Cycloadditionl Ab initio, B3LYP, Electron density topological analyais