Ab initio HF/6-31G^* and density functional theory(DFT) B3LYP/6-31+ +G^*^* have been used to study the mechanism of the cycloaddition riaction between thioketene and thioformaldehyde. These two reactions that give different four-memabered heterocyclic products; both proceed;via a two-step mechanism involving a zwitterionic intermediate. Both processes proceed easily,but reaction(1) is more favorable.In addition, Bader type electron density topological analysis was used to study the electronic structures of stationarypoints for reactions(1)and(2).

Key words: KENONE P, AB INITIO CALCULATION, THIOFORMALDEHYDE, CYCLOADDITION REACTION, REACTION MECHANISM, ELECTRON DENSITY, TOPOLOGY