COMPARISON OF SEVERAL SEMIEMPIRICAL AND AB-INITIO METHODS FOR TRANSITION-STATE STRUCTURE CHARACTERIZATION - ADDITION OF CO2 TO CH3NHCONH2

The potential energy surfaces for the addition of CO2 to CH3NHCONH2 have been the subject of intensive research. Stationary points representing reactants, transition state, and products have been located and characterized using MNDO, MINDO/3, AM1, and PM3 semiempirical methods and ab initio 3-21G, 6-31G, and 6-31G* basis sets within Hartree-Fock (HF) procedures. The electron correlation has been estimated at MP2/3-21G, MP2/6-31G*//HF-6-31G*, and MP2/6-31G**//MP2/3-21G levels. The pathway connecting stationary points was traced by the IRC procedure. An analysis of the results shows that the addition is an asynchronous process, and the transition state structure can be described as a four-membered ring.