The mechanism of the reaction HNCO+OH--NH~2+CO~2 has been studied by using ab initio MO method. The geometries of reactants, transition states,intermediates and products have been optimized with UHF/6-31G basis set and verified by frequency analysis. Furthermore,the correlation energies are corrected by Mφller-Plesset perturbation theory up to 4th order. The zero-pint energies are also corrected. The results show that the reaction is a multi-step complex one. Along the reaction path there are three transition states, two internal rotational barriers and four intermediates. The step IM3--TS2 is the rate-controlling step. Moreover, the calculated activation energy for the rate controlling step of the reaction channel studied in this paper i.e. the E~a of the reaction channel HNCO+OH--NH~2+CO~2(202. 388kJ/mol) is much greater than that of the other reaction channel HNCO+OH--H~2O+NCO (which equals to 69.038kJ/mol). Hence, the latter reaction channel is the main product channel. This is in good agreement with the experimental results.

Key words: ISOCYANIC ACID, HYDROXY GROUP, REACTION MECHANISM, AB INITIO CALCULATION, TRANSITION STATE THEORY, ACTIVATION ENERGY