The reaction mechanisms of HNCO with CX (X = F, Cl, Br) have been investigated using density functional theory (DFT) at the B3LYP/6- 31+G~* level. The transition states and intermediates of these reactions are verified by frequency analysis. The relationship of reactants, transition states, intermediates and products is affirmed by IRC. The energy barriers of these reactions have also been calculated. The results show that reactions of HNCO with CX are different from reactions of HNCO with other small molecular radicals. The hydrogen transfer between molecules is the first step in the case of HNCO with CX reactions. Then the N atom approaches the C(1) atom forming the N-C(1) bond and the stable intermediate (IM) is obtained. After that the N atom transfers to C(1) forming TS2. The TS2 decomposes into product finally. The reaction pathway is reactants→ TS1→IM→TS2→products. These reactions are exothermic.

Key words: ISOCYANIC ACID, FREE RADICAL, REACTION MECHANISM, density functional theory