Cooperative effect of water molecules in the self-catalyzed neutral hydrolysis of isocyanic acid: a comprehensive theoretical study

The detailed reaction mechanism for the water-assisted hydrolysis of isocyanic acid, HNCO + (n + 1) H₂O → CO₂ + NH₃ + nH₂O (n = 0−6), taking place in the gas phase, has been investigated. All structures were optimized and characterized at the MP2/6-31 + G* level of theory, and then re-optimized at MP2/6-311++G**. The seven explicit water molecules participating in the hydrolysis can be divided into two groups, one directly involved in the proton relay, and the other located in the vicinity of the substrate playing the cooperative role by engaging in hydrogen-bonding to HN = C = O. Two possible reaction pathways, the addition of water molecule across the C = N bond or across the C = O bond, are discussed, and the former is proved to be more favorable energetically. Our calculations suggest that, in the most kinetically favorable pathway for the titled hydrolysis, three water molecules are directly participating in the hydrogen transfer via an eight-membered cyclic transition state, while the other four water molecules catalyze the hydrolysis of HN = C = O by forming three eight-membered cooperative loops near the substrate. This strain-free hydrogen-bond network leads to the best estimated rate-determining activation energy of 24.9 kJ mol⁻¹ at 600 K, in excellent agreement with the gas-phase kinetic experimental result, 25.8 kJ mol⁻¹.