College of Chemistry, and Key State Laboratory of Biotherapy, Sichuan University, Chengdu 610064, Peoples Republic of China.
J Mol Model. 2011 Aug;17(8):2069-82. doi: 10.1007/s00894-010-0917-z. Epub 2010 Dec 14.
The detailed reaction mechanism for the water-assisted hydrolysis of isocyanic acid, HNCO + (n + 1) H(2)O → CO(2) + NH(3) + nH(2)O (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).
水助异氰酸水解的详细反应机理，HNCO + (n + 1) H(2)O → CO(2) + NH(3) + nH(2)O (n = 0-6)，在气相中进行，已经被研究过了。所有结构都是在 MP2/6-31 + G 理论水平上进行优化和表征的，然后在 MP2/6-311++G 上重新进行优化。参与水解的七个显式水分子可以分为两组，一组直接参与质子转移，另一组位于底物附近，通过与 HN = C = O 形成氢键来发挥协同作用。讨论了两种可能的反应途径，水分子跨越 C = N 键或跨越 C = O 键的加成，证明前者在能量上更有利。我们的计算表明，在标题水解的最动力学有利途径中，三个水分子通过一个八元环过渡态直接参与氢键转移，而另外四个水分子通过在底物附近形成三个八元协同环来催化 HN = C = O 的水解。这个无应变的氢键网络导致最佳估计的速率决定步骤活化能为 24.9 kJ mol(-1)，在 600 K 时，与气相动力学实验结果 25.8 kJ mol(-1)非常吻合。
