HNCO热解为CO_2和HNCNH的反应机理理论研究

A Theoretical Study on the Thermo Decomposition of HNCO to HNCNH and CO_2

利用从头算RHF／3－21G方法研究了HNCO二聚后生成HNCNH和CO2的反应机理,计算表明，该反应是分步反应，由反应物经第一过渡态生成四元环中间体，再经过第二过渡态分解为产物，与实验得到的结论一致，反应的第一步是速度控制步骤，计算得到的活化位垒为172．55kJ·mol-1，与实验上测得的176．40±16．30kJ·mol-1相吻合,反应的第二位垒为83．68kJ·mol-1，在实验条件下是一个快速步骤.因此，四元环中间体不能作为最终产物．整个反应是包含两个非同步过程的加成反应．

Ab initio RHF/3-21G method has been employed to study the mechanism of the bimolecular thermolysis of HNCO. The results show that this reaction is a stepwise reaction. The first step is the dimerization of HNCO to form a four-membered ring in termediate, the second step is the dissociation of the intermediate to form the products HNCNH and CO2. The calculated activation barrier for the steps are 172.55kJ·mol-1 and 83.68kJ·mol-1, respectively. It can be seen that the first step is the rate determining step,its barrier is in good agreement with experimental data (activation energy: 176.40±16.30 kJ·mol-1). The second step is a fast step, the four-membered ring intermediate can not stay as a final product. The reaction can be described as a stepwise non-synchronous reaction.