The thermal reaction of HNCO at moderate temperatures

The thermal reaction of HNCO has been studied in a static cell at temperatures between 873 and 1220 K and a constant pressure of 800 torr under highly diluted conditions. The reaction was measurable above 1000 K by FTIR spectrometry. The products detected include CO, CO 2 , HCN, NH 3 , and the unreacted HNCO. In this moderate temperature regime, the rates of product formation and HNCO decay cannot be accounted for by a previously established high‐temperature mechanism, assuming HNCO → NH + CO (1) as the initiation process. Instead, a new bimolecular reaction, 2HNCO → CO 2 + HNCNH (2), has been invoked to interpret the disappearance of HNCO as well as the formation of various products, most importantly CO 2 . The concentration profiles of all measured species can be quantitatively modeled, throughout the temperature range analyzed, by varying k 2 using a modified mechanism. The kinetically modeled values of k 2 can be effectively represented by\documentclass{article}\pagestyle{empty}\begin{document}$$ k_{\rm 2} = 10^{10.84 \pm 0.07} {\rm \,exp}(- 21,240 \pm 1,960/{\rm T}){\rm\, cm}^3 /{\rm mol\, s}{\rm .} $$\end{document}This result agrees closely with that computed with the conventional transition‐state theory using the TST parameters predicted by the BAC‐MP4 method:\documentclass{article}\pagestyle{empty}\begin{document}$$ k_2 ({\rm BAC} - {\rm MP}4) = 10^{11.13} {\rm \,exp(- 21,600/T) cm}^{\rm 3} /{\rm mol s}{\rm .} $$\end{document}The bimolecular reaction takes place via a stable 4‐membered ring intermediate which is isoelectronic with diketene; viz.