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, CO2, HCN, NH3, 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 --> CO2 + HNCNH (2), has been invoked to interpret the disappearance of HNCO as well as the formation of various products, most importantly CO2. The concentration profiles of all measured species can be quantitatively modeled, throughout the temperature range analyzed, by varying k2 Using a modified mechanism. The kinetically modeled values of k2 can be effectively represented by k2 = 10(10.84 +/- 0.70) exp(-21,240 +/- 1,960/T) cm3/mol s. This result agrees closely with that computed with the conventional transition-state theory using the TST parameters predicted by the BAC-MP4 method: k2(BAC-MP4) = 10(11.13) exp(-21,600/T) cm3/mol s. The bimolecular reaction takes place via a stable 4-membered ring intermediate which is isoelectronic with diketene; viz.(~)[GRAPHICS]