Kinetics of the NCO radical reacting with atoms and selected molecules

Rate constants for the reaction of isocyanate radicals (NCO) in its electronic ground state (X̃2Π) with oxygen atoms were determined at 2.5 Torr total pressure in the temperature range 302–757 K. Excimer laser photolysis (ELP) of chlorine isocyanate (ClNCO) produced NCO radicals detected by laser-induced fluorescence (LIF). The reaction NCO + O exhibits a negative temperature dependence, described by the two-parameter equation: kNCO+O(T) = (4.3−2.2+3.2) × 10−8 × T−1.14−0.12+0.08 cm3 molecule−1 s−1. Measurements at 298 K and total pressures of 2.5 and 9.9 Torr, respectively, indicated a slight pressure dependence. For the reaction of NCO radicals with hydrogen atoms, the rate constant kNCO+H = (2.2 ± 1.5) × 10−11 cm3 molecule−1 s−1was obtained at 298 K and a total pressure of 2.6 Torr for the first time by a direct measurement. From a single measurement k = (3.8 ± 1.6) × 10−11 cm3 molecule−1 s−1 was determined at 548 K and 2.4 Torr total pressure. In addition, rate constants for the reactions of NCO radicals with molecular oxygen (O2), carbon dioxide (CO2), molecular hydrogen (H2), and carbon monoxide (CO), which is a dissociation product of CO2 in a microwave discharge, were measured at two different temperatures. At room temperature these reactions were slow and at the detection limit of the ELP/LIF technique. However, at elevated temperatures at least the rate constants of the reactions NCO + O2 and NCO + H2 become significantly larger and, therefore, should be taken into account, when modeling combustion processes under certain conditions.