Kinetics of the NCO radical reacting with atoms and selected molecules

Rate constants for the reaction of isocyanate radicals (NCO) in its electro nic ground state ((X) over tilde(2)II) 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 detec ted by laser-induced fluorescence (LIF). The reaction NCO + O exhibits a ne gative temperature dependence, described by the two-parameter equation: [GRAPHICS] 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 k(NCO+H) = (2.2 +/- 1.5) x 10(-11) cm(3) molecule(-1) s(-1) was obtained at 298 K and a total pressure of 2.6 Torr for the first time b y a direct measurement. From a single measurement k = (3.8 +/- 1.6) x 10(-1 1) cm(3) molecule(-1) s(-1) was determined at 548 K and 2.4 Torr total pres sure. In addition, rate constants for the reactions of NCO radicals with mo lecular oxygen (O-2), carbon dioxide (CO2), molecular hydrogen (H-2), and c arbon 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 techni que. However, at elevated temperatures at least the rate constants of the r eactions NCO + O-2 and NCO + H-2 become significantly larger and, therefore , should be taken into account, when modeling combustion processes under ce rtain conditions. (C) 2000 by The Combustion Institute.