A shock tube study of H+HNCO→H 2+NCO and the thermal decomposition of NCO

Abstract

The reaction of atomic hydrogen with isocyanic acid (HNCO) to produce molecular hydrogen and the NCO radical, H+HNCO→H 2 +NCO (3a) and the thermal dissociation reaction NCO+Ar→N+CO+Ar (4) have been studied in shock-heated mixtures of HNCO dilute in argon. Quantitative time histories of the NCO radical were measured behind the shock waves using cw, narrow-linewidth laser absorption at 440 nm. The second-order rate coefficients of reactions (3a) and (4) were determined to be k 3a =5.5×10 14 exp(−13.700/ T , K) ( f =0.50, F =1.5) T =2260−3250 K k 4 =2.2×10 14 exp(−27,200/ T , K) ( f =0.40, F =1.8) T =2370−3050 K cm 3 mol −1 s −1 , where f and F define the lower and upper uncertainty limits, respectively. The results for reaction (3a) were combined with the lower-temperature results from two previous studies for the reverse reaction (−3a), resulting in the expression: k 3a =9.0×10 7 T 1.66 exp(−7000/ T , K) cm 3 mol −1 s −1 , T =591−3250 K The experimental results for k 4 were used to calculate values of the collision efficiency ( β x ) and threshold energy ( E 0 ) for reaction (4) using unimolecular rate constant theory: β x =0.020 (−65%.+100%) E 0 =61.4±1.5 kcal/mol