Branching ratios for competing channels in the reaction of HOCO radicals with NO

The yields of CO2 from the reactions of HOCO radicals with O2 and with NO have been compared using the technique of time-resolved infrared absorption. In two separate series of experiments, HOCO radicals have been generated by (i) broadband ultraviolet photolysis of acrylic acid (CH2CHCOOH) and (ii) the reaction, Cl + HCOOH → HCl + HOCO, following pulsed laser photolysis of Cl2 at 351 nm. In the latter experiments, the observed CO2 yields were corrected for the loss of Cl atoms by the reactions Cl + NO + He → ClNO + He and Cl + ClNO → Cl2 + NO. Our measurements on the rate of Cl + NO + He yield a value of the third-order rate constant of (5.1 ± 0.7) × 10−32 cm6 molecule−2 s−1. The results of the main experiments show that the yield of CO2 from the reaction between NO + HOCO is appreciably less than that from O2 + HOCO and is dependent on the total pressure in the range 10 to 100 Torr. We interpret this as showing that the reactions HOCO + NO → HNO + CO2 and HOCO + NO(+M) → HOC(O)NO(+M) occur at similar rates, with the latter reaction being favoured at the higher total pressure.