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.