Infrared Chemiluminescence Study of the Reactions of Hydroxyl Radicals with Formaldehyde and Formyl Radicals with H, OH, NO, and NO2

Abstract

The infrared chemiluminescence in the 1800−3900 cm-1 range was observed from vibrationally excited products generated by the reactions of OH and OD with H2CO in a fast flow reactor with 0.5−1.0 Torr of Ar carrier gas. Computer simulation of the emission spectra from H2O and HOD molecules generated by the primary reaction gave an inverted vibrational distribution in the ν3(O−H) stretching mode of HOD, with a maximum population in v3 = 1; the distribution for the ν1 and ν3 stretch modes of H2O was similar. The vibrational energy disposal to H2O and HOD was 〈fv〉 = 0.54−0.56 with 63% in the newly formed OH bond and 34% in the bending mode. This vibrational distribution is characteristic for a direct abstraction mechanism. The excitation in the bending mode exceeds that from OH reactions with hydrocarbons (<20%), but it is similar to that from the reaction of OH with dimethyl sulfide and HBr. By adjustment of the reaction conditions, infrared emission could be observed from secondary reactions of HCO radical with NO2, NO, OH, and H atoms. The vibrational distributions of H2O and HOD from the primary reaction plus the vibrational distributions of CO2 and CO from the NO2 + HCO reaction, HNO from the NO + HCO reaction, H2O and CO from the OH + HCO reaction, and CO from the H + HCO reaction were analyzed using information theory. The results support an addition mechanism followed by unimolecular decomposition for the HCO radical reactions. The vibrational of H2O distribution from OH + HCO is especially noteworthy, since it can be used to distinguish between direct abstraction vs recombination followed by the decomposition of HCOOH.