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

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 H 2 CO in a fast flow reactor with 0.5−1.0 Torr of Ar carrier gas. Computer simulation of the emission spectra from H 2 O and HOD molecules generated by the primary reaction gave an inverted vibrational distribution in the ν 3 (O−H) stretching mode of HOD, with a maximum population in v 3 = 1; the distribution for the ν 1 and ν 3 stretch modes of H 2 O was similar. The vibrational energy disposal to H 2 O and HOD was 〈 f v 〉 = 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 NO 2, NO, OH, and H atoms. The vibrational distributions of H 2 O and HOD from the primary reaction plus the vibrational distributions of CO 2 and CO from the NO 2 + HCO reaction, HNO from the NO + HCO reaction, H 2 O 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 H 2 O 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.