Infrared Chemiluminescence Study of the Reaction of Hydroxyl Radical with Formamide and the Secondary Unimolecular Reaction of Chemically Activated Carbamic Acid

Abstract

Reactions of OH and OD radicals with NHCHO and NDCHO were studied by Fourier transform infrared emission spectroscopy of the product molecules from a fast-flow reactor at 298 K. Vibrational distributions of the HOD and HO molecules from the primary reactions with the C-H bond were obtained by computer simulation of the emission spectra. The vibrational distributions resemble those for other direct H atom abstraction reactions, such as with acetaldehyde. The highest observed level gives an estimate of the C-H bond dissociation energy in formamide of 90.5 ± 1.3 kcal mol. Observation of CO, ammonia, and secondary water chemiluminescence gave evidence that recombination of OH and NHCO forms carbamic acid (NHCOOH) with excitation energy of 103 kcal mol, which decomposes through two pathways forming either NH + CO or HO + HNCO. The branching fraction for ammonia formation was estimated to be 2-3 times larger than formation of water. This observation was confirmed by RRKM calculation of the decomposition rate constants. A new simulation method was developed to analyze infrared emission from NH, NHD, NDH, and ND. Dynamical aspects of the primary and secondary reactions are discussed based on the vibrational distributions of CO and those of H/D isotopes of water and ammonia.

Rate constants and vibrational distributions for water‐forming reactions of OH and OD radicals with thioacetic acid, 1,2‐ethanedithiol and tert‐butanethiol

“…Vibrational distributions of H 2 O and HOD provide information about how the potential energy of a reaction is released to products . Calculation of the mean vibrational energy of H 2 O and HOD, < f v > = < E v >/< E av >, showed that in all three title reactions about half of the reaction energy is released as H 2 O and HOD vibrations, being slightly lower for the reaction with TAA.…”

“…Thus, the experimental spectra are modeled as a superposition of emission from the vibrational v 1,3 v 2 states of H 2 O from the OH + HR reactions, and v 3 v 1,2 states of HOD from the OD + HR reactions . The improved fundamental, hot and combinations bands constructed using the most recent spectroscopic data from the HITRAN database and ab initio calculations were described in recent work . Least‐squares fitting of the experimental spectrum gives vibrational distributions.…”

“…The surprisal is defined as I ( v ) = –ln[ P ( v )/ P 0 ( v )], and surprisal plots show I ( v ) versus f v = E v /< E av >. Vibrational surprisal plots are usually linear for H‐atom abstraction reactions, ( f v ) = λ v 0 + λ v f v ; the slope, λ v , is a global measure of the deviation from the statistical prediction, and the intercept provides an estimate for the P v ( v = 0) population.…”

“…10 The improved fundamental, hot and combinations bands constructed using the most recent spectroscopic data from the HITRAN database and ab initio calculations were described in recent work. 11 Least-squares fitting of the experimental spectrum gives vibrational distributions. The uncertainty associated with least-squares fitting has been discussed earlier, 9 where the most important factors are highlighted-the need to estimate the populations of the dark states and position of a baseline.…”

“…Vibrational distributions of H 2 O and HOD provide information about how the potential energy of a reaction is released to products. [9][10][11][12] Calculation of the mean vibrational energy of H 2 O and HOD, <f v > = <E v >/<E av >, showed that in all three title reactions about half of the reaction energy is released as H 2 O and HOD vibrations, being slightly lower for the reaction a v 1,2 = max(v 2 ) in a series of equilibrated (v 1 /2i 2 ) levels; P 0 3 is the statistical distribution b P 3 (0) is not included. c From simulation of the spectra in the 2400-2900 cm −1 range.…”