Formation of Nitric Acid in the Gas-Phase HO2 + NO Reaction: Effects of Temperature and Water Vapor

International audienc

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

Reactions of OH and OD radicals with NH₂CHO and ND₂CHO 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 H₂O 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^–1. Observation of CO₂, ammonia, and secondary water chemiluminescence gave evidence that recombination of OH and NH₂CO forms carbamic acid (NH₂COOH) with excitation energy of 103 kcal mol^–1, which decomposes through two pathways forming either NH₃ + CO₂ or H₂O + 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 NH₃, NH₂D, ND₂H, and ND₃. Dynamical aspects of the primary and secondary reactions are discussed based on the vibrational distributions of CO₂ and those of H/D isotopes of water and ammonia

Branching Ratios and Vibrational Distributions in Water-Forming Reactions of OH and OD Radicals with Methylamines

Reactions of OH and OD radicals with (CH₃)₃N, (CH₃)₂NH, and CH₃NH₂ were studied by Fourier transform infrared emission spectroscopy (FTIR) of the water product molecules from a fast-flow reactor at 298 K. The rate constants (4.4 ± 0.5) × 10^–11, (5.2 ± 0.8) × 10^–11, and (2.0 ± 0.4) × 10^–11 cm³ molecule^–1 s^–1 were determined for OD + (CH₃)₃N, (CH₃)₂NH, and CH₃NH₂, respectively, by comparing the HOD emission intensities to the HOD intensity from the OD reaction with H₂S. Abstraction from the nitrogen site competes with abstraction from the methyl group, as obtained from an analysis of the HOD and D₂O emission intensities from the OD reactions with the deuterated reactants, (CD₃)₂NH and CD₃NH₂. After adjustment for the hydrogen–deuterium kinetic isotope effect, the product branching fractions of the hydrogen abstraction from the nitrogen for di- and monomethylamine were found to be 0.34 ± 0.04 and 0.26 ± 0.05, respectively. Vibrational distributions of the H₂O, HOD, and D₂O molecules are typical for direct hydrogen atom abstraction from polar molecules, even though activation energies are negative because of the formation of pre-transition-state complexes. Comparison is made to the reactions of hydroxyl radicals with ammonia and with other compounds with primary C–H bonds to discuss specific features of disposal of energy to water product