CH2NH2 + O2 and CH3CHNH2 + O2 Reaction Kinetics: Photoionization Mass Spectrometry Experiments and Master Equation Calculations

Abstract

Two carbon centered amino radical (CH2NH2 and CH3CHNH2) reactions with O2 were scrutinized by means of laboratory gas kinetics experiments together with quantum chemical computations and master equation modeling. In the experiments, laser photolysis of alkylamine compounds at 193 nm was used for the radical production and photoionization mass spectrometry was employed for the time-resolved detection of the reactants and products. The investigations were performed in a tubular, uncoated borosilicate glass flow reactor. The rate coefficients obtained were high, ranging from 2.4 × 10(-11) to 3.5 × 10(-11) cm(3) molecule(-1) s(-1) in the CH2NH2 + O2 reaction and from 5.5 × 10(-11) to 7.5 × 10(-11) cm(3) molecule(-1) s(-1) in the CH3CHNH2 + O2 reaction, showed negative temperature dependence with no dependence on the helium bath gas pressure (0.5 to 2.5 Torr He). The measured rate coefficients can be expressed as a function of temperature with: k(CH2NH2 + O2) = (2.89 ± 0.13) × 10(-11) (T/300 K)(-(1.10±0.47)) cm(3) molecule(-1) s(-1) (267-363 K) and k(CH3CHNH2 + O2) = (5.92 ± 0.23) × 10(-11) (T/300 K)(-(0.50±0.42)) cm(3) molecule(-1) s(-1) (241-363 K). The reaction paths and mechanisms were characterized using quantum chemical calculations and master equation modeling. Master equation computations, constrained by experimental kinetic results, were employed to model pressure-dependencies of the reactions. The constrained modeling results reproduce the experimentally observed negative temperature dependence and the dominant CH2NH imine production in the CH2NH2 + O2 reaction at the low pressures of the present laboratory investigation. In the CH3CHNH2 + O2 reaction, similar qualitative behavior was observed both in the rate coefficients and in the product formation, although the fine details of the mechanism were observed to change according to the different energetics in this system. In conclusion, the constrained modeling results predict significant imine + HO2 production for both reactions even at atmospheric pressure.