The Reaction Kinetics of H Atoms with Methylamine: Measurements and Theory

Abstract Understanding the reactivity of methylamine (CH3NH2), the simplest primary amine, is of relevance in a variety of contexts, ranging from atmospheric chemistry to pyrolysis and combustion. In this work we determined experimentally and theoretically the rate of the H-abstraction reaction from CH3NH2 and its methyl-deuterated analogue CD3NH2 by atomic hydrogen. Experimentally, we used pulsed laser photolysis to generate H atoms in the presence of excess CH3NH2 and CD3NH2 and measured their decay via resonance fluorescence at 121 nm over 294–620 K. Rate constants were determined from the fluorescence signal decays. Theoretically, rate constants were computed both for the methyl and the amino abstraction channels using the ab initio transition-state theory-based master equation approach. Quantum chemistry calculations were performed at different levels of theory, determining structures and Hessians through double-hybrid density functional theory, CCSD(T), and CASPT2 simulations performed using the aug-cc-pVTZ basis set. Energies were refined by performing CCSDT(Q) calculations, with zero-point energy corrections computed using the rigid-rotor harmonic approximation (RHHO), estimating anharmonic corrections using degeneracy-corrected second-order perturbation theory. Theoretical rate constants were computed using conventional transition state theory, as it was found that variational corrections are minor. Tunneling contributions, which play a relevant role at low temperatures, were determined using both Eckart and small-curvature tunneling models. The measured rate constant has Arrhenius type behavior for 350–620 K, where it can be properly fitted with the expression k1(T) = (2.5 ± 0.9) × 10–11 exp(−4.7 ± 0.3 kcal mol–1/RT) cm3 molecule–1 s–1. Kinetic isotope effects are between a factor of 2 and 6 in the investigated temperature range, decreasing with the temperature, thus indicating that the main abstraction channel is from methyl. This is supported by the theoretical estimates, in good agreement with experiments. The best fit of the calculated rate constant for CH3NH2 + H → CH2NH2 + H2, performed in the 500–2500 K temperature range and recommended for combustion simulations, is k1a(T) = 1.6 × 10–19T2.734 exp(−2.74 kcal mol–1/RT) cm3 molecule–1 s–1, whereas for CH3NH2 + H → CH3NH + H2 the recommendation is k1b(T) = 1.26 × 10–20T2.908 exp(−5.04 kcal mol–1/RT) cm3 molecule s–1.