Combustion kinetics of amines: Exploring hydrogen atom abstraction reactions from primary amines by ṄH2 radicals

Abstract

Amines are often used as model compounds to investigate the combustion chemistry of the nitrogen-containing compounds in biomass. The amino radical (ṄH₂) plays a pivotal role in the initial stages of biomass pyrolysis and oxidation. To gain insight into nitrogen conversion chemistry, it is necessary to understand the cross-reactions between amine-bearing molecules and ṄH₂ radicals. In this study, a comprehensive investigation was performed on the chemical kinetics of hydrogen atom abstraction reactions of twelve C₁ – C₅ amines by ṄH₂ radicals. Geometry optimizations and frequency analyses were conducted for all species involved in fifty reaction channels at the M06–2X/6–311++G(d,p) level of theory. Single-point energies were calculated at the QCISD(T)/CBS level of theory, and subsequently corrected by zero-point energy. Conventional transition state theory with asymmetric Eckart tunneling corrections and the one-dimensional hindered rotor approximation was used to calculate the high-pressure limit rate constants for these targeted reactions over a temperature range of 500 – 2000 K. The average rate constants for hydrogen atom abstraction from primary, secondary, and tertiary carbons at different positions relative to the amino functional group, labelled as α, β, γ, and δ, were provided, following the order: primary < secondary < tertiary. The hyperconjugation effect of the amino group on the α C–H bond lowers the electronic energy barrier; therefore, rate constants for abstraction from the α C–H site largely dominate. A comparison of the average rate constants for amines and the previous studies on alkanes, alcohols, ethers, and esters was performed to reveal the influence of different functional groups on kinetic parameters. The updated rate constants were then employed in the target fuel mechanism to investigate their effect on the prediction of species concentrations associated with H-atom abstraction reactions in a jet-stirred reactor.