Hydrogen Abstraction from Hydrocarbons by NH₂

Abstract

This contribution investigates thermokinetic parameters of bimolecular gas-phase reactions involving the amine (NH₂) radical and a large number of saturated and unsaturated hydrocarbons. These reactions play an important role in combustion and pyrolysis of nitrogen-rich fuels, most notably biomass. Computations performed at the CBS-QB3 level and based on the conventional transition-state theory yield potential-energy surfaces and reaction rate constants, accounting for tunnelling effects and the presence of hindered rotors. In an analogy to other H abstraction systems, we demonstrate only a small influence of variational effects on the rate constants for selected reaction. The studied reactions cover the abstraction of hydrogen atoms by the NH₂ radical from the C-H bonds in C₁-C₄ species, and four C₅ hydrocarbons of 2-methylbutane, 2-methyl-1-butene, 3-methyl-1-butene, 3-methyl-2-butene, and 3-methyl-1-butyne. For the abstraction of H from methane, in the temperature windows 300-500 and 1600-2000 K, the calculated reaction rate constants concur with the available experimental measurements, i.e., k_calculated/k_experimetal = 0.3-2.5 and 1.1-1.4, and the previous theoretical estimates. Abstraction of H atom from ethane attains the ratio of k_calculated/k_experimetal equal to 0.10-1.2 and 1.3-1.5 over the temperature windows of available experimental measurements, i.e., 300-900 K and 1500-2000 K, respectively. For the remaining alkanes (propane and n-butane), the average k_experimental/k_calculated ratio remains 2.6 and 1.3 over the temperature range of experimental data. Also, comparing the calculated standard enthalpy of reaction (Δ_rH°₂₉₈) with the available experimental measurements for alkanes, we found the mean unsigned error of computations as 3.7 kJ mol^-1. This agreement provides an accuracy benchmark of our methodology, affording the estimation of the unreported kinetic parameters for H abstractions from alkenes and alkynes. On the basis of the Evans-Polanyi plots, calculated bond dissociation enthalpies (BDHs) correlate linearly with the standard enthalpy of activation (Δ^⧧H°₂₉₈), allowing estimation of the enthalpy barrier for reaction of NH₂ with other hydrocarbons in future work. Finally, we develop six sets of the generalized Arrhenius rate parameters for H abstractions from different C-H bond types. These parameters extend the application of the present results to any noncyclic hydrocarbon interacting with the NH₂ radical.