Theoretical study of hydrogen abstraction by HO₂ radicals from primary straight chain amines CH₂-NH₂ (n = 1-4)

Hydrogen abstraction reactions by HO₂ radicals from four primary amines including methylamine (MA), ethylamine (EA), n-propylamine (PA), and n-butylamine (BA), are investigated and the effect of the functional group on rate constants at different reaction sites is examined. A hybrid functional BH&HLYP coupled with cc-pVTZ as the basis set is utilized to determine geometry optimizations, frequencies, and connections between transition states and corresponding local minima. By comparing the reaction energies obtained by several density functional theory methods to those obtained using the gold-standard CCSD(T)/CBS(T-Q) method, the M08-HX/maug-cc-pVTZ combination is identified as the best suitable method with a mean unsigned deviation of 0.81 kcal mol^-1. This method is then applied to construct the potential energy surface for all the reaction systems. High-pressure limit rate constants at 500-2500 K are calculated through variation transition state theory and conventional transition state theory, including a one-dimensional hindered rotor treatment and asymmetrical Eckart tunneling correction. The branching ratio analysis suggests that the hydrogen abstraction at the C site adjacent to the NH₂ functional group (α reaction site) dominates the reactions. Linear Bell-Evans-Polanyi and Bell-Evans correlations are observed for the hydrogen abstractions at the C reaction sites. Furthermore, a scheme to estimate the rate constants for the CH₂-NH₂ + HO₂ reaction system is presented.