Experimental and Theoretical Study of the OH-Initiated Degradation of Ethylenediamine (NH2CH2CH2NH2) under Simulated Atmospheric Conditions. Part 1: Kinetics of the Ethylenediamine + OH Gas Phase Reaction

The ethylenediamine (NH₂CH₂CH₂NH₂) reaction with OH radicals was studied under natural sunlight conditions in a large outdoor atmospheric simulation chamber and in theoretical calculations based on CCSD(T*)-F12a/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ quantum chemistry results. The OH rate coefficient was determined in relative rate experiments employing online PTR-ToF-MS detection to be k_OH = (2.8 ± 0.8) × 10^–10 cm³ molecule^–1 s^–1 at 308 ± 3 K and 1013 ± 3 hPa. The theoretical study includes a conformational mapping of ethylenediamine showing nine conformers to be considered in reaction kinetics modeling, CCSD(T*)-F12a/aug-cc-pVTZ optimized geometries, and results from vibrational anharmonicity calculations for all conformers. The rate coefficients for the OH radical reaction with each of the nine ethylenediamine conformers were obtained in master equation calculations showing the kinetics being governed by both the formation of prereaction adducts and by tight transition states giving a Boltzmann conformer weighted rate coefficient k_OH = 2.9 × 10^–10 cm³ molecule^–1 s^–1, and a 1:1 branching between H-abstraction from the NH₂ and CH₂ groups at 298 K and 1013 hPa. The calculated rate coefficient shows a negative temperature dependence and a negligible variation with pressure under atmospheric conditions. The theoretical kinetics data were aligned to the experimental result, and the rate coefficient in the 200–400 K region can be approximated by a modified Arrhenius expression k(T) = 1.23 × 10^–10 × (T/298)^0.6 × exp(249 K/T) cm³ molecule^–1 s^–1.