Atmospheric oxidation of imine derivative of piperazine by OH radical

Amines are emitted to the Earth's atmosphere by several biogenic and anthropogenic sources. One such source, expected to increase in importance in the coming decades, is Carbon-Capture (CC), which often employs amine solvents as CO₂ filters. Given that atmospheric oxidation of amines have the potential to produce nitrosamines (R₁R₂NNO) and nitramines (R₁R₂NNO2), known carcinogenic compounds, several chemical kinetics studies have investigated these reactions aiming to assess the impact on air quality from CC emissions. Piperazine is a widely employed CC amine solvent whose reaction with OH radical, the main atmospheric oxidant, has been the target of previous works, revealing a low yield of hazardous products. However, almost nothing is known about the fate of the major oxidation product, the cyclic imine 1,2,3,6-tetrahydropyrazine (THP). In fact, only a few studies focused on the atmospheric chemistry of imines in general, despite consistently appearing as major products of amine oxidation. In this work, we employed quantum chemistry and theoretical kinetics methods to investigate the mechanism and kinetics of reaction between THP and OH radical. Our findings predict that this reaction has a low, but not negligible potential to produce nitrosamines and nitramines, with a maximum yield of ~18% under high NO_x conditions. The major reaction channels involve the formation of a second imine functional group, leading to the diimines 2,3-dihydropyrazine and 2,5-dihydropyrazine. Our calculations also revealed two new oxidation pathways, both involving fast C-C bond scissions. One of these pathways produce an isocyanate (RN=C=O), which is also potentially toxic. While this channel is minor for THP + OH radical (maximum yield of 14%), we argue that it could be more important during the OH radical-initiated oxidation of other imines relevant to the atmosphere.