Atmospheric Oxidation of Imine Derivative of Piperazine Initiated by OH Radical

Abstract

The cyclic imine 1,2,3,6-tetrahydropyrazine (THPyz) has been observed to be the major atmospheric photo-oxidation product of piperazine, a widely used solvent in carbon-capture technology, yet little is known about its own fate. Very few studies have focused on the atmospheric chemistry of imines in general, despite consistently appearing as major products of amine oxidation. In this work, we explore the reaction mechanism of THPyz oxidation initiated by OH radicals, as well as the fate of the first-generation C-centered (alkyl) and N-centered (aminyl) radical products, with quantum chemistry and theoretical kinetics methods. We predict that the major initial reaction steps involve H-abstraction from a carbon adjacent to the amine nitrogen, leading to subsequent formation of a second imine function via the O2-addition/HO2-elimination pathway. Calculated yields of potentially hazardous products are low but non-negligible. Typically carcinogenic compounds, nitrosamines and nitramines, are expected to have a maximum yield of ∼7% and ∼11%, respectively, under high NOx regimes, considering the uncertainties in the obtained rates. Low yield (1–14%) of an isocyanate is also predicted, formed in a channel following initial H-abstraction from the imine carbon. The aminyl radical formed from OH radical addition to the imine carbon undergoes fast C–C bond scission, leading to an imidic acid. These pathways are minor for OH radical oxidation of THPyz but could be more competitive for other Schiff bases.