Oxygenate-Induced Tuning of Aldehyde-Amine Reactivity and Its Atmospheric Implications

Atmospheric aerosols often contain a significant fraction of carbon-nitrogen functionality, which makes gas-phase aldehyde-amine chemistries an important source of nitrogen containing compounds in aerosols. Here we use high-level ab initio calculations to examine the key determinants of amine (ammonia, methylamine, and dimethylamine) addition onto three different aldehydes (acetaldehyde, glycolaldehyde, and 2-hydroperoxy acetaldehyde), with each reaction being catalyzed by a single water molecule. The model aldehydes reflect different degrees of oxygenation at a site adjacent to the carbonyl moiety, the α-site, and represent typical oxygenates that can arise from atmospheric oxidation especially under conditions where the concentration of NO is low. Our results show that the reaction barrier is influenced not only by the nature of the amine but also by the nature of the aldehyde. We find that, for a given amine, the reaction barrier decreases with increasing oxygenation of the aldehyde. This observed trend in barrier height can be explained through a distortion/interaction analysis, which reveals a gradual increase in internal hydrogen bonding interactions upon increased oxygenation, which, in turn, impacts the reaction barrier. Further, the calculations reveal that the reactions of methylamine and dimethylamine with the oxygenated aldehydes are barrierless under catalysis by a single water molecule. As a result, we expect these addition reactions to be energetically feasible under atmospheric conditions. The present findings have important implications for atmospheric chemistry as amine-aldehyde addition reactions can facilitate aerosol growth by providing low-energy neutral pathways for the formation of larger, less volatile compounds, from readily available smaller components.