Unusual Confinement-Induced Basicity and Proton-Mediated CH Activity of an Adipic Acid-Ammonium Cluster

Organic acids play an important role in atmospheric chemistry, particularly in the formation of aerosol particles. They are often detected with hydronium, ammonium, or nitrate chemical ionization sources, which are generally soft ionization sources but may cause fragmentation. Here we explore the reactivity of adipic acid, an analogue to the alpha-pinene oxidation product pinic acid, upon complexation with ammonium. Mass spectra and mass-selective vibrational spectra show that even mild activation, consistent with normal operation of an electrospray ionization source, yields (adipic acid)H+ and neutral ammonia. This is traced to a specific structural motif in which both protonated carboxylic acid carbonyl groups combine to form a site that is locally more basic than ammonia. Further mild collisional activation yields sequential loss of two water molecules, the known behavior of carboxylic acids in superacids, necessitating abstraction of at least one hydrogen from a CH group. Deuterium labeling experiments confirm that the second step indeed involves CH hydrogen atoms. Comparison of vibrational spectra and quantum chemical calculations allows us to assign structures for each step, identifying several ring structures but notably not forming the minimum energy structure upon the first loss of water. ESI of a range of linear dicarboxylic acids shows this to be a common feature of this class of organic acids. We propose a mechanism that explains this reactivity and discuss possible atmospheric implications of these observations.