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. Here we explore the reactivity of adipic acid, an analogue to the alpha-pinene oxidation product pinic acid, upon complexation with ammonium. Collision-induced dissociation mass spectra and mass-selective vibrational spectra show that relatively mild activation, consistent with breaking hydrogen bonds, yields (adipic acid)H+ and neutral ammonia. This is consistent with a higher proton affinity for adipic acid than ammonia that we trace to a specific structural motif in which both protonated carboxylic acid carbonyl groups combine to form a basic site that supports an additional bridging proton. Further mild collisional activation yields sequential loss of two water molecules, similar to the 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. We propose a mechanism that explains this reactivity and discuss possible atmospheric implications of these observations.