Sprayed water microdroplets containing dissolved pyridine spontaneously generate pyridyl anions.

Abstract

The anion of pyridine, C5H5N−, has been thought to be short lived in the gas phase and was only previously observed indirectly. In the condensed phase, C5H5N− is known to be stabilized by solvation with other molecules. We provide in this study striking results for the formation of isolated C5H5N− from microdroplets of water containing dissolved pyridine observed in the negative ion mass spectrum. The gas-phase lifetime of C5H5N− is estimated to be at least 50 ms, which is much longer than previously thought. The generated C5H5N− captured CO2 molecules to form a stable (Py-CO2)− complex, further confirming the existence of C5H5N−. We propose that the high electric field at the air–water interface of a microdroplet helps OH− to transfer an electron to pyridine to form C5H5N− and the hydroxyl radical •OH. Oxidation products of the Py reacting with •OH are also observed in the mass spectrum recorded in positive mode, which further supports this mechanism. The present study pushes the limits of the reducing and oxidizing power of water microdroplets to a new level, emphasizing how different the behavior of microdroplets can be from bulk water. We also note that the easy formation of C5H5N− in water microdroplets presents a green chemistry way to synthesize value-added chemicals.