Tracking the Structural Evolution of 4-Aminobenzoic Acid in the Transition from Solution to the Gas Phase

Here, cryogenic ion mobility-mass spectrometry (cryo-IM-MS) is used to investigate intracluster proton transfer reactions of 4-aminobenzoic acid during the transition from solution to the gas phase. Previous studies have shown that protonation of the amine group of 4-aminobenzoic acid (4-ABAH⁺) is favored in solution (N-protomer), whereas protonation of the carboxylic acid group is favored in the gas phase (O-protomer). Results from cryo-IM-MS (80 K) studies of hydrated 4-ABAH⁺ ions, 4-ABAH⁺(H₂O), are interpreted as evidence that the proton transfer reaction occurs through a water bridge at n = 6 connecting the -NH₃⁺ and -COOH groups, that is, a Grotthuss mechanism. The weak binding energy of water molecules imposes limits for obtaining first-principles collisional cross sections (CCSs) of hydrated ions; consequently, candidate structures for 4-ABAH⁺(H₂O)_0-6 ions are derived by correlating experimental arrival-time distributions to theoretically determined CCSs. To our knowledge, these are the first first-principles determinations of CCS for hydrated ions. Apolar cosolvents, particularly acetonitrile, have been postulated to inhibit proton transfer by blocking the Grotthuss mechanism, but our data suggest that acetonitrile simply stabilizes the ammonium ion.