Long-distance proton transfer induced by a single ammonia molecule: Ion mobility mass spectrometry of protonated benzocaine reacted with NH₃

Long-distance proton transfer is a ubiquitous phenomenon in chemical and biological systems. Two mechanisms of proton transfer in solids are well established; the Grotthuss mechanism (proton-relay) and the vehicle mechanism. Previously, intramolecular proton transfer has been extensively studied in the gas phase to understand the proton transfer mechanism microscopically. However, only the Grotthuss mechanism was proposed so far for intramolecular proton transfer. Here we show the first evidence for long-distance proton transfer (ca. 0.7 nm) via the vehicle mechanism in a gas-phase protonated molecule. Using ion mobility mass spectrometry, we observed that intramolecular proton transfer between two structural isomers with different protonation sites of protonated benzocaine (BC; p-NH₂C₆H₄COOC₂H₅) is induced by a single NH₃ molecule. In combination with theoretical calculations of the reaction pathway for the bimolecular reaction of BC·H⁺ + NH₃, it was concluded that intramolecular proton transfer to produce the O-protomer (protonated BC at the CO group) proceeds in the N-protomer (protonated BC at the NH₂ group) by NH₃ coordination. In the calculated pathway, the NH₄⁺ ion formed by proton transfer from the NH₂ group of the N-protomer to NH₃ donates a proton to the CO group after hopping on the benzene ring of BC. Our results demonstrate that we can investigate microscopically not only the Grotthuss mechanism but also the vehicle mechanism using gas-phase spectroscopic methods.