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.
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