Switching of Protonation Sites in Hydrated Nicotine via a Grotthuss Mechanism
Yuika Okura, Garrett D. Santis, Keisuke Hirata, Vasilios S. Melissas, Shun‐ichi Ishiuchi, Masaaki Fujii, Sotiris S. Xantheas
Abstract
The switching of the protonation sites in hydrated nicotine, probed by experimental infrared (IR) spectroscopy and theoretical ab initio calculations, is facilitated via a Grotthuss instead of a bimolecular proton transfer (vehicle) mechanism at the experimental temperature ( T = 130 K) as unambiguously confirmed by experiments with deuterated water. In contrast, the bimolecular vehicle mechanism is preferred at higher temperatures ( T = 300 K) as determined by theory. The Grotthuss mechanism for the concerted proton transfer results in the production of nicotine’s bioactive and addictive pyrrolidine-protonated (Pyrro-H + ) protomer with just 5 water molecules. Theoretical analysis suggests that the concerted proton transfer occurs via hydrogen-bonded bridges consisting of a 3 water molecule “core” that connects the pyridine protonated (Pyri-H + ) with the pyrrolidine-protonated (Pyrro-H + ) protomers. Additional water molecules attached as acceptors to the hydrogen-bonded “core” bridge result in lowering the reaction barrier of the concerted proton transfer down to less than 6 kcal/mol, which is consistent with the experimental observations.