posted on 2024-01-23, 16:05authored byYuika Okura, Garrett D. Santis, Keisuke Hirata, Vasilios S. Melissas, Shun-ichi Ishiuchi, Masaaki Fujii, Sotiris S. Xantheas
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.