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First Observation of Ultrafast Intramolecular Proton Transfer Rate between Electronic Ground States in Solution

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journal contribution
posted on 10.05.2012, 00:00 by Yuichi Masuda, Tomoko Nakano, Midori Sugiyama
Despite the importance of ultrafast (time scale exceeding 10–11 s) intramolecular proton transfer (PT) events between electronic ground states in solution, experimental determination of the rates of such reactions has not yet been accomplished because of the limitations of the utilized methods. The objective of this study was to evaluate the PT rates of intramolecular O···H···O hydrogen-bonded systems in solution through the 1H spin-lattice relaxation times of the hydroxyl protons, induced by the 1H–17O dipolar interactions (T1dd(OH)), taking into account the contribution of the OH reorientational motion to T1dd(OH). Solutions of the benzoic acid dimer (BA dimer), 1-benzoyl-6-hydroxy-6-phenylfulvene (Fulvene), and dibenzoylmethane (DBM) were chosen as test systems. For Fulvene in CCl4, the PT time, τPT, was deduced to be 7 × 10–11 s. In the case of the BA dimer in CCl4, the τPT value was considerably greater than the OH reorientational correlation time, τR(OH) = 4.3 × 10–11 s. In contrast, the experimental results for DBM in CCl4 indicated that the proton is located about midway between the two oxygen atoms, that is, the PT potential energy surface is a single well or a double well with a PT barrier near or below the zero-point energy.