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Excited-State Diproton Transfer in [2,2′-Bipyridyl]-3,3′-diol: the Mechanism Is Sequential, Not Concerted

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journal contribution
posted on 30.07.2009, 00:00 by Felix Plasser, Mario Barbatti, Adélia J. A. Aquino, Hans Lischka
The excited-state mono- and diproton transfer has been investigated in the S1 state of [2,2′-bipyridyl]-3,3′-diol using the quantum mechanical resolution-of-identity second-order approximate coupled-cluster (RI-CC2) and time-dependent density functional theory (TDDFT) methods. Static investigation of stationary points and scans of the ππ* and nπ* energy surfaces have been performed. These calculations show that the concerted diproton transfer in S1 proceeds along a ridge thus making this process highly unlikely since it will stabilize toward the unsymmetrical monoproton transfer. A small energy barrier of about 0.11 eV (RI-CC2 result) between the mono- and diketo structures is obtained allowing rapid continuation of the proton transfer to the diketo form. On-the-fly dynamics simulations performed at the RI-CC2 level confirm this picture. The first proton transfer step is so fast (7 fs) that it probably cannot be resolved by experimental techniques. Important participation of the nπ* state is predicted. The present results shed a completely new light on the interpretation of the experimental results. The simulations clearly show that what has been experimentally determined as concerted transfer is in fact a combination of two sequential proton transfers separated by a small delay below the present experimental resolution. Concerning the second step of the sequential proton transfer the dynamics calculations indicate the existence of a highly dynamic system. Both the forward and reverse reactions of a monoketo/diketo equilibrium were found within the 300 fs period of the simulation. Environmental effects will certainly lead to a substantial cooling of the initially hot molecule and a concomitant decrease in the monoketo/diketo conversion rates, which will result in the experimentally observed overall time scale of 10 ps for the second proton transfer step.

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