"Ultrafast double proton transfer: symmetry breaking wavepacket motion and absence of deuterium isotope effect"S. Lochbrunner, K. Stock, C. Schriever, and E. Riedle
Ultrafast Phenomena XIV, T. Kobayashi, T. Okada, T. Kobayashi, K. Nelson, S. De Silvestri, eds. (Springer-Verlag, Berlin, 2005), 491 - 495
The double proton transfer of [2,2'-Bipyridyl]-3,3'-diol is investigated by UV-visible pump-probe spectroscopy with 30 fs time resolution. The first step of the sequential transfer proceeds with a 40 fs delay and the second step with a time constant of 10 ps. The concerted transfer of the two protons takes 50 fs. Both transfer paths are accompanied by characteristic wavepacket motions, a bending motion for the sequential transfer and symmetric stretching motions for the concerted. The nontotally-symmetric bending vibration cannot be excited by the optical transition. This demonstrates that the reactive process itself and not only the optical excitation drives the vibrational motions. We show by absence of a deuterium isotope effect on the proton transfer that the ESIPT dynamics is entirely determined by the skeletal modes and that it should not be described by tunneling of the protons.
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