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"Direct Observation of the Nuclear Motion during Ultrafast Intramolecular Proton Transfer" S. Lochbrunner, C. Schriever, and E. Riedlein "Hydrogen-Transfer Reactions", J. T. Hynes, J. P. Klinman, H.-H. Limbach, and R. L. Schowen, eds. (Wiley-VCH, Weinheim, 2006), 349-375 Abstract: The evolution of the ultrafast excited state intramolecular proton transfer (ESIPT) and the associated motion of the nuclei can be observed in real time by transient absorption experiments with a time resolution of 30 fs. For many compounds containing a H-chelate ring similar dynamics is found. The ultrafast ESIPT proceeds as a ballistic nuclear wavepacket motion. The wavepacket stays confined during the whole process and moves from the Franck-Condon region to the product minimum within a time period of about 50 fs given by the inertia of the involved nuclei. Subsequently, a ringing of the molecule in as many as four vibrational modes is observed. They are coherently excited by the ESIPT and reflect the structural changes during the reaction. From the corresponding signal contributions the nuclear degrees of freedom taking part in the process are identified, resulting in a detailed description of the reaction coordinate. The slope of the PES at the Franck-Condon point corresponds to a relaxation of the chromophore. However, the further evolution is dominated by an in-plane bending motion of the molecular skeleton, resulting in a reduction of the donor-acceptor distance. An electronic configuration change occurs when this distance is sufficiently shortened, and the bonds are altered from the enol to the keto configuration. Afterwards the molecular geometry relaxes along several skeletal modes towards the keto minimum of the S_1 state. The proton itself stays at its local potential minimum and is passively shifted from the enol to the keto site by the skeletal motions during the transfer. The irreversibility of the transfer results first of all from its multidimensional character. This model accounts for the observation of a ballistic wavepacket motion, the coherent excitation of skeletal in-plane vibrations and the lacking excitation of high frequency modes. The topology of the S_1 PES is given by the interaction between the enol and the keto configuration which is extremely sensitive to the donor-acceptor distance. It can be related to the correlation between the strength of a hydrogen bond and the corresponding donor acceptor distance. The results indicate that also in the ground state skeletal modes and intermolecular motions modulating the proton transfer barrier govern the kinetics. In the case of intramolecular double proton transfer a wavepacket motion is found which depends via the excess energy on the branching ratio between concerted double and single proton transfer. This demonstrates that the coherent wavepacket dynamics in ESIPT molecules is specific for the reaction path and is driven by the ESIPT itself. BMO authors (in alphabetic order): Stefan Lochbrunner Eberhard Riedle Christian Schriever |
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Letzte Änderung: 2016-09-14 13:34
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