Franck-Condon proton transfer

In an outer-sphere electron transfer the actual electron transfer is iso-energetic and takes place in 10 16 s, during which time, by the Franck-Condon principle, the heavier nuclei do not move. Since protons are known to tunnel (Bell et ah, 1956 Bell, 1973 Caldin, 1969 Lewis, 1975) and since the electron is 2000 times lighter than the proton, it is almost certain that in the actual electron transfer the electron tunnels. For instance, in an isotopic mixing experiment (16) the barrier to the reaction (k = 8 m-1 s-1) lies in achieving the... 

In the second chapter, Appleby presents a detailed discussion and review in modem terms of a central aspect of electrochemistry Electron Transfer Reactions With and Without Ion Transfer. Electron transfer is the most fundamental aspect of most processes at electrode interfaces and is also involved intimately with the homogeneous chemistry of redox reactions in solutions. The subject has experienced controversial discussions of the role of solvational interactions in the processes of electron transfer at electrodes and in solution, especially in relation to the role of Inner-sphere versus Outer-sphere activation effects in the act of electron transfer. The author distils out the essential features of electron transfer processes in a tour de force treatment of all aspects of this important field in terms of models of the solvent (continuum and molecular), and of the activation process in the kinetics of electron transfer reactions, especially with respect to the applicability of the Franck-Condon principle to the time-scales of electron transfer and solvational excitation. Sections specially devoted to hydration of the proton and its heterogeneous transfer, coupled with... 

In summary, the spectroscopy of jet-cooled BBXHQ, and of its thiazole analogue BBTHQ, likely reveals two kinds of potentials for the excited state intramolecular proton transfer reaction. The first case is characterized by a small reaction enthalpy and by a small but significant energy barrier for the reaction. In the second case, the reaction enthalpy exceeds 1.5 kcal/mole there is no energy barrier and the excited proton—transferred molecule may be directly reached by optical excitation, albeit with small Franck-Condon factors for non-vertical transitions. 

Useful information can be obtained from the experimental determination of the activation energy and preexponential factor separately, since they are determined by different factors. According to the quantum mechanical theory, the value of the preexponential factor in proton transfer reactions is determined mainly by the characteristics of the chemical bond to be broken, whereas the activation energy is determined by the Franck-Condon potential barrier associated with solvent polarization. Experimental investigation of the hydrogen evolution reaction from the ions and CHaCNH" in water and in... 

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