Isotope excitation energy transfer between

Schemes for laser isotope enrichment all have some general requirements in common. Firstly, the isotomers of the starting material must have some discrete spectral absorptions that do not overlap. Secondly, the laser has to be sufficiently tunable and monochromatic to excite only one of these isotomers. Thirdly, a chemical or photochemical process is needed that acts selectively on the excited species, preferably to lock up the selected isotope in a new stable species. Finally, the effects of processes that destroy the isotopic selectivity must be minimized. One of the chief ways in which isotopic selectivity can be lost is by transfer of the excitation energy from one isotomer to another. The interplay between reactions of selectively excited species and inelastic collisions is a recurring theme in this chapter (see Section 1.3.4). Because their energy states are very close, the exchange of energy between different isotomers is nearly resonant and is likely to be rapid. In isotope enrichment experiments, these processes are detrimental, since they destroy the isotopic selectivity of the initial excitation.

It must be remembered, furthermore, that the identification of the H-atom translocation mode is not equivalent to the identification of the reaction coordinate. We have attributed the absence of a deuterium isotope effect on the excited-state H-atom transfer (for the 10-ps component in hypericin and hypo-crellin A) to the zero-point energy in the proton coordinate lying above the barrier, with the H-atom being effectively delocalized between the two oxygen atoms. Consequently, the reaction coordinate for the excited-state H-atom transfer cannot be identified with the proton coordinate, and it must be concluded that other intramolecular motions are in fact responsible for the process. Temperature-dependent measurements indicate that these motions are extremely low amplitude, Ea 0.05 kcal/mol for hypericin [37]. Because the nature of this motion is not yet identified, we refer to it as the skeleton coordinate [48, 71, 82]. We propose that it is the time scale for this latter conformational change... 

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