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"A Comprehensive Microscopic Picture of the Benzhydryl Radical and Cation Photo-Generation and Interconversion through Electron Transfer" C. F. Sailer, B. P. Fingerhut, S. Thallmair, C. Nolte, J. Ammer, H. Mayr, I. Pugliesi, R. de Vivie-Riedle, E. RiedleChemPhysChem 14, 1423 - 1437 (2013). Abstract: Bond cleavage and bond formation are central to organic chemistry. Carbocations play a key role in our understanding of nucleophilic substitution reactions that involve both processes. The precise understanding of the mechanism and dynamics of the photo-generation of carbocations and carbon radicals is therefore an important quest. Especially the role of electron transfer for the generation of carbocations from the radical pair is still unclear. We present a quantitative femtosecond absorption study with ultrabroad probing on selected donor- and acceptor-substituted benzhydryl chlorides irradiated with 270 nm (35 fs) pulses. The ultrafast bond cleavage within 300 fs is almost exclusively homolytic, leading to a radical pair. The carbocations observable in the nanosecond regime are generated from these radicals by electron transfer from the benzhydryl to the chlorine radical within the first tens of picoseconds. Their concentration is reduced by geminate recombination within hundreds of picoseconds. In moderately polar solvents this depletion almost extinguishes the cation population, in highly polar solvents free ions are still observable on the nanosecond time scale. The explanation of our experimental findings requires to account for the microscopic realm of the intermediates, including their spatial and environmental distributions. We combine distance dependent electron transfer described by Marcus theory with Smoluchowski diffusion. The depletion of the radical pair distribution at small distances causes a temporal increase of the mean distance and the observed stretched exponential electron transfer. A close accord with the experiment can only be reached for a broad distribution of the nascent radical pairs. The increase in the inter-radical and inter-ion pair distance is directly measured as a shift of the UV-Vis absorption of the products. Our results demonstrate that at least for aprotic solvents traditional descriptions of reaction mechanisms based on the concept of contact and solvent-separated pairs have to be reassessed. BMO authors (in alphabetic order): Igor Pugliesi Eberhard Riedle Christian Sailer Sebastian Thallmair Order Hardcopy  WWW-Version |
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Letzte Änderung: 2016-09-14 13:34
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