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Radical cage effects: A method for measuring recombination efficiencies of secondary geminate radical cage pairs using pump-probe transient absorption methods

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Abstract

A method is reported for measuring the recombination efficiency of secondary geminate radical cage pairs. The procedure involves measuring the recombination efficiency for primary geminate recombination (Fc1) using pump-probe laser methods and measuring the “apparent” (or net) recombination efficiency (FcP) for all geminate pairs (primary and secondary) using steady-state irradiation methods. A mathematical relationship between FcP, Fc1, and Fc2 (where Fc2 is the recombination efficiency for secondary geminate recombination) is derived and demonstrated using the photolysis reactions of the [(CpR)Mo(CO)3]2 molecules, where CpR = η5-C5H4CH3 and η5-C5H4(CH2)2C(O)NCH3(CH2)nCH3 (n = 3, 8, 13, 18). As an example of the results obtained using the new method, it was found that Fc1 = 0.43 and Fc2 = 0.68 for the molecule with CpR = η5-C5H4CH2CH2N(CH3)C(O)(CH2)18CH3. The value of Fc2 decreased as the side-chain on the Cp ring got shorter; Fc2 is equal to 0.0 for the molecules with n = 3 and for CpR = η5-C5H4CH3. It is hypothesized that a longer side-chain prevents facile diffusion of the radicals out of the secondary cage, whereas the smaller side-chains permit more facile diffusion apart of the radicals. A general conclusion is that the reactions of large radicals in particular may be especially impacted by secondary geminate cage recombination.

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  1. Contribution from the Department of Chemistry, University of Oregon, Eugene, Oregon, 97403, USA

    Alan B. Oelkers & David R. Tyler

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Oelkers, A.B., Tyler, D.R. Radical cage effects: A method for measuring recombination efficiencies of secondary geminate radical cage pairs using pump-probe transient absorption methods. Photochem Photobiol Sci 7, 1386–1390 (2008). https://doi.org/10.1039/b804399j

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