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