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 (F_c1) using pump–probe laser methods and measuring the “apparent” (or net) recombination efficiency (F_cP) for all geminate pairs (primary and secondary) using steady-state irradiation methods. A mathematical relationship between F_cP, F_c1, and F_c2 (where F_c2 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 F_c1 = 0.43 and F_c2 = 0.68 for the molecule with CpR = η⁵-C₅H₄CH₂CH₂N(CH₃)C(O)(CH₂)₁₈CH₃. The value of F_c2 decreased as the side-chain on the Cp ring got shorter; F_c2 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.