Radical cage effects: a method for measuring recombination efficiencies of secondary geminate radical cage pairs...

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 = eta(5)-C(5)H(4)CH(3) and eta(5)-C(5)H(4)(CH(2))(2)C(O)NCH(3)(CH(2))(n)CH(3) (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 = eta(5)-C(5)H(4)CH(2)CH(2)N(CH(3))C(O)(CH(2))(18)CH(3). 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 = eta(5)-C(5)H(4)CH(3). 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.