The Solvent Cage Effect: Is There a Spin Barrier to Recombination of Transition Metal Radicals?

This investigation explored whether there is a spin barrier to recombination of first- and second-row transition metal-centered radicals in a radical cage pair. To answer this question, the recombination
efficiencies of photochemically generated radical cage pairs (denoted as FcP) were measured in the presence
and absence of an external heavy atom probe. Two methods were employed for measuring the cage effect.
The first method was femtosecond pump−probe transient absorption spectroscopy, which directly measured
FcP from reaction kinetics, and the second method (referred to herein as the “steady-state” method) obtained
FcP from quantum yields for the radical trapping reaction with CCl4 as a function of solvent viscosity. Both
methods generated radical cage pairs by photolysis (λ = 515 nm for the pump probe method and λ = 546
nm for the steady-state method) of Cp‘2Mo2(CO)6 (Cp‘ = η5-C5H4CH3). In addition, radical cage pairs
generated from Cp‘2Fe2(CO)4 and Cp*2TiCl2 (Cp* = η5-C5(CH3)5) were studied by the steady-state method.
The pump−probe method used p-dichlorobenzene as the heavy atom perturber, whereas the steady-state
method used iodobenzene. For both methods and for all the radical caged pairs investigated, there were
no observable heavy atom effects, from which it is concluded there is no spin barrier to recombination.