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Several methods are employed to find out if there exists a spin barrier to the recombination of the first- and second-row transition metal-centered radicals in a radical cage pair, which are normally photochemically generated. The techniques used proves that no heavy atom effects take place, which then finally helps in concluding that the recombination doesnEt have to actually face any kind of spin barrier.

The photochemistry of the [(CpR)Mo(CO)3]2 molecules, where CpR = η5-C5H4(CH2)2C(O)NCH3(CH2)nCH3 (n = 3, 8, 13, and 18), was examined using femtosecond pump—probe transient absorption spectroscopy. The goal of this study was to investigate the importance of radical size and mass on the dynamics and efficiency of geminate radical—radical recombination. The femtosecond results demonstrated the lack of any size/mass dependence of the recombination efficiency. This finding contrasts with results from a prior study that did find a size/mass dependence using a steady-state photochemical technique. To explain these conflicting results, it is proposed that the femtosecond pump—probe results are a measurement of the efficiency of primary geminate recombination whereas the steady-state method results are a measurement of the sum of primary and secondary geminate recombination efficiencies. The size/mass dependence is evident in the latter because secondary geminate recombination is a slower diffusive recombination process and therefore depends on the steric properties of the radicals. Although the existence of primary and secondary recombination channels is often taken for granted, experimental differentiation of primary and secondary caging has proven to be difficult because it is not possible for a single experimental technique to span the entire timescale for recombination of a radical cage pair and adequately resolve these recombination pathways. [ABSTRACT FROM AUTHOR]

Femtosecond pump-probe studies of the photodissociation and subsequent radical cage pair recombination dynamics of the organometallic dimer [Cp'Mo(CO)3]2 (Cp' = eta5-C5H4CH3) are reported. The dynamics following photodissociation were studied in numerous noncoordinating hydrocarbon solvents. The results indicate that primary geminate recombination occurs on an ultrafast time scale (tau approximately 5 ps) and the efficiency of cage escape is inversely proportional to solvent viscosity. Investigation of the time-dependent anisotropy in this system allowed for an estimate of the rotational correlation time of the radical fragments (tau approximately 5-25 ps). Comparison of the rates of rotational motion with the population kinetics shows that the primary solvent cage dynamics and recombination efficiency are controlled by radical diffusion and not by radical rotation.