Fourier transform-electron paramagnetic resonance study of the photochemical reaction of acetone with 2-propanol

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Abstract
Fourier transform-electron paramagnetic resonance (FT-EPR) was used to study the pulsed-laser induced reduction of acetone with 2-propanol. By monitoring the EPR signal of the acetone ketyl radical as function of delay time (τd) between laser pulse and microwave pulse, with τd ranging from nanoseconds to 100 μs, information was obtained on the kinetics of free radical formation and decay. The time evolution of the signal also gave an insight into the chemically induced dynamic electron polarization (CIDEP) mechanisms that affect signal amplitudes. It was found that the spectra obtained with τd settings ranging from 0 to 400 ns contain a dispersive signal contribution that is due to the presence of spin-correlated radical pairs (SCRP) at the time of the microwave pulse. For acetone(D6) in 2-propanol(D8) the rate constants of formation and decay of the SCRP are found to be 7.5±3.7×106 and ∼5×107 s−1, respectively. The SCRP lifetime in 2-propanol(D8) at room temperature corresponds to what would be expected for diffusion controlled cage escape. The rate constant for ketyl(D7) radical formation is found to be 5.8±0.5×106 s−1. The decay rate constant kdN(0), where N(0) is the maximum radical concentration, is found to be 1.0×105 s−1. The rate constant for radical formation increases to 1.1×107 s−1 for the acetone(H6)/2-propanol(H8) system. The pronounced isotope effect on radical formation is consistent with a transition state involving C–H bond stretching. The FT-EPR spectra show the effects of radical pair CIDEP. However, a detailed analysis of signal growth and decay kinetics shows that there is no triplet mechanism CIDEP contribution. This finding is in agreement with the relative magnitudes of acetone triplet spin lattice relaxation and radical formation rate constants. The results also demonstrate that the rate constant for hydrogen abstraction is triplet sublevel independent.