Fourier transform-electron paramagnetic resonance study of the photochemical reaction of acetone with 2-propanol
- Department of Chemistry, University of Massachusetts at Boston, Boston, Massachusetts 02125 (USA)
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 ({tau}{sub {ital d}}) between laser pulse and microwave pulse, with {tau}{sub {ital d}} ranging from nanoseconds to 100 {mu}s, 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 {tau}{sub {ital d}} 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{plus minus}3.7{times}10{sup 6} and {similar to}5{times}10{sup 7} s{sup {minus}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{plus minus}0.5{times}10{sup 6} s{sup {minus}1}. The decay rate constant {ital k}{sub {ital d}}{ital N}(0), where {ital N}(0) is the maximum radical concentration, is found to be 1.0{times}10{sup 5} s{sup {minus}1}. The rate constant for radical formation increases to 1.1{times}10{sup 7} s{sup {minus}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.
- DOE Contract Number:
- FG05-88ER75441
- OSTI ID:
- 5555948
- Journal Information:
- Journal of Chemical Physics; (USA), Vol. 95:2; ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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