Tak-ming Chiu

Date of Award


Degree Type


Degree Name

Doctor of Philosophy


Time-Resolved Flash Photolysis Electron Paramagnetic Resonance Spectroscopy has been used to probe the early electron polarization events in two different photochemical systems: (1) acetone in 2-propanol {lcub}acetone system{rcub} and (2) di-t-butyl peroxide in 2-propanol {lcub}(t-BuO)(,2) system){rcub}. On photolysis both systems give rise to the same radical species, the acetone ketyl radicals.;It was found that the observed polarization in these two systems are markedly different, reflecting different origins. In the acetone system where geminate triplet acetone ketyl radical pairs (G pairs) are expected, the polarization is of the E/A type as predicted by the Radical Pair Mechanism (RPM). In contrast, a much weaker polarization was observed in the (t-BuO)(,2) system where free random acetone ketyl radical pairs (F pairs) are formed. The observed persistence of a large initial polarization (even with a ten fold decrease in the free radical concentration) clearly confirms the dominance of the geminate pairs in the polarization process.;It is further suggested that even in the (t-BuO)(,2) system where F pairs are expected, "pseudo-geminate" radical pairs can be formed which can contribute to the observed polarization.;In measurements of the fast kinetics of photochemically produced radical transients, the measured output of the detection system is a convolution of the exciting pulse profile and the instrumental response with the undistorted kinetic response of the chemical system under study. However, the latter can only be obtained with a delta exciting pulse and a detection system of infinite bandwidth. We have chosen as a test system the spin-trapping reaction of acetone ketyl radicals by the cyclic spin trap 5,5-dimethyl-1-pyrroline-1-oxide (DMPO). The measured EPR signal was deconvolved using an assumed "theoretical" spectrometer response profile which was found qualitatively to agree well with the experimental profile obtained separately. The absolute bimolecular trapping rate constant obtained at 100 kHz field modulation (time resolution (TURN) 10 (mu)s) was found to be (1.2 (+OR-) 0.1) x 10('8) M('-1) s('-1). This compares favourably with the value of (1.1 (+OR-) 0.1) x 10('8) M('-1) s('-1) obtained at 2 MHz field modulation (time resolution (TURN) 1.5 (mu)s).



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