The mechanism of the photochemistry of p-benzoquinone (BQ) in aqueous solution has been investigated by using optical flash photolysis. The results agree very well with the mechanism developed from the EPR study in the preceding paper. The second-order rate constant for oxidation of water by triplet BQ at pH 7 is estimated to be ~2 X 10s M'1 s'1, while a value of (6.5 ± 0.3) X 107 M'1 s'1 is found for the secondary formation of the semiquinone radical (BQH") according to BQ + BQ-OH" -BQH" + HO-BQ. The Arrhenius plot for this reaction gives an activation energy of 37 ± 2 kj mol"1. Also the rate constant of the reaction 2BQ'" + 2H+ -* BQH2 + BQ is found to be (1.5 ± 0.1) X 10s M"1 s'1. The yield of semiquinone increases with increase in pH. This is explained by a decrease in the redox potential of the reaction OH" + H+ + e" -H20 with increase in pH which makes it more favorable for 3BQ* to oxidize H20. The quantum yield for BQ"' formation at pH 7 was found to be 0.47 ± 0.04, independent of the initial concentration of BQ in the concentration range 0.3-1.0 mM.
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