We have used double-laser nanosecond laser flash photolysis to study the kinetics of bimolecular quenching of the lowest doublet excited state of the transient diphenylmethyl radical (2Ph2CH·) in acetonitrile at 25 °C. We chose a series of cyanoaromatic compounds as acceptor-type quenchers and a series of 4-substituted phenols as donor-type quenchers. The observed bimolecular quenching rate constants (k Q) are in the range 2 × 106–2 × 1010 M −1 s−1 for cyanoaromatics and 1 × 105–3 × 109 M −1 s−1 for phenols. The Hammett plots of the k Q values for these two series have positive and negative slopes, respectively, and establish the enhanced behavior of the photo-excited diphenylmethyl radical as both a donor and an acceptor. For cyanoaromatics, the trend in the dependence of 2Ph2CH·* quenching rate constants on the free energy change of electron transfer (ΔG el) coincides well with that of Rehm–Weller data on acceptor-type and donor-type quenchers of singlet excited states. An approximate fit of the k Q data for cyanoaromatics to the Agmon–Levine model of electron transfer gave a value of 4.5 kcal mol−1 for the free energy of activation at ΔG el = 0. For phenols, the trend in the dependence of k Q on the free energy change of electron transfer deviates significantly from that of cyanoaromatics data. This deviation was traced to the use of irreversible oxidation potentials of phenols to calculate ΔG el. No isotope effect is observed from substitution of phenolic hydrogen by deuterium, i.e., k Q,H/k Q,D ≈ 1. Phenolate ions quench 2Ph2CH·* with large rate constants, 1–2 × 1010 M −1 s−1, i.e., well in the limit of diffusion control.
Research on Chemical Intermediates – Springer Journals
Published: Jan 21, 2015
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