In methanol/water, dpph(•) bleaching (519 nm) by quercetin, QH(2), exhibits biphasic kinetics. The dpph(•) reacts completely with the quercetin anion within 100 ms. Subsequent slower bleaching involves solvent and QH(2) addition to quinoid products. The fast reaction is first-order in dpph(•) but only ca. 0.38 order in [QH(2)]. This extraordinary nonintegral order is attributed to reversible formation of π-stacked {QH(-)/dpph(•)} complexes in which electron transfer to products, {QH(•)/dpph(-)}, is slow (k(ET) ≈ 10(5) s(-1)).
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| http://dx.doi.org/10.1021/ol2019086 | DOI Listing |
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