engleski

Reaction of aquachromium(II) ions with 2, 2'-bipyrimidine. A pulse radiolysis study

Chromium(II) ion was generated from hexaamminochromium(III) and hydrated electrons in the presence of 2, 2'-bipyridine (bpm). The reaction of hexaamminochromium(III) with e- (aq) has k = 4.9 E10 (1/Ms) at 25 °C. The hexaaminochromium(II) so produced loses all the ammonia ligands with k > 1 E5 (1/s). A product complex with an absorption maximum at around 350 nm was formed in the reaction of chromium(II) ion and bpm. In the presence of a moderate excess of bpm an equilibrium is established on the 0.1 ms timescale with K(app) = k(f) [bpm]/k(b). Both the forward (k(f)) and back (k(b)) reactions are pH dependent in the range 3.5 to 8.6. The UV spectra of the reaction product(s) indicate that the absorbing product contains chromium(III) and bipyrimidine radical ligand. The first acid dissociation constant of chromium(II) ion was determined as 1.4 E5 M. On the 0.1 ms timescale, chromium(III) and bipyrimidine radical ligand is stable but undergoes changes at longer times. The protonated bpm molecule is reduced by 1-hydoxy-1-methylethyl radicals, yielding a diprotonated bpm radical. No reaction was observed between hexaaquochromium(III) and diprotonated bpm radical in pulse radiolysis experiments, which places the rate constant for this reaction at < 2 E6 (1/Ms).

excited states ; pulse radiolysis ; radicals

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