Cross-reaction Rate Constants Research Articles

Kinetics of the oxidation of ascorbic acid and substituted 1,2- and 1,4-dihydroxybenzenes by the hexacyanoruthenate(III) ion in acidic perchlorate media

The kinetics of oxidation of ascorbic acid and a series of substituted 1,2- and 1,4-dihydroxybenzene compounds (H2Q) by [Ru(CN)6]3– have been investigated in acidic perchlorate media. The inverse dependences of the rate constants on acid concentrations for 2,3-dicyano-1,4-dihydroxybenzene, 4,5-dihydroxybenzene-1,3-disulphonate, and ascorbic acid have been attributed to concurrent rate-determining pathways involving the one-electron oxidations of H2Q or HQ– by [Ru(CN)6]3– to the corresponding semiquinone or ascorbate radical intermediate. The cross-reaction rate constants have been correlated with the semiquinone or ascorbate reduction potentials in terms of the Marcus relationship to yield a [Ru(CN)6]3––[Ru(CN)6]4– electron self-exchange rate constant of (1.0 ± 0.8)× 105 dm3 mol–1 s–1. This is compared with those for other low-spin d5–d6 transition-metal complex couples and discussed in terms of the inner-sphere and solvent reorganization energies.

Rate constants for electron-transfer reactions of cuboidal [Mo 4S 4(edta) 2] 4−, 3−, 2− cluster complexes

Green [Mo 4 S 4 (edta) 2 ] 3− is obtained by reduction of the di-μ-sulfido Mo(V) dimer, [Mo 2 O 2 S 2 (edta)] 2− with NaBH 4 . Rate constants for cross reactions were obtained by stopped-flow and in one case conventional spectrophotometry

Rate constants for the reactions of CH3 radicals with C2H5, i-C3H7 and t-C4H9 radicals

Molecular modulation spectrometry has been used to measure the overall cross-reaction rate constants for the reactions of CH3 radicals with C2H5, i-C3H7 and t-C4H9 radicals at 308 K. The values of the rate constants obtained are (in cm3 mol–1 s–1): CH3+ C2H5, k=(28.5 ± 2.2)× 1012; CH3+ i-C3H7, k=(24.3 ± 2.3)× 1012; CH3+ t-C4H9, k=(12.8 ± 1.1)× 1012. Mutual reaction rate constants and absorption cross-sections for these radicals have also been measured in the same system. The values obtained are (k in cm3 mol–1 s–1 and σ in cm2 molecule–1): CH3, k=(20.8 = 0.8)× 1012 and σ=(30.8 ± 0.8)× 10–18; C2H5, k=(10.2 ± 0.9)× 1012 and σ=(1.89 ± 0.10)× 10–18; C3H7, k=(6.78 ± 0.58)× 1012 and σ=(3.89 ± 0.19)× 10–18; t-C4H9, k=(4.96 ± 0.20)× 1012 and σ=(7.47 ± 0.24)× 10–18. Combining these results with the appropriate disproportionation-combination ratios gives values for the corresponding combination and disproportionation rate constants, and cross-reaction and cross-combination ratios have been evaluated. An evaluation of the data for i-C3H7+ i-C3H7 leads to k=(6.69 ± 2.14)× 1012 cm3 mol–1 s–1 being recommended as the best value for the overall rate constant in the temperature range 300–400 K.

Kinetics studies of oxidations of catechol and l-dopa as mediated by IrCl i2−6 at polymer-coated glassy carbon electrodes

Rotating disk techniques were employed to measure the kinetics of the oxidations of catechol and l-dopa as mediated by IrCl 2− 6 at a glassy carbon rotating disk electrode coated with a mixture of two polycationic electrolytes. For both reactants the kinetic behavior matched that described by Andrieux et al. [1] for an “SR” case. Products of the cross-reaction rate constants and substrate partition coefficients derived from the data had values consistent with very similar reactivities of both substrates toward IrCl 2− 6 inside and outside of the coatings.

The oxidation of ascorbic acid by tris(2,2′-bipyridine) complexes of osmium(III), ruthenium(III) and nickel(III) in aqueous media: Applications of the Marcus cross-relation

The kinetics of the outer-sphere oxidation of L-ascorbic acid (H 2A) by tris(2,2′-bypyridine) and tris-(4,4′-dimethyl-2,2′-bipyridine) complexes of osmium(III), ruthenium(III), and nickel(III) have been investigated in acidic aqueous media at 9–25 °C. The observed inverse acid dependence, -d[ML 3+ 3]/dt = 2(k 1 + k 2K 1/[H +])(ML 3+ 3][H 2A], suggest a mechanism involving H 2A and HA − pathways, for which the specific rate and activation parameters have been determined. These and previously reported rate constants for cross-reactions involving ascorbate species are correlated in terms of the Marcus relationships. Estimates of the self-exchange parameters for the one-electron couples: H 2A/H 2A +·, k 11 = 2 × 10 3 M − s −1; HA −/HA·, k 11 = 1 × 10 5 M − s −1; A 2−/A−·, k 11 = 2 × 10 5 M − s −1], based on ascorbate radical reduction potentials calculated from proton and redox equilibrium constant, are presented.