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A kinetic ESR study(flow-stop method)was performed on the redox reaction betweenascorbic acid(1)and 4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy radical(6)in oxygen-free aqueoussolution.Both 6 and the oxidation intermediate of 1-ascorbate radical anion(3)were observedsimultaneously in alkaline solution(pH 9-11)and their decay rates monitored by ESR spectroscopyafter stopping the flow.Based on the kinetic data a mechanism involving two step one-electrontransfer reactions and the disproportionation of 3 is proposed.The rate constants at 25℃ are 1.95,1.42×10~4 and 2.22×10~6 M~(-1)s~(-1) with activation energies 39.4,32.5 and 24.7 kJ·mole~(-1) and activa-tion entropies -29.9,-17.6 and -13.6 e.u.respectively.The results indicate that the formation ofstrictly oriented activation complexes may be involved during these reactions.
A kinetic ESR study (flow-stop method) was performed on the redox reaction between ascorbic acid (1) and 4-hydroxy-2,2,6,6-tetramethylpiperidinyl- 6 and the oxidation intermediate of 1-ascorbate radical anion (3) were observedsimultaneously in alkaline solution (pH 9-11) and their decay rates monitored by ESR spectroscopy after stopping the flow. Based on the kinetic data a mechanism involving two step one-electrontransfer reactions and the disproportionation of 3 are proposed. The rate constants at 25 ℃ are 1.95,1.42 × 10 -4 and 2.22 × 10 ~ 6 M -1 s -1 with activation energies 39.4, 32.5 and 24.7 kJ · Mole ~ (-1) and activa-tion entropies -29.9, -17.6 and -13.6 eurespectively. The results indicate that the formation of strictly oriented activation complexes may be involved during these reactions.