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用流动-停止ESR技术研究了4-羟基-2,2,6,6-四甲基哌啶-1-氧自由基(6)对维生素C(1)在水溶液中的氧化反应,发现在碱性水溶液中(pH=9～11),6与1的氧化反应中间体—维生素C负离子基(3)可以同时检测到。动力学的测定结果表明,该反应包含着两步电子转移反应和3的歧化反应。25℃时,三步反应的速率常数分别为1.95,1.42×10~4,2.22×10~6Ms~(-1)活化能分别为39.4,32.5,24.7kJmol~(-1),活化熵分别为-29.9,-17.6,-13.6e.u.电子转移反应中很低的速率常数和很负的活化熵表明反应中可能形成了严格定向的活化络合物。
The oxidation of vitamin C (1) in aqueous solution by 4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl radical (6) was studied by flow-stop ESR technique. In aqueous solution (pH = 9 ~ 11), 6 and 1 of the oxidation reaction intermediates - vitamin C anion (3) can be detected simultaneously. Kinetic determination results show that the reaction contains two-step electron transfer reaction and 3 disproportionation reaction. At 25 ℃, the rate constants of the three reactions were 1.95, 1.42 × 10 ~ 4 and 2.22 × 10 ~ 6Ms -1, respectively. The activation energies were 39.4, 32.5 and 24.7 kJ mol -1, respectively. The activation entropies were -29.9, -17.6, -13.6eu The very low rate constants and very negative activation entropy in the electron transfer reaction suggest that a tightly oriented activating complex may form in the reaction.