維他命C還原Fe(CN)5L2-(L=CN-, 4,4'-bpy)的反應屬外圈電子轉移反應機構，反應速率受溶液pH影響很大，主要乃由於維他命C為雙質子酸，在不同pH條件下分別以H2A，HA-及A2-型態出現，[H+]=0.01~0.10M及pH=4~9條件下動力學探討結果得k0(H2A)=0.42 ±0.02 (L=CN-)，2.5 ±0.2 (L=4,4'-bpy)；k1(HA-)= (5.2±0.1)´102 (L=CN-)，(9.6±0.1)´103 (L=4,4'-bpy) ；k2 (A2-)= (2.03±0.07)´107 (L=CN-) M-1s-1，所得結果符合Marcus外圈電子轉移反應理論，且根據進一步理論分析，發現在不同pH條件下之所以有如此大的還原速率差異k0:k1:k2~1:103:108，與反應物之電荷效應無多大關聯，主要乃由維他命C在不同型態中反應活性(kex)及整個反應能障之差異所引起。 The reductions of ascorbic acid on Fe(CN)5L2-(L=CN-, 4,4'-bpy) complexes undergo outer-sphere electron transfer mechanism. The rates of the reduction depend greatly on the pH of the solution for the reason that ascorbic acid exists in different forms, namely H2A，HA- and A2-, at various pH values. The kinetic studies of the reactions at [H+]= 0.01~0.10 M and pH=4~9 yield specific rate constants of reduction as k0(H2A)=0.42 ±0.02 (L=CN-), 2.5 ±0.2 (L=4,4'-bpy), k1 (HA-)= (5.2 ±0.1)´102 (L=CN-), (9.6 ±0.1)´103 (L=4,4'-bpy) ; k2 (A2-)= (2.03 ±0.07)´107 (L=CN-) M-1s-1，respectively at m=0.50 M HClO4/LiClO4 and T=25℃.Our kinetic results conform with Marcus theory for outer-sphere electron transfer. Moreover, the detailed theoretical analysis indicates that this tremendously large rate difference with k0:k1:k2~1:103:108 (L=CN-) arises predominantly from the equilibrium barrier and the intrinsic reactivity of the different acid forms of the ascorbic acid, and is rather insensitive to the charge effect of the reactants.
