| Abstract: | 本文乃探討在pH 5-9條件下，原兒茶素與一系列五氰鐵錯合物 [Fe(CN)5L]2- ( L = 4-cyano-pyridine, isonicotinamide, 4,4’-bipyridine, pyridine, pyrazine )之外圈電子轉移反應動力學，反應速率受到溶液pH值影響，主要乃由於原兒茶素在不同pH條件下分別以H2cat-COO-(k1)、Hcat-COO2-(k2)及cat-COO3-(k3)存在，動力學結果分別為k1= 1.5×102(cp)，3.9×101(isn)，8.0×101(bpy)，5.4×102 M-1s-1(pz)；k2= 3.0×105(cp)，1.2×105(isn)，1.1×105(bpy)，3.4×104(py)，2.9×105 M-1s-1(pz)；k3= 3.2×108(cp)，1.6×108(isn)，9.2×107(bpy)，5.5×107(py)，k1、k2及k3值皆符合外圈電子轉移反應之Marcus理論值。根據進一步Marcus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k1:k2:k3?1:103:106，主要乃由原兒茶素在不同型態中與[Fe(CN)5L]2-錯合物之反應能障差異所引起，而與反應物之電荷效應及原兒茶素在不同形態反應活性並無太大關聯。
The reductions of [Fe(CN)5L]2-(L=4-cyano-pyridine, isonicotinamide, 4,4’-bipyridine, pyridine, pyrazine)complexes by protocatechuic acid have been subjected to a detailed kinetic study in the range of pH 5 – 9. The rate law of the reaction is interpreted as a rate determining reaction between Fe(III) complexes and the protocatechuic acid in the form of H2cat-COO-(k1), Hcat-COO2-(k2), and cat-COO3-(k3), depending on the pH of the solution, follow by a rapid scavenge of the protocatechuic acid radicals by Fe(III) complex. With given Ka1, Ka2, and Ka3, the rate constants are k1 = 1.5×102(cp), 3.9×101(isn), 8.0×101(bpy), and 5.4×102 M-1s-1(pz)；k2 = 3.0×105(cp), 1.2×105(isn),1.1×105(bpy), 3.4×104(py), and 2.9×105 M-1s-1(pz)；k3 = 3.2×108(cp), 1.6×108(isn), 9.2×107(bpy), and 5.5×107(py), respectively, atμ= 0.10 M LiClO4, T = 25℃. The kinetic results are compatible with the Marcus theory for out-sphere electron transfer. Moreover, the detailed theoretical analysis indicates that, the tremendously large rate difference with k1:k2:k3?1:103:106, arises predominantly from the equilibrium barrier of the different acid forms of the protocatechuic acid, and is rather insensitive to the intrinsic reactivities and the charge effect of the reactants. |
