Evaluation of Ultrafast Intramolecular Electron Transfer Rate by Means of NMR Relaxation

采用核磁共振自旋-晶格弛豫测量方法，评价了不同体系溶液中的分子内电子转移速率。从旋转弛豫时间和电子自旋弛豫时间等方面证实了该方法的局限性。对三种混价双二茂衍生物（Fe（II）、Fe(III)）的皮二级反应速率进行了观察，发现反应速率具有一定的特征性。双二茂单价化合物的速率常数比其衍生物1,1 ' '-二碘衍生物大约3倍，但这两种混价化合物具有相似的价间能带能和强度。这一结果可归因于外球和内球重组能的比值不同。L-M-L拉伸模式的频率差异是二碘衍生物的ET速率延迟的另一个原因，另一方面，乙炔桥接的二茂铁单位的ET速率比二茂铁单位的ET速率高3倍。这一结果与IT波段测量结果不一致。乙炔桥接配合物的速率随温度的变化表明其速率有相当大的核隧穿效应。这可能是由混合价络合物的刚性桥接结构引起的。只要把溶剂看作介质连续体，本研究所给出的速率都超过了包括溶剂动力学效应在内的任何模型所预测的速率。更快的局部溶剂动力学可能（至少部分地）是控制速率的原因。

The intramolecular electron transfer rates in solution were evaluated in several systems by NMR spin-lattice relaxation measurements. A limitation of this method was confirmed referring the rotational relaxation time and the electron spin relaxation time, etc.The reaction rates with pico second order were observed for three mixed-valence biferrocene derivatives (Fe(II), Fe(III)) and the rates showed a characteristic behavior. The biferrocene monocation showed ca. three times larger rate constant than its 1,1 "'- diiodo derivative, nevertheless these two mixed-valence compounds show similar intervalence band energy and strength. This results can be attributed to difference in the ratio of the outersphere and the innersphere reorganization energy. The frequency difference in the L-M-L stretching modes is another reason of the retardation of the ET rate of diiodo derivatives, On the other hand, an acetylene bridged biferrocene monocation gave three times higher rates than those boferrocene monocation. This result is disagreement with the IT band measurements. The temperature dependence of the rate of the acetylene bridged complex suggested considerable nuclear tunneling effect on the rates. This may be arisen by the rigid bridging structure of the mixed-valence complex.All the rates given in the present studies exceed those predicted any model including solvent dynamical effect as long as solvent is regarded as a dielectric continuum. Faster local solvent dynamics may be, at least partially, responsible to control the rates.