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

通过核磁共振自旋-晶格弛豫测量，在几个体系中评价了溶液中的分子内电子转移速率。结合转动弛豫时间和电子自旋弛豫时间等数据证实了该方法的局限性，并对三种混合价双二茂铁衍生物（Fe（II），Fe（III））的反应速率进行了研究，发现反应速率均为皮科二级，表现出特征性行为.双二茂铁一价阳离子显示ca。速率常数比其1，1-二碘代衍生物大三倍，然而这两种混合价化合物显示出相似的价带间能量和强度。这一结果可归因于内外球重组能比值的差异。L-M-L伸缩模的频率差是二碘代衍生物电子转移速率延迟的另一个原因。另一方面，乙炔桥连的双二茂铁一价阳离子的电子转移速率比bofercene一价阳离子高三倍。这一结果与IT带测量结果不一致。乙炔桥络合物的反应速率与温度的关系表明，反应速率受核隧穿效应的影响。这可能是由于混合价络合物的刚性桥连结构引起的.只要溶剂被看作是一个介电连续体，本文所给出的所有速率都超过了任何考虑溶剂动力学效应的模型所预测的速率.更快的局部溶剂动力学可能至少部分地负责控制速率。

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.