Insights into the free-energy dependence of intramolecular dissociative electron transfers.
Insights into the free-energy dependence of intramolecular dissociative electron transfers.
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DOI：
10.1021/ja0263644
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发表时间：
2002-08
影响因子：
15
通讯作者：
S. Antonello;M. Crisma;F. Formaggio;A. Moretto;F. Taddei;C. Toniolo;Flavio Maran-
中科院分区：
文献类型：
--
作者：
S. Antonello;M. Crisma;F. Formaggio;A. Moretto;F. Taddei;C. Toniolo;Flavio Maran-
为了研究分子内离解电子转移速率与驱动力之间的关系，设计并合成了一系列供体-间隔体-受体（D-Sp-A）体系。顺式1,4-环己基和一个烯醚官能团分别作为间隔和受体保持不变。通过改变邻苯二甲酸亚胺部分的芳基取代基作为供体，驱动力可变化0.74 eV。x射线衍射晶体学和从头算构象计算表明，D- sp -A分子具有顺式-（环己烷）赤道式（邻苯二甲酸）轴向式（聚醚酯）和相同的D/A取向。用电化学方法研究了N，N-二甲基甲酰胺分子内的解离电子转移过程，并与用受体和给体模型得到的热力学和动力学信息进行了比较。分子内过程包括电子从电化学生成的邻苯二胺部分自由基阴离子转移到过氧化物官能团。电化学分析提供了明确的证据，表明协同解离电子转移机制导致O-O键的断裂。通过从头算MO计算得到了这一机制的支持，该计算提供了关于受体的LUMO和供体的SOMO的信息。测定了分子内速率常数，并与相应的分子间值进行了比较，后者的数据是通过使用模型分子得到的。只要供体SOMO质心的有效位置不因改变芳基取代基而发生显著变化，分子内解离电子转移遵循与已经强调的相应分子间过程相同的主要规则。另一方面，硝基的引入使SOMO远离受体，因此，分子内速率比期望值下降了1.6个数量级。因此，对于分子内速率的定量预测，必须考虑到比分子间电子转移更大的溶剂重组和有效的D/ a距离以及电子耦合。
To study the relationship between rate and driving force of intramolecular dissociative electron transfers, a series of donor-spacer-acceptor (D-Sp-A) systems has been devised and synthesized. cis-1,4-Cyclohexanedyil and a perester functional group were kept constant as the spacer and acceptor, respectively. By changing the aryl substituents of the phthalimide moiety, which served as the donor, the driving force could be varied by 0.74 eV. X-ray diffraction crystallography and ab initio conformational calculations pointed to D-Sp-A molecules having the cis-(cyclohexane) equatorial(phthalimido)-axial(perester) conformation and the same D/A orientation. The intramolecular dissociative electron-transfer process was studied by electrochemical means in N,N-dimethylformamide, in comparison with thermodynamic and kinetic information obtained with models of the acceptor and the donor. The intramolecular process consists of the electron transfer from the electrochemically generated phthalimide-moiety radical anion to the peroxide functional group. The electrochemical analysis provided clear evidence of a concerted dissociative electron-transfer mechanism, leading to the cleavage of the O-O bond. Support for this mechanism was obtained by ab initio MO calculations, which provided information about the LUMO of the acceptor and the SOMO of the donor. The intramolecular rate constants were determined and compared with the corresponding intermolecular values, the latter data being obtained by using the model molecules. As long as the effective location of the centroid of the donor SOMO does not vary significantly by changing the aryl substituent(s), the intramolecular dissociative electron transfer obeys the same main rules already highlighted for the corresponding intermolecular process. On the other hand, introduction of a nitro group drags the SOMO away from the acceptor, and consequently, the intramolecular rate drops by as much as 1.6 orders of magnitude from the expected value. Therefore, a larger solvent reorganization than for intermolecular electron transfers and the effective D/A distance and thus electronic coupling must be taken into account for quantitative predictions of intramolecular rates.