Institut für Anorganische Chemie, Universität Würzburg, Germany.
J Am Chem Soc. 2009 Nov 11;131(44):16292-302. doi: 10.1021/ja9070859.
Structures, dipole moments, electron-transfer barriers, and spin density distributions of a series of mixed-valent bistriarylamin radical cations have been studied systematically by hybrid density functional methods with variable exact-exchange admixture combined with a continuum solvent model. The chosen systems differ in their bridging units and are all relatively close, from both sides, to the class II/III borderline of the Robin-Day classification of mixed-valence systems. Solvent effects are found to have a dramatic influence on the localized vs delocalized character of these cations. While gas-phase calculations or computations in a nonpolar solvent place all systems on the delocalized class III side, a more polar solvent like acetonitrile enables observation of symmetry breaking and charge localization with moderate exact-exchange admixtures in a hybrid functional for the systems on the class II side (with diphenylbutadiyne and diphenylethyne bridges). In contrast, the cations with the shortest bridges (phenylene, biphenylene) are characterized as class III. The comparison of computed intervalence charge-transfer excitation frequencies with experiment confirms the system with the diphenylbutadiyne bridge, and probably the system with the diphenylethyne bridge, to be class II, whereas in the dichloromethane solvent employed for spectroscopic measurements, the two other systems are on the class III side. Nonstandard hybrid density functional calculations with 35% Hartree-Fock-like exchange combined with continuum solvent models are suggested as a practical protocol for the quantum-chemical characterization of organic mixed-valence systems. This approach should allow closer examinations and provides a basis for the evaluation of other computational methods.
通过混合密度泛函方法与可变精确交换混合相结合的连续溶剂模型，系统地研究了一系列混合价双三联芳基自由基阳离子的结构、偶极矩、电子转移势垒和自旋密度分布。所选择的体系在其桥接单元上有所不同，并且都相对接近混合价体系罗宾-戴分类的 II/III 边界的两侧。溶剂效应被发现对这些阳离子的局部化与离域特征有显著影响。虽然气相计算或非极性溶剂中的计算将所有体系都置于离域的 III 类，但更极性的溶剂如乙腈可以在混合功能中观察到对称性破缺和电荷定域，并且具有中等的精确交换混合，对于 II 类体系（具有二苯丁二炔和二苯乙炔桥）。相比之下，具有最短桥的阳离子（亚苯基、联苯）的特征为 III 类。计算的价间电荷转移激发频率与实验的比较证实了具有二苯丁二炔桥的体系，可能还有具有二苯乙炔桥的体系，为 II 类，而在用于光谱测量的二氯甲烷溶剂中，另外两个体系位于 III 类。建议使用 35% Hartree-Fock 样交换与连续溶剂模型相结合的非标准混合密度泛函计算作为有机混合价体系量子化学表征的实用方案。这种方法应该能够更仔细地检查，并为评估其他计算方法提供基础。
