The Spectral Elucidation versus the X-ray Structure of the Critical Precursor Complex in Bimolecular Electron Transfers: Application of Experimental/Theoretical Solvent Probes to Ion-Radical (Redox) Dyads

The mechanistic conundrum is commonly posed by the intrinsic structural disconnect between
a bimolecular (reactive) intermediate that is fleetingly detected spectroscopically in solution versus that
rigorously defined by isolation and X-ray crystallography. We resolve this ambiguity by the combined
experimental and theoretical application of the solvent media probe to the transient (1:1) precursor complex
in the simplest chemical reaction involving direct adiabatic electron transfer (ET) among various donor/acceptor pairs. Of particular help in our resolution of such an important ET problem is the characterization
of the bimolecular precursor complex as Robin−Day class II (localized) or class III (delocalized) from either
the solvent-dependent or the solvent-independent response of the diagnostic intervalence absorption bands
for the quantitative evaluation of the electronic coupling elements. The magnitudes of these intracomplex
bindings are confirmed by theoretical (ab initio and DFT) computations that derive from X-ray structures
and Marcus−Hush theories. Most importantly, the experimental solvent-induced ET barriers evaluated from
the intervalence absorption bands are also quantitatively verified by the calculated outer-shell reorganization
energies to establish unambiguously the intimate interconnection between the loosely bound bimolecular
intermediate identified concurrently in solution and in the solid state.

该机理谜题通常源于溶液中通过光谱法瞬态检测得到的双分子（反应性）中间体，与经分离及X射线晶体学（X-ray crystallography）严格表征的对应物种之间存在的内在结构脱节。我们结合溶剂介质探针法的实验与理论手段，对参与涉及各类供体/受体对间直接绝热电子转移（electron transfer, ET）的最简单化学反应的瞬态（1:1）前驱体配合物展开研究，从而厘清了这一歧义。在解决这类重要电子转移问题时，关键的辅助方法是通过诊断性价间吸收带的溶剂依赖或溶剂非依赖响应，将双分子前驱体配合物表征为Robin-Day II类（localized）或III类（delocalized），以此定量评估电子耦合系数。这些配合物内部的结合强度，通过基于X射线晶体结构与Marcus-Hush理论的从头算（ab initio）及密度泛函理论（DFT）计算得到验证。最为关键的是，由价间吸收带得到的实验溶剂诱导电子转移势垒，也通过计算得到的外层重组能得到定量验证，从而明确证实了在溶液与固态中同时观测到的弱结合双分子中间体之间存在紧密的内在关联。