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.