Resonance Raman spectra of the N,N ‐ dihydro‐4,4′‐bipyridinium radical cation

The Raman spectrum of diprotonated 4,4′‐bipyridine in the solid state as the dibromide salt is reported. The resonance Raman spectrum of the radical cation derived by one‐electron reduction of the dication is also reported. Viscous glycerol was employed as a solvent in order to protect the radical by inhibiting oxygen diffusion. The spectra are interpreted on the basis of a comparison with those of related compounds. A large shift in frequency (+70 cm−1) of the inter‐ring stretching vibration between the parent dication and radical monocation is observed. This has been interpreted in terms of a major contribution to the delocalized radical structure of a quinoid‐type form.