Infrared, Raman and resonance Raman investigations of methylviologen and its radical cation

Abstract The infrared and Raman spectra of methylviologen (1,1′‐dimethyl‐4,4′‐bipyridine dication, MV 2+ ) have been measured from both the solid dichloride and aqueous solutions. The preresonance behaviour of MV 2+ Raman bands showed that the absorption at ˜196 nm is mainly responsible for the resonance scattering process in MV 2+ . In the region below 1700 cm −1 , nine polarized ( p ˜1/3) resonance Raman bands of the methylviologen radical cation, MV + , were assigned to in‐plane ring modes. Arguments are given that the chromophore of MV + may also partially include the methyl groups. The excitation profiles of the MV + fundamentals, measured with laser wavelengths covering the range 350.6–676.4 nm, reveal three main groups of modes, each behaving differently for the excitation in this range. Modes containing largely ν(CC) ring contributions are mainly coupled with electronic transitions at ˜395, ˜605 and ˜720 nm and yield simple excitation profiles. Modes involving principally the vibrations of the N + CH 3 group show complicated interference peaks in the whole range of the excitation profiles. γ(CCC) is mainly coupled with the absorption at ˜620 nm. Resonance Raman bands of the (MV + ) 2 dimer have been characterized through studies of the concentration dependence of the spectra.