An electron spin resonance, ENDOR, and TRIPLE resonance study of methyl-substituted N,N′-diphenyl-4,4′-bipyridylium dichloride radical cations

Abstract

The e.s.r. and ENDOR spectra of the radical cations of N,N′-diphenyl-4,4′-bipyridylium di-chloride (DPQ), N,N′-bis-(4-methylphenyl)-4,4′-bipyridylium dichloride (p-mDPQ), N,N′-bis-(3-methylphenyl)-4,4′-bipyridylium dichloride (m-mDPQ), N,N′-bis-(2-methylphenyl-4,4′-bipyridylium dichloride (o-mDPQ), and N,N′-bis-(2,4,6-trimethylphenyl)-4,4′-bipyridylium dichloride (T-mDPQ) in methanol were studied over a range of temperatures. Values for the hyperfine couplin constants are given at –40 °C. It was found that the radical cation concentrations of DPQ⁺˙, p-mDPQ⁺˙, m-mDPQ⁺˙, and o-mDPQ⁺˙ decreased with decreasing temperature, this change being reversible, whilst that of T-mDPQ⁺˙ did not. We explain the reversible change with temperature in terms of a monomer–dimer equilibrium, and the absence of such an equilibrium in the case of T-mDPQ⁺˙ as due to the lack, of planarity of the molecule, because of steric hindrance between the methyl groups and the pyridyl ortho-protons. The thermodynamic constants ΔG°, ΔH°, and ΔS°, obtained for the dimerisation equilibria, are reported.