Bipyridyl radical cations. Part I. Electron spin resonance study of the dimerisation equilibrium of morphamquat radical cation in methanol

Abstract

We have studied the e.s.r. spectra of methanol solutions of the morphamquat {bis-1,1′-[(2,6-dimethylmorpholin-4-yl)carbonylmethyl]bipyridyl} radical cation (I) from +40 to –80°. The value of the splitting constants obtained are A(pyridyl N) 0.400, A[pyridyl H(3)] 0.142, A[pyridyl H(2)] 0.048, A(NCH₂) 0.245, and A(morpholine N) 0.024 mT. It is found that as the temperature decreases the concentration of the radical cation decreases until at –80° there is no e.s.r. spectrum. This process is reversible. Concentration experiments show that the morphamquat radical cation is in equilibrium with a dimeric diamagnetic species. The ΔH° value for the equilibrium is found to be –45.05 ± 0.3 kJ mol^–1. The ΔG° and ΔS° values at 25 °C are –10.6 kJ mol^–1 and –115.6 J mol^–1 K^–1 respectively.