Electron spin resonance study of the 1 : 1 hydrogen-bonded complex of di-t-butylnitroxide with methanol. An analysis of an acceptor : self-associated donor equilibrium

Abstract

The formation constant K_f for the 1:1 complex of di-t-butylnitroxide with methanol has been determined over the temperature range –23°C to + 65°C by measuring the variation of a^N with methanol concentration. The solvent was carbon tetrachloride. The Newton–Raphson (N.R.) method was used in treating the results. Proper account was taken of the self-associated nature of the methanol. With suitable modifications the analysis should be applicable to other spectroscopic techniques used to study complex formation between two species, one of which is self-associated. Evidence has been obtained which favours the existence of linear rather than cyclic tetramers of methanol in equilibrium with monomers. The enthalpy of formation of the hydrogen-bonded complex was low, namely –6.7 ± 1.8 kJ mol^–1. The temperature coefficients of a^N in the free and complexed nitroxide are of opposite sign. For the complex, this is shown to be the consequence of proton hopping in an asymmetric double-minimum potential with an energy difference between the minima of 9.4 ± 0.8 kJ mol^–1.