Formation of intramolecular hydrogen bond in hydroxy-substituted nitroxide radicals as evidenced by electron spin resonance

The nature of the intramolecular hydrogen bond in hydroxysubstituted nitroxide radicals was examined. The hydroxysubstituted radical was produced by the spin trapping reaction of hydroxyalkyl radicals with a-phenyl-N-t-butylnitrone. The magnitude of the hyperfine splitting caused by β-hydrogen of the radical is strongly dependent on the composition of the solvent, which is attributed to the formation of the intramolecular hydrogen bond. The equilibrium constant of the intramolecular hydrogen bond in an aprotic solvent was determined by observing the magnitude of the hyperfine splitting of β-hydrogen as a function of the amount of added methanol. Based on the temperature dependence of the equilibrium constant, the standard entropy and the standard enthalpy of the formation of the hydrogen-bonded chelate ring was estimated. The same method of analysis was applied to the system of diastereomeric nitroxide and the difference of the internal energy of the chelate ring in the isomers was detected. Moreover, the difference was utilized to separate the ESR spectra of the diastereomers. (author)