Electron spin resonance study of liquid during photolysis. XXIII. The 1-hydro-3,5-pyridinedicarboxylate anion radical in basic aqueous solutions

Abstract

An electron spin resonance study of the 1-hydro-3,5-pyridinedicarboxylate anion radical is reported for a broad range of NaOH concentrations in aqueous solutions. Observed effects are attributed to a reversible, base-catalyzed dissociation of the NH-hydrogen. At high concentrations of NaOH the radical interchanges rapidly between its undissociated and dissociated forms, resulting in a single sharp-lined spectrum whose couplings depend upon the degree of dissociation. This made it possible to determine the thermodynamic equilibrium constant for the base-catalyzed equilibrium, from which the constant for the ionization in aqueous solution was calculated. The latter for an aqueous solution containing 20 ml of isopropyl alcohol per liter of solution at 25°C, expressed in p K units, is 16.82 ± 0.05. At lower concentrations of NaOH the radical is essentially completely undissociated, but the NH-hydrogen is reoriented by the equilibrium reactions producing lineshape effects which depend upon the second-order rate constant for the reaction between OH − and the undissociated form of the radical. This constant was determined for a range of dilute NaOH concentrations with the help of spectral simulations. The value of the rate constant extrapolated to infinite ionic dilution was found to be (5.0 ± 0.5) × 10 7 M −1 sec −1 at 30°C. The g values and couplings of both forms of the radical are reported.