Electron spin resonance study of liquids during photolysis. XXII. The 1-hydro-2,5-pyridinedicarboxylate anion radical in basic aqueous solutions

Abstract

An electron spin resonance study of the 1-hydro-2,5-pyridinedicarboxylate anion radical in basic aqueous solutions is reported. The radical undergoes a reversible, base-catalyzed dissociation of the NH-hydrogen. At high NaOH concentrations the rate of interconversion of the dissociated and undissociated radical is rapid, resulting in a single sharp-lined spectrum whose parameters depend upon the degree of dissociation. The equilibrium constant of the base-catalyzed reaction was determined, and from it the equilibrium 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 32°C, expressed in pK units, is 16.64 ± 0.1. At lower concentrations of NaOH the dynamic equilibrium serves to reorient the NH-hydrogen spin state of the undissociated radical, and thus produces lineshape effects which depend upon the rate of dissociation. The second-order rate constants at infinite ionic dilution for the forward and backward directions, respectively, of the base-catalyzed dissociation reaction for an aqueous solution containing 100 ml of isopropyl alcohol per liter of solution at 32°C are (2.2 ± 0.6) × 10 7 and ∼3 × 10 8, M −1 sec −1. The g values and couplings of both radicals were determined, and couplings are compared with values estimated from MO calculations.