ESR MEASUREMENT OF THE PKA OF CARBOXYL RADICAL AND ABINITIO CALCULATION OF THE C-13 HYPERFINE CONSTANT

The pKa value for the equilibrium CO2H ⇄ H+ + CO2- has been determined by ESR methods to be -0.2 ±0.1. Formate that had been enriched in 13C was reacted with SO4•- in steady-state photolysis experiments to produce the radical. The positions of the hyperfine lines did not vary over the pH range 1.6-10 but were found to change continuously as the acid concentration was increased over the range pH 1.2 to about pH 0, where the intensity became too small for study. The change in average hyperfine constant followed the behavior expected for a simple equilibrium under the assumption that exchange of the acid proton is rapid. The 13C hyperfine constants in aqueous solution were found to be 146.4 ± 0.1 G for CO2- and 166 ± 5 G for CO2H. Ab initio molecular orbital calculations generally agree with the measured hyperfine constants and predict a higher value for the neutral form of the radical. ESR lines from a reduced oxalate radical containing 13C were also detected. © 1990 American Chemical Society.