13C Hyperfine Interactions in Radicals from Some Carboxylic Acids

Abstract

The ESR spectra of four radicals of the type RĊHCO2−, four of the type RĊ (O−) CO2−, and one each of the types R2ĊCO2− and ROHĊCO2− have been studied and the 13C hyperfine constants determined. The radicals were produced by continuous, in situ radiolysis of basic aqueous solutions of the appropriate acids. In all cases the radicals bore at least two units of negative charge, and radicals labeled with 13C at the natural abundance level were detectable because of the lowering of the recombination rate by this multiple charge. A typical radical of the first type is ĊH (CO2−)2 (aαC = 31.44 G, aβC = 11.92 G) derived from malonate. Also studied were radicals from succinate, citrate, and the anion of 1,2,3-propanetricarboxylic acid. In all cases where aαC could be determined, the values are close to 30 G. The values of aβC are also rather constant, being in all cases in the range 10–13 G. The hyperfine constants of the γ carbons (substituents on β carbons) in this type of radical are consistent with an angular dependence aγ ≅ 20 Ccos2 θ. A typical radical of the second type is CH3Ċ(O−;) CO2− (a α = 14.09 CG, aβC = 8.19, 6.35 G). The small 13C hyperfine constants of this type of radical (∼ 7–14 G) demonstrate the planarity of these radicals at the α carbon. Also studied was the radical O2−CCH2OĊHCO2− for which aαC was found to be 26.45 G. The low value of aαC shows that this radical is planar at the radical site and hence can serve as a reference for the values of aαC in hydroxyalkyl and ketyl radicals. The hyperfine constants of the radicals ĊH2CO2−, ĊH (CO2−)2, and HĊ(O−)CO2− are compared with values from INDO molecular orbital calculations.