13C Hyperfine Interactions in Radicals from Some Carboxylic Acids

The ESR spectra of four radicals of the type $\mathrm{R}{\mathrm{ĊHCO}}_2^{-}$ , four of the type $\mathrm{RĊ}({\mathrm{O}}^{-}){\mathrm{CO}}_2^{-}$ , and one each of the types ${\mathrm{R}}_2{\mathrm{ĊCO}}_2^{-}$ and ${\mathrm{ROHĊCO}}_2^{-}$ 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 $\mathrm{ĊH}({\mathrm{CO}}_2^{-}{)}_2{\mathrm{\hspace{0.17em}(a}}_{\alpha }^{\mathrm{C}}\text{= 31.44\hspace{0.17em}}\mathrm{G}{\mathrm{,\; a}}_{\beta }^{\mathrm{C}}\text{= 11.92\hspace{0.17em}}\mathrm{G})$ derived from malonate. Also studied were radicals from succinate, citrate, and the anion of 1,2,3‐propanetricarboxylic acid. In all cases where $a_{\alpha }^{\mathrm{C}}$ could be determined, the values are close to 30 G. The values of $a_{\beta }^{\mathrm{C}}$ 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 $a_{\gamma }{}^{\mathrm{C}}\text{≅ 20\hspace{0.17em}}{\cos }^2\theta$ . A typical radical of the second type is ${\mathrm{CH}}_3\mathrm{Ċ}({\mathrm{O}}^{\mathrm{-;}}){\mathrm{CO}}_2^{-}{\mathrm{\hspace{0.17em}(a}}_{\alpha }{}^{\mathrm{C}}\text{= 14.09\hspace{0.17em}}\mathrm{G}{\mathrm{,\; a}}_{\beta }^{\mathrm{C}}\text{= 8.19, 6.35\hspace{0.17em}}\mathrm{G})$ . The small ¹³C hyperfine constants of this type of radical ($\text{∼ 7–14}\mathrm{G}$ ) demonstrate the planarity of these radicals at the α carbon. Also studied was the radical ${}^{-}{\mathrm{O}}_2{\mathrm{CCH}}_2\mathrm{O}{\mathrm{ĊHCO}}_2^{-}$ for which $a_{\alpha }{}^{\mathrm{C}}$ was found to be 26.45 G. The low value of $a_{\alpha }{}^{\mathrm{C}}$ shows that this radical is planar at the radical site and hence can serve as a reference for the values of $a_{\alpha }{}^{\mathrm{C}}$ in hydroxyalkyl and ketyl radicals. The hyperfine constants of the radicals ${\mathrm{ĊH}}_2{\mathrm{CO}}_2^{-}\mathrm{,\hspace{0.17em}}\mathrm{ĊH}({\mathrm{CO}}_2^{-}{)}_2$ , and $\mathrm{HĊ}({\mathrm{O}}^{-}){\mathrm{CO}}_2^{-}$ are compared with values from INDO molecular orbital calculations.