Semiempirical and Theoretical Derivation of the Oxygen-17 σ–π Interaction Constants in Carbonyl-Containing Anion Radicals

Oxygen‐17 hyperfine splitting constants $a_O$ are determined in ̄a number of carbonyl radicals including simple and substituted semiquinones, semidiones, and ketyls. The values for $a_O$ range between 7.5 and 10.4 G. The results are analyzed in terms of the $\pi$ spin‐density distribution in the radicals: Neglecting overlap spin densities the analysis shows that the most important contribution to $a_O$ comes from spin density on the oxygen itself and a semiempirical relation of the type $a_O{\mathrm{\hspace{0.17em}=\hspace{0.17em}Q}}_{\mathrm{OC}}^O{\rho }_O$ where $Q_{\mathrm{OC}}^O\text{\hspace{0.17em}=\hspace{0.17em}45\hspace{0.17em}±\hspace{0.17em}5}\mathrm{G}$ accounts satisfactorily for the observed $a_O\text{'}\mathrm{s}$ . The over‐all range of ¹⁷O hfs and the accuracy of the spin densities are not sufficient for a semiempirical determination of the effect of overlap spin density. The semiempirical results are compared with an independent calculation of the $\mathrm{\sigma –\pi }$ interaction constants of the carbonyl oxygen using the theory of McLachlan, Dearman, and Lefebvre and the approximate Hartree–Fock wavefunctions for formaldehyde calculated by Newton and Palke. These calculations give the following theoretical expression for $a_O{\mathrm{\hspace{0.17em}:\hspace{0.17em}a}}_O{\text{\hspace{0.17em}=\hspace{0.17em}48.70ρ}}_O{\text{\hspace{0.17em}−\hspace{0.17em}0.46ρ}}_C{\text{\hspace{0.17em}−\hspace{0.17em}6.03ρ}}_{\mathrm{CO}}$ (where ${\rho }_{\mathrm{CO}}$ is the overlap spin density in the C–O bond), in good agreement with the results of the semiempirical analysis. The origin of the oxygen $\mathrm{\sigma –\pi }$ interaction constants is discussed and it is shown that they result mainly from spin polarization of the lone‐pair ${\text{2s}}_O$ electrons, but there is also a small (negative) contribution from the inner‐shell ${\text{1s}}_O$ orbital.