SEMIEMPIRICAL AND THEORETICAL DERIVATION OF OXYGEN-17 SIGMA-PI INTERACTION CONSTANTS IN CARBONYL-CONTAINING ANION RADICALS

Oxygen-17 hyperfine splitting constants ao are determined in a number of carbonyl radicals including simple and substituted semiquinones, semidiones, and ketyls. The values for aO range between 7.5 and 10.4 G. The results are analyzed in terms of the π spin-density distribution in the radicals: Neglecting overlap spin densities the analysis shows that the most important contribution to aO comes from spin density on the oxygen itself and a semiempirical relation of the type aO=QOC°ρO where QOC° = 45±5 G accounts satisfactorily for the observed aO'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 σ-π interaction constants of the carbonyl oxygen using the theory of McLachlan,Dearman, and Lefebvre and the approximateHartree-Fock wavefunctions for formaldehyde calculated by Newton and Palke. These calculations give the following theoretical expression for aO:a O=48.70ρO-0.46ρC-6.03 ρCO (where ρCO 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 σ-π interaction constants is discussed and it is shown that they result mainly from spin polarization of the lone-pair 2so electrons, but there is also a small (negative) contribution from the inner-shell Iso orbital.