Showing papers by "George K. Fraenkel published in 1962"

Solvent Effects in Electron Spin Resonance Spectra

Abstract: It has been shown that a simple model can account for the available data on the solvent dependence of the hyperfine splittings in the ESR spectra of organic free radicals. We have assumed that the changes in splittings arise entirely from a redistribution of the pi‐electron spin density, and that the spin density is affected only by localized complexes between the solvent and polar substituents or heteroatoms in the radical. This model predicts that the magnitudes of the changes in proton splittings should often be small, although large fractional changes at positions of small spin density can sometimes occur. The large variations found for the nuclei of many electron atoms are shown to arise because their splittings are very critical functions of the spin density.The effect of the solvent on the proton hyperfine splittings in the semiquinones has been treated by assuming that the solvent alters the electronegativity of the oxygen atoms and by performing molecular‐orbital calculations to estimate spin den...

Spin‐Density Distribution in Nitrile Anion Radicals

Abstract: Molecular‐orbital calculations of the pi‐electron spin densities in a series of aromatic and aliphatic nitrile anion radicals have been performed using the Huckel—LCAO method and the approximate configuration‐interaction correction of McLachlan. Coulomb and resonance integrals for the nitrile group were estimated by comparing calculated spin densities with proton and carbon‐13 hyperfine splittings obtained from electron‐spin resonance measurements. The predicted spin densities were in generally good agreement with the experimental results, but for some of the compounds it is impossible to get an exact fit between theory and experiment within the framework of the simple valence‐theory calculations if the sigma—pi interaction parameters relating splittings to spin densities are taken as fixed quantities. Semiempirical treatments of the N14 and C13 splittings in the cyano groups give excellent correlations of the experimental splittings with the calculated spin densities, and estimates have been made of some of the sigma—pi interaction parameters relating to the N14 splitting. Polarographic half‐wave potentials have also been compared with calculated pi‐electron energies. A discussion is given of the relation between the spin densities predicted by the valencebond and molecular‐orbital theories.

Electron Spin Resonance Spectra of Carbonyl Anion Radicals

Abstract: Electron spin resonance studies are reported on the anion radicals of single‐ring aromatic ompounds containing aldehyde, acetyl, or amide groups, as well as other substituents. The radicals were generated by electrolytic reduction in N,N‐dimethylformamide solution. Many of the radicals have spectra which indicate that the carbonyl group is locked in a conformation planar with the ring for times of the order of a microsecond or longer. The para dicarbonyls and the 3‐cyanoacetophenone anion were found to be present in both the cis and trans modifications. A simple modification of conventional molecular‐orbital theory has been used with considerable success to account for the loss of symmetry in the pi‐electron spin density for compounds with a locked carbonyl group, and the calculated energy differences for the cis and trans isomers are in good agreement with experiment. Molecular‐orbital calculations of spin densities were made for most of the radicals, often with excellent results, and comparisons are made with the predictions of valence‐bond theory. The benzaldehyde, acetophenone, and 4‐fluoroacetophenone anions have spectra with abnormally small ring‐proton splitting constants, and no satisfactory explanation of these anomalous results has been found. The appearance or nonappearance in all the radicals but these three of twofold symmetry in the pattern of splitting constants is interpreted qualitatively in terms of competing effects determined by the bond order of the bond between the ring and the carbonyl group, and steric factors in the neighborhood of the carbonyl group. A number of features of the experimentally determined spin‐density distributions have been correlated with the relative electron‐withdrawing effects of the substituents.

Electron Spin Resonance of Azulene Anion Radicals

Abstract: The electron spin resonance spectra of the anion radicals of azulene, azulene‐1,3‐d2, 4,6,8‐trimethylazulene, and 4,6,8‐trimethylazulene‐1,3‐d2 have been examined. These hydrocarbons are nonalternant and have aromatic ring structures with C–C–C bond angles that depart from 120°. The radicals were generated electrolytically in N,N‐dimethylformamide using tetra‐n‐propylammonium perchlorate as supporting electrolyte. Sigma‐pi‐interaction calculations of the parameter QCHH [in the relation aiH=QCHHρiπ between the isotropic proton hyperfine splitting (aiH) and pi‐electron spin density (ρiπ)] have been performed as a function of the bond angles at the carbon atom bonded to the proton. These calculations indicate a strong dependence of this parameter on angle. By using recent x‐ray data on azulene, predictions could be made of the Q's at the different positions in the radicals. In contrast, the proton splitting at a methyl‐group substituent, determined by the parameter QCCH3H, is predicted to be independent of t...