Showing papers by "George K. Fraenkel published in 1964"
TL;DR: The pairing theorem for alternant hydrocarbons predicts that the pi-electron spin densities at corresponding positions of the negative and positive ions of a hydrocarbon should be the same, but significant differences have been found previously for the proton hyperfine splittings in electron spin resonance spectra as discussed by the authors.
Abstract: The pairing theorem for alternant hydrocarbons predicts that the pi‐electron spin densities at corresponding positions of the negative and positive ions of a hydrocarbon should be the same, but significant differences have been found previously for the proton hyperfine splittings in electron‐spin resonance spectra In order to examine the validity of the theorem more thoroughly, we have measured the 13C splittings in the ESR spectra of the anthracene negative and positive ions These have been obtained by studying the satellites arising from 13C nuclei present in natural abundance and also by investigating anthracene labeled with 50% 13C in the 9‐position All of the four possible splittings were determined for the negative ion and assigned to the appropriate positions in the radical, and three of the splittings were obtained for the positive ion The assignments of the splittings, and also their signs, were established by linewidth studies, counter‐ion effects, and calculation The negative ions were pro
137 citations
TL;DR: In this article, it was shown that the parameter which determines the effect on 14N splittings of the spin density on an adjacent carbon atom has the value QCNN=2.62 G. This result was obtained without making any assumptions about the magnitudes of the sigma-pi parameters and without using valence-theory calculations of pi-electron wavefunctions.
Abstract: Electron spin resonance spectra have been obtained for the cation radicals of dihydropyrazine and its methyl‐substituted derivatives, and of dihydroquinoxaline, 2,3‐dimethyldihydroquinoxaline, dihydrophenazine, and N,N′‐dihydro‐4,4′‐dipyridyl. It was found that the parameter which determines the effect on 14N splittings of the spin density on an adjacent carbon atom has the value QCNN=2.62 G. This result was obtained without making any assumptions about the magnitudes of the sigma—pi parameters and without using valence‐theory calculations of pi‐electron wavefunctions. The positive sign for QCNN is opposite to that expected from the analogous parameters QCHH and QC CC. The small value of QCNN, and the approximate constancy of the ratio (sum of the spin densities on atoms adjacent to the nitrogen atom) to the (spin density on the nitrogen atom) in most of the compounds for which data are available, makes it possible to represent the nitrogen splittings by the expression aN=—0.940 aNHH, where aNHH is the NH...
94 citations
TL;DR: In this article, it was shown that most of the linewidth differences can be accounted for by modulation through molecular tumbling of the intramolecular anisotropic dipolar and g-tensor interactions.
Abstract: Large variations have been found among the linewidths of the different hyperfine lines in the low‐temperature electron spin resonance spectra of the p‐ and o‐dinitrobenzene anions generated electrolytically in N,N‐dimethylformamide solutions. The magnitude of the variations in the para compound is so great that the spectrum is superficially uninterpretable. Detailed analysis of the spectrum of this radical shows, however, that most of the linewidth differences can be accounted for by modulation through molecular tumbling of the intramolecular anisotropic dipolar and g‐tensor interactions. There may also be a small contribution from modulation of the isotropic proton hyperfine splittings, the mechanism that accounts for the alteranting linewidth phenomenon in a number of radicals, but an alternation of the widths is not observed in the p‐dinitrobenzene anion spectrum because the contribution from this interaction is small. The sign of the isotropic nitrogen hyperfine splitting aN has been determined by a n...
88 citations
TL;DR: The electron spin resonance spectra of radicals obtained from acetophenone, benzaldehyde, and 4fluoroacetophenone were found in a previous investigation to be inconsistent with those of other carbonyl-anion radicals as mentioned in this paper.
Abstract: The electron spin resonance spectra of radicals obtained from acetophenone, benzaldehyde, and 4‐fluoroacetophenone were found in a previous investigation to be inconsistent with those of other carbonyl‐anion radicals. These compounds have been reinvestigated using new techniques for the electrolytic generation of the radicals. For the first two substances, unstable radicals were detected with spectra that were different from those previously obtained. The new spectra are completely consistent with the results for the other related radicals, and also show that the carbonyl groups are locked into the plane of the aromatic ring. The nature of the radicals investigated previously has not been ascertained. A new spectrum was not obtained for the 4‐fluoroacetophenone radical under the new conditions employed for the other two radicals presumably because this species is too unstable. A preliminary interpretation has been given to linewidth variations found in the spectra.
78 citations
TL;DR: In this article, the electron spin resonance spectra of a number of nitro-substituted benzene anions were obtained over a range from above to below room temperature.
Abstract: Studies of linewidths in the electron spin resonance spectra of a number of nitro‐substituted benzene anions are reported The radicals were generated electrolytically in N,N‐dimethylformamide solutions, and spectra were obtained over a range from above to below room temperature Pronounced alternations in the linewidths of the nitrogen lines in many of the spectra were observed If MN is the total z component of the nuclear spin angular momentum in compounds containing two equivalent nitrogen nuclei, the alternating linewidth phenomenon causes a broadening of the lines for which MN=±1 and a reduction in the amplitude of the central line Large alternating linewidths were observed in the room‐temperature spectra of the anions of dinitrodurene, dinitromesitylene, and 2,6‐dinitro‐3,5‐dimethyl‐4‐acetyl‐t‐butylbenzene; and also in a spectrum which shows splittings from two equivalent nitrogen nuclei that was obtained from trinitromesitylene Slight alternating linewidth effects at room temperature that became more marked at low temperatures were found in the spectra of the 2,6‐dinitrotoluene and m‐dinitrobenzene anions No linewidth alternations were detectable even at low temperatures in the spectra of the o‐dinitrobenzene, p‐dinitrobenzene, and 2,6‐dinitrophenolate anions The 2,6‐dinitroaniline anion, which was investigated only at room temperature, showed no evidence of an alternating linewidth effect None of the spectra of the mononitrobenzene anions examined exhibited any anomalous linewidth phenomena The experimental observations are in general agreement with the recently developed theory of linewidths The theory attributes the alternation in widths in these radicals to an out‐of‐phase correlation of a modulation of the isotropic hyperfine splittings of the two equivalent nitrogen nuclei This out‐of‐phase modulation probably arises from either fluctuating solvent complexes with the nitro groups, or from internal rotations of these groups relative to the plane of the benzene ring, and both types of motion may occur simultaneously Steric hindrance of the nitro groups was found to enhance the magnitude of the alternations in width, as did lowering of the temperature
59 citations
48 citations
Abstract: The theory of the linewidths in the electron spin resonance spectra of free radicals recently developed by Freed and Fraenkel predicts that, in general, composite lines arising from a set of degenerate nuclear spin states should not be Lorentzian in shape, and that the shapes of different lines in the same spectrum should be different. The earlier theory of Kivelson predicted that all the lines should be Lorentzian. To test the differences between the two theories, experimental studies of the line shape in the spectrum of the tetracyanoethylene anion have been made in a solvent consisting of a mixture of absolute ethanol and glycerine. Different lines were found to have different shapes, and the shapes were well represented by a sum of Lorentzian‐shaped components, in agreement with the newer theory. Studies of dimethylsulfoxide solutions of the p‐benzosemiquinone ion were also made. The spectra obtained in this system show only small linewidth variations and, in agreement with the theory, the variations ...
36 citations
27 citations
27 citations
TL;DR: In this article, the electron spin resonance spectrum of the dihydropyrazine cation radical was used to determine the signs of the isotropic hyperfine splittings.
Abstract: Linewidth studies of the electron spin resonance spectrum of the dihydropyrazine cation radical have been employed to determine the signs of the isotropic hyperfine splittings. It was found that the nitrogen splitting aN is positive and the proton splitting aNHH of the protons bonded to the nitrogen atoms is negative. The sigma—pi parameter QNHH in McConnell's relation for NH bonds (aNHH=QNHHρNπ) was found to be negative.
24 citations
TL;DR: In this article, a number of classical dynamical models are developed for describing the effects of internal rotational motions and solvent-complex formation on the ESR hyperfine linewidths of free radicals in solution.
Abstract: A number of classical dynamical models are developed for describing the effects of internal rotational motions and solvent‐complex formation on the ESR hyperfine linewidths of free radicals in solution. These dynamical processes can lead to linewidth effects because they cause time‐dependent modulations of the isotropic hyperfine interactions of the different magnetic nuclei. An alternating linewidth effect, which has been observed in a number of recent studies, is predicted to result when there is an out‐of‐phase correlation between the hyperfine splittings ai(t) of equivalent nuclei. By equivalent nuclei here are meant those for which the time‐average splittings 〈ai(t) 〉Av are equal, and an out‐of‐phase correlation is one in which an increase in the instantaneous splitting from Nucleus i, ai(t), is correlated with a decrease in the splitting aj(t) from nucleus j. This out‐of‐phase correlation can result from a coupling of the mechanical motions of different rotating groups and also from the effects that...
TL;DR: In this article, a method was developed for the assignment to nuclear positions of the hyperfine splittings in the electron spin resonance spectra of free radicals in solution, based on linewidth effects.
Abstract: A method is developed for the assignment to nuclear positions of the hyperfine splittings in the electron spin resonance spectra of free radicals in solution. The procedure is applicable to the splittings from the nuclei of atoms with a nonvanishing pi‐electron spin density, and it is thus useful for 13C and 14N splittings, but cannot be employed for proton splittings. The technique used depends on linewidth effects. It is shown that one contribution to the linewidths is approximately proportional to the square of the spin density on the atom containing the nucleus of interest (the local spin density), and that, under appropriate conditions, splittings from nuclei at positions with high spin density show greater linewidth effects than those from positions with low spin density. The technique is applied to two of the 13C splittings from nuclei present in natural abundance in the spectrum of the anthracene negative ion radical.