Magnetic Resonance in Chemistry

Fawthrop, Susan A.; Gillies, Duncan G.; Sutcliffe, Leslie H.; Symms, Mark R.

doi: 10.1002/mrc.1260331317pmid: N/A

ESR spectroscopy can be used to measure the translational diffusion constants of spin probes in a variety of media using a capillary method. Following preliminary work, it has been possible to improve upon the accuracy of determining diffusion constants in two ways. Both methods require an experimental measurement of the resonant cavity sampling profile. In the first procedure, this cavity profile is convolved with a trial Gaussian distribution and the result is compared with the experimental data obtained from the ESR measurements; the diffusion constant is then determined by graphical optimization. The second method is a graphical one based on a knowledge of the experimental diffusion profile and the cavity sampling profile. Both methods have been shown to give similar results but the second less rigorous corrected‐zero method is generally simpler to apply. For a medium such as water, it takes several days to acquire the necessary experimental data but the time taken to carry out the experiment can be shortened (or slower rates of diffusion can be measured) by using a shielded resonant cavity.

Mombourquette, M. J.; McEachern, R. J.; Weil, J. A.

doi: 10.1002/mrc.1260331313pmid: N/A

Germanium 3+ occurs in α‐quartz substitutionally for silicon 4+, either as complex anions [GeO4]− or these anions charge‐compensated with interstitial alkali cations, i.e. as two primary types of species denoted by [GeO4/M] A, C0, where M = Li, Na. A brief review of the relevant literature is presented. Using the molecular‐orbital spin population for the unpaired electron in each center, available from 73Ge and 17O hyperfine data, an evaluation of the likely M+ positions within the quartz structure has been performed. In part, the present calculations were done to test the well known McConnell‐Strathdee approach with well defined experimental hyperfine data. A generalized discussion of atom‐position determination by EPR, and problems encountered in the process, is included.

Binmore, Gavin T.; Walton, John C.; Adcock, William; Clark, Christopher I.; Kristic, Alexander R.

doi: 10.1002/mrc.1260331310pmid: N/A

4‐Substituted bicyclo [2.2.2] oct‐1‐yl radicals were generated by bromine atom abstraction from the corresponding 1‐bromobicyclo [2.2.2] octanes and observed in solution by EPR spectroscopy. A tendency towards lower a(Hβ) and a(Hγ) values for inductively electron‐withdrawing substituents such as OMe and F and towards higher values for electron‐releasing groups such as Me3Ge and Me3Sn was observed. For a series of bridgehead radicals, the hfs of β‐hydrogens showed a monotonic increase as the extent of flattening at the bridgehead increased. The EPR data indicated that 4‐substitutents exercised a significant effect at the radical centre, mainly by a through‐bond mechanism. 10‐Substituted triptycl radicals were generated in a similar way but showed no hfs from magnetic nuclei of the substituents. Thus, the triptycyl cage transmitted spin density much less effectively than the bicyclo [2.2.2] octyl cage. Bicyclo [2.2.2] oct‐1‐yl and adamant‐1‐yl radicals added to benzene, tert‐butylbenzene and 1,3‐di‐tert‐butylbenzene to give cyclohexadienyl radicals which were characterized by EPR spectroscopy. Triptycyl radicals and strained bridgehead radicals such as cubyl and bicyclo [1.1.1] pent‐1‐yl gave no detectable cyclohexadienyl radicals under similar conditions. Both bicyclo [2.2.2] oct‐1‐yl and adamant‐1‐yl radicals generated in tert‐butylbenzene showed exclusive meta addition with formation of the corresponding 1‐polycyclo‐3‐tert‐butylcyclohexadienyl radical.

Li, Lijuan; Morton, John R.; Preston, Keith F.

doi: 10.1002/mrc.1260331305pmid: N/A

The oxidation of Fe(NO)2(CO)2 by tetracyanoethylene (TCNE), by the nitrosonium ion (NO+), or by electrolysis in polar solvents was studied by EPR spectroscopy. A number of long‐lived (τ1/2 of the order of hours) free radicals were detected in the early stages of reaction, all having similar isotropic g‐factors and 14,15N hyperfine interactions. These have been identified as the mononuclear cations, Fe(NO)2(CO)2−nStn+ (St = solvent molecule). They slowly disappear and are replaced by persistent radicals (τ1/2 of the order of days) that show equal isotropic hyperfine interactions with four‐, six‐ or eight‐nitrogen nuclei. Infrared and EPR spectroscopic data suggest that two of these species are trinuclear nitrosyl clusters, Fe3(NO)2n(CO) (12−3n)+ (n = 3, 4), in which the unpaired spin is localized on one iron nucleus, but which are fluxional on the EPR time‐scale. The third nitrosyl radical with four equivalent NO ligands is tentatively identified as a binuclear intermediate that may be a precursor of the trinuclear clusters.

doi: 10.1002/mrc.1260331301pmid: N/A

Harris, Robin K.

doi: 10.1002/mrc.1260331303pmid: N/A

doi: 10.1002/mrc.1260331328pmid: N/A

Batchelor, Stephen N.; Sun, Licheng; Möbius, Klaus; Kurreck, Harry

doi: 10.1002/mrc.1260331307pmid: N/A

Time‐resolved EPR spectra are reported for photoexcited porphyrin—quinones with butylene and cis‐ or trans‐ cyclohexylene spacers between porphyrin donors and crown ether—quinone acceptors. In the nematic phases of liquid crystals, polarized triplet EPR spectra of the porphyrin moiety could be observed at early delay times (0.1–0.6 μs) after laser flash irradiation. At later delay times (1–2 μs) the triplet EPR spectra of the charge‐separated radical pair state, created by intramolecular electron transfer from the porphyrin donor to the crown ether quinone acceptor, could be detected. The similarity of the zero‐field splitting parameters D of the cyclohexylene‐ and butylene‐linked compounds shows that the radical pair of the flexible butylene‐linked compound is in a stretched conformation. However, longer laser irradiation of the species with the flexible bridge results in the formation of a novel triplet radical pair species with a shorter porphyrin—quinone separation indicating a photoinduced backfolding of the quinone over the porphyrin plane. Comparison of the cis‐ and trans‐cyclohexylene‐linked compounds reveals a distortion of the cyclohexylene spacer in the cis compound.