Duncan G. Gillies

NMR DETERMINATION OF EPR HYPERFINE COUPLING CONSTANTS OF SOME 5-(N-ALKYL)-1,1,3,3-TETRAKIS(TRIDEUTERIOMETHYL)ISOINDOLIN-2-YLOXYLS

2D 1H–13C chemical shift correlation spectroscopy has been used to determine some small 1H and 13C EPR hyperfine interaction constants in some 5-(n-alkyl)isoindolin-2-yloxyl radicals. In the methodology used, it was necessary to measure the chemical shifts of both the isoindolines and of the cognate isoindolin-2-yloxyl radicals. The latter n-alkyl-substituted radicals have considerable potential as EPR spin probes in lipid systems.

Molecular motion of a water-soluble nitroxyl radical in gelatin gels

Electron spin resonance spectroscopy (ESR) has been used to measure the rotational correlation times and translational diffusion constants of a radical spin probe in gelatin gels. The radical used is NaTMIOS, the sodium salt of sulphonated 1,1,3,3-tetramethylisoindolin-2-yloxyl. It was found that the mobility of the radical was influenced by the gelatin concentration and by temperature. It was concluded that the spin probe was constrained by the polymer chains and, when conditions were such that the distance between the chains was of the order of the size of the probe, rotational motion was severely impeded.

ESR measurements of the partitioning of some new spin probes in n-octanol–water

The stable free radical 1,1,3,3‐tetramethylisoindolin‐2‐yloxyl (TMIO) has proved to be very suitable for use as a spin probe for a number of applications. Because it is soluble mainly in non‐polar liquids, there is a need for new derivatives that can be used in a variety of environments. This has been done by introducing substituents in the 5‐position of the aromatic ring, namely carboxyl (CTMIO), trimethylamino (TMTMIOI) and sodium sulphonate (NaTMIOS). An accurate ESR method was developed for the measurement of partition coefficients in n‐octanol–water. For comparison purposes the method was also applied to some Tempo derivatives. The effect of temperature on the rotational correlation times and the nitrogen‐14 hyperfine coupling constant of some of the spin probes was investigated. There is evidence for dimerization of CTMIO to form a biradical. Copyright

An EPR and NMR study of some tetramethylisoindolin-2-yloxyl free radicals

Isoindolin-2-yloxyls are stable free radicals whose potential as spin probes/labels has not yet been exploited. We have prepared 1,1,3,3-tetramethylisoindoline, its 5-nitro derivative (paving the way to adding prosthetic groups) and a version with fully deuteriated methyl groups. These isoindolines have been oxidized to give the corresponding free radicals. It was found that the isotropic EPR spectra of the radicals have linewidths much smaller than those of the TEMPO family of radicals. The parent radical has a spectrum characterized by hyperfine interactions from nitrogen and from protons of the methyl groups. By measuring the NMR chemical shift differences between the isoindolines and the isoindolinyloxyls, values and signs of hyperfine splittings have been determined for the aromatic protons (too small to measure even in well resolved EPR spectra) and for some of the carbon-13 nuclei (at natural abundance). An important advantage of the NMR method is that assignments can be made with near certainty. This advantage has been taken further by obtaining 1H–13C chemical shift correlation spectra of the free radicals.For spin probe measurements, accurate values of the anisotropic g-factors and nitrogen hyperfine interactions are needed: these have been derived from the EPR powder spectrum of the deuteriated radical.

Effects encountered in EPR spectroscopy and imaging at small magnetic fields

In vivo EPR studies of biological systems and other ‘wet’ systems are often performed at radiofrequencies (low static magnetic fields) in order to minimize conductive losses incurred with relatively large samples. Here, attention is drawn to several consequences which can arise from carrying out EPR spectroscopy and imaging (EMRI) at low fields. The first causes a distortion of the gradient when the applied gradient is not a small fraction of the external magnetic field. These ‘concomitant gradients’ could pose problems if high-resolution imaging experiments are attempted. Other phenomena stem from the Breit–Rabi effect which causes spectral distortions and has implications for data acquisition and image processing. A third effect is the near removal of g factor anisotropy resulting from using low magnetic fields. Thus the powder spectrum for a typical aminoxyl radical is completely different from that observed at X-band: this has consequences for both imaging and for the measurement of rotational diffusion constants. It is shown that there are disadvantages in using 15N- labelled spin probes/labels for EPR measurements at radiofrequencies.

MATCHED SPIN PROBES FOR THE STUDY OF THE OVERALL MOTION OF MODEL LUBRICANTS

For ESR studies of molecular rotation in liquids, it is desirable to match the spin probe as closely as possible to the solvent molecules. In this work the solvents were model lubricants whose overall motion has been characterized from carbon‐13 NMR relaxation measurements. Two compounds were studied, the approximately cylindrical mol‐ecule 5‐(n‐octyl)naphthalene (ON) (a model low‐traction fluid) and the approximately spherical molecule dicyclohexy‐3,4‐furan dicarboxylate (DCFDC) (a model high‐traction fluid). For these two compounds the matched spin probes 5‐(n‐octyl)‐1,1,13,3‐tetrakis(trideuteriomethyl)isoindolin‐2‐yloxyl (OTMIOD) and 4,5‐bis(cyclohexyloxycarbonyl)‐1,3,2‐dithiazol‐2‐yl (BCCD), respectively, were prepared. ESR measurements were made on OTMIOD in ON and in toluene at two frequencies, namely 9 GHz (X band) and 94 GHz (W band), and the importance of making the high‐frequency measurements was demonstrated for evaluation of the anisotropy parameters. The results are different for the two solvents, thus proving the necessity for matching the spin probe to the solvent. BCCD was shown to be tumbling isotropically in DCFDC. Activation energies were derived for the overall motions from the ESR and NMR measurements. These activation energies were compared with those for the viscosities of the solvents. The viscosities of both ON and DCFDC were measured over a wide temperature range using an ESR method.

Magnetic resonance measurements of solubilizate distribution coefficients and surface probabilities in some micelles

Proton relaxation rates have been used to measure (i) distribution coefficients of solubilizates in a variety of micelles, (ii) surface probabilities (which provide information on the orientation of a surfactant molecule with respect to the micelle surface). For both types of work, paramagnetic ions are added to the solutions and it has been established that for (i) the paramagnetic ions must not complex with the head groups and be completely insoluble in the micelle: Fremys salt has been used with some negatively charged surfactants to refine a method for the measurement of distribution coefficients. For (ii) the paramagnetic ion must be associated with the head groups hence transition metal ions such as those of copper(II) and chromium(III) are suitable. The surface probabilities obtained for most of the surfactants are consistent with a simple linear orientation; however, some types of surfactant have unusual conformations. It has also been shown that the location of solubilizates can be determined from surface probability measurements.

Simple EPR method for measuring translational diffusion constants

The capillary method has been adapted to provide a simple EPR method for measuring translational diffusion constants of spin probes.

X-ray crystallography and NMR spectroscopy of some cyclohexyl esters

Abstract The X-ray structures and proton and carbon-13 NMR spectra have been obtained for two dicyclohexyl esters and one tricyclohexyl ester: namely, di-2-methylcyclohexylsuccinate (D2MCS), dicyclohexyl-3,4-furandicarboxylate (DCFDC) and tricyclohexyl-1,2,3-propanetricarboxylate (TCPTC). These esters are models for high-traction fluids. It was shown that the molecular modelling calculations we performed (on DCFDC and TCPTC) are not adequate for determining the minimum energy configuration of the compounds investigated.

A radiofrequency ESR spectrometer/imager

A Radiofrequency ESR spectrometer/imager is described. Modified from a Surrey Medical Imaging Systems NMR spectroscopy/imaging console, it uses a PC with associated waveform controller cards to perform CW ESR. Digital phase-sensitive detection is provided by a vector processor. The “target-field”-designed magnet and gradients are driven by audio amplifiers. The dual-channel broad-band RF bridge has a reflection homodyne configuration. Birdcage coils have been built that resonate near 200 MHz. Spectra and images of lossy samples of up to 4 cm diameter have been acquired.