Electron spin resonance study of nitroxide spin probes dissolved in the discotic mesophase of (p)-hexabis(n-hexyloxy)triphenylene (original) (raw)
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Nitroxide spin probes for magnetic resonance characterization of ordered systems
Colloids and Surfaces A: Physicochemical and Engineering Aspects, 2001
Stable free radicals belonging to different classes have been synthesized in view of potential applications in studies of organized systems. Among them a new pyrroline nitroxide with a high fluorine content, 2,2,5,5-tetramethyl-3pyrrolin-1-yloxy-3-[(1H,1H-perfluoro-1-octoxy)carbonyl] (3), apt to probe the hydrophobic perfluoro-rich domains inside macroaggregates, has been prepared by a rarely encountered approach namely the spin-labelling of the very cosurfactant participating in the self-assembling process. Emphasis has then been laid upon the synthesis of further stable free radicals from the group of substituted nitronyl nitroxides. A general experimental procedure has been adapted according to the nature of the 2-substituent, resulting in a series of nitronyl nitroxides with a wide range of hydrophobicity and showing spectral parameters in good agreement with literature values. Using the spin probe-electron spin resonance technique, 3 and the 1-H-imidazol-1-yloxy-4,5-dihydro-4,4,5,5-tetramethyl-2-ethyl-3-oxide (7, R=C 2 H 5 ) or the 1-H-imidazol-1-yloxy-4,5-dihydro-4,4,5,5-tetramethyl-2-undecyl-3-oxide (7, R =C 11 H 23 ) have been applied in ESR investigations on vesicle phases of a zwitterionic surfactant (TDMAO)/perfluorocosurfactant (PFC) system and on Triton X-100 reverse micelles, respectively. While spin probe 3 proved to be insensitive to differences between the charged and uncharged phases, probes 7 were able to detect changes in the micelle micropolarity induced by hydration. The results corroborate well with previous UV measurements in the Triton X-100 system using the same probes 7.
The Journal of Physical Chemistry B, 2010
Application of a new integrated computational approach for two widely used nitroxide spin probes allows to show unequivocally that proper account of stereoelectronic, environmental, and dynamical effects leads to magnetic properties in quantitative agreement with experimental results without the need of any empirical parameter. Together with their specific interest, our results point out, in our opinion, the importance of developing and validating computational approaches able to switch on and off different effects, including environmental and dynamical ones, in order to evaluate their specific role in determining the overall experimental outcome.
Metal-nitroxyl interactions. 12. Nitroxyl spin probes in the presence of tris(oxalato)chromate(III)
Inorg Chem, 1979
C r ( o~)~~is more effective than the commonly used Fe(CN)lin broadening the EPR spectra of nitroxyl spin probes. The extent of broadening of the EPR spectrum of 4-amino-2,2,6,6-tetramethylpiperidinyl-1 -oxy by C r ( o~)~~-depends on pH, the nature of the buffer, and the concentration of the nitroxyl as well as on the identity and concentration of the metal species PtS was found to undergo a high-pressure phase transformation at 30 kbar to a phase isomorphous with PdS. The ambient and high-pressure phases are diamagnetic semiconductors with electrical band gaps of 0.32 and 0.19 eV, respectively. The extent of solid solution of NiS in both phases of PtS was found to be less than 10 atom %.
The Journal of Physical Chemistry A, 2001
The isotropic and anisotropic hyperfine coupling constants and g-values of the nitroxide spin label (1-oxyl-2,2,5,5-tetramethylpyrroline-3-methyl)methanethiosulfonate (MTSSL) were determined from 9-GHz and 95-GHz electron paramagnetic resonance (EPR) measurements in various solvents with a large distribution in polarity and proticity. The parameters A iso , g iso , A zz , and g xx of MTSSL were found to be sensitive to changes in solvent properties, where A-values increased and g-values decreased due to increased solvent polarity or proticity. A linear correlation was found for the isotropic (g iso , A iso ) and anisotropic (g xx , A zz ) parameters, respectively. Furthermore, density functional theory (DFT) calculations of the same parameters were performed for a model spin label with the possibility to vary the dielectric constant ( ) of the medium and the number of hydrogen bonds formed with the nitroxide oxygen. From a qualitative analysis of experimental and calculated results, it was possible to specify the causes of the parameter shifts in more detail. In the "apolar region" ( < 25), the sensitivity of A iso and A zz to is large. However, in the "polar region" ( > 25), the sensitivity to is small, and the shifts in A iso and A zz are mainly determined by the proticity of the solvent. Methanol was found to form ∼1 and water ∼2 hydrogen bonds to the nitroxide on average. The DFT method determined the shifts in g iso and g xx due to hydrogen bonding more accurately compared with the restricted open-shell Hartree-Fock method. The anisotropic spin label-solvent data can be used in the interpretation of rigid-limit data from spin-labeled proteins to gain further insight in local environmental properties.
International Journal of Quantum Chemistry, 1996
We describe by approximate MO calculation a number of species for which high spin states are either the ground state or lie very low in energy. These are models for the spin coupling in projected ferromagnetic organic materials. The theory guiding their construction is based on planar conjugated systems, while experimentally realized systems are often far from planarity. We can by appeal to steric decoupling explain the failure of the prediction that methoxy subsitution on metaphenylene-coupled nitroxides should stabilize the triplet. However, we find in general that the qualitative rules derived from discussion of planar systems are robust; drastic departures from planarity are required before they lose their value. 0 1996 John Wiley & Sons, Inc. netic (i.e., spin-aligning) coupling between sites. A number of simple theoretical methods and principles have helped to guide and rationalize experimental design. These include the Huckel analysis of Longuet-Higgins [2] and the valence-bond treatment of Ovchinnikov [3], which permit the prediction of possible high-spin states in planar conjugated hydrocarbon species. The topology of rr systems is the foundation of all these analyses, and lntroduction he possibility of producing ferrmagnetic or-T ganic materials has attracted great interest [l]. The preparation of these materials requires sites with high local spin densities and ferromag-
Effect of solvent nature on spin exchange in rigid nitroxide biradicals
Applied Magnetic Resonance, 2006
Intramolecular electron spin exchange as a function of temperature and solvent viscosity and polarity has been studied by X-band electron paramagnetic resonance (EPR) spectroscopy in two rigid nitroxide biradicals existing in one spatial conformation only. Temperature variations of the isotropic hyperfine splitting constant a and exchange integral value IJ/a] were measured from EPR spectra and subsequently analyzed. The interaction of polar solvent molecules with >N-O fragments of nitroxide groups led to a slight decrease of the [J/al value with the increase of temperature T. In contrast, the interaction of polar solvent molecules with functional groups inside the bridge resulted in a noticeable increase of [J/a[ vs. T. In the last case, a converse relationship between the values of U/al and the hyperfine splitting constant a has been observed for solvents with different polarity.
Inversion-Recovery of Nitroxide Spin Labels in Solution and Microheterogeneous Environments
Journal of the American Chemical Society, 1996
Two modifications of a conventional inversion-recovery experiment which exclude the effect of spectral diffusion on the measured spin-lattice relaxation times of rapidly tumbling nitroxide spin labels are described. In the first approach an almost uniform inversion is achieved by means of specially designed pulse trains with excitation patterns which match the three-line nitroxide ESR spectrum. In the second approach we essentially combine an inversion-recovery for the M I ) 0 line with an analog of a pulsed ELDOR technique for M I ) (1 lines in a single experiment. This allows us to reconstruct the recovery of the total magnetization of the ensemble of radicals which is not affected by spectral diffusion. The spin-lattice relaxation times of several nitroxide spin labels in different homogeneous and microheterogeneous environments are measured and compared. The temperature dependence of T 1 times is compared for two solvents, methylcyclohexane and carbon disulfide, which differ in nuclear spin concentration by almost a factor of 1000. The reported experimental evidence suggests that interactions with the nuclear spins of a solvent do not significantly contribute to the spin-lattice relaxation of nitroxide spin labels.
A DNP Investigation of π and ς Transitions for Nitroxide Spin Probes at 1.53 mT
Journal of Magnetic Resonance, 1998
A weak field DNP study of 4-hydroxy-2,2,6,6-tetramethyl-1-piperidinyloxy (4-hydroxy-TEMPO), 4-amino-2,2,6,6-tetramethyl-1-piperidinyloxy (4-amino-TEMPO), and 2,2-Di(4tert-octylphenyl)-1-picrylhydrazyl (DPPH) free radicals in various solvents was performed at 1.53 mT using a double resonance spectrometer. transitions, in which the RF field has a component perpendicular to the static magnetic field B 0 , and transitions, in which the RF field has a component parallel to B 0 , were recorded. In order to observe the transitions a new probe head was designed. The transitions were obtained at the frequencies expected from theoretical calculations. However, the results obtained for the observed enhancements deviated from the values expected theoretically.
Applied Magnetic Resonance, 1992
The structure and conformation of carboxylic acid, formyl, and propenoic acid derivafives of the nitroxyl spin-label 2,2,5,5-tetramethyl-l-oxypyrroline have been determined by electron nuclear double resonance 0ENDOR) spectroscopy. From ENDOR spectra of the spin-label compounds in frozen solutions, we have assigned the resonance absorption features for each class of protons. The ENDOR spectra were analyzed on the basis of their dependence on H 0. The maximum and mŸ ENDOR shifts for each proton were shown to correspond to axially symmetric principal hyperfine coupling (hfc) components, from which the dipolar cont¡ were estimated to calculate electron-proton separations. Conformational analysis on the basis of torsion angle search calculations constrained by the ENDOR determined electron-proton distances revealed that in all three spin-label compounds the side chains ate in a planar conformation with respect to the oxypyrrolinyl ring. In the carboxylic acid and formyl derivatives the C=O group is in a s-trans conformation with respect to the vinyl group of the spin-label, while in the spin-labeled propenoic acid the conformation is found to be all planar trans-s-cis.