Nitroxide spin radical

MiniDium Number of Steps n. Needed to Reach the Convergence of the C.F. and (in brackets) Elapsed Computing Times (s) to Obtain ESR Spectra" of a Nitroxide Spin Radical as a Function of Parameters P2 and r. 

Nitronyl and imino nitroxide free radicals are also among the most versatile spin carriers which are widely used in the design of molecular magnets. Their delocalised unpaired electrons make them convenient building blocks and ideal magnetic bridges between magnetic metals, to achieve new compounds with particular... 

Zheludev, A., Bonnet, Luneau, D. et al. (1995) The spin density in an imino nitroxide free radical a polarized-neutron study, Physica B, 213-214, 268-271. 

Ressouche, E., Zheludev, A., Boucherle, J.X. et al. (1993) Spin densities in nitronyl nitroxide free radicals, Mol. Cryst. Liqu. Cryst., 232, 13-25. 

Zheludev, A., Baron, V., Bonnet, M. et al. (1994) Spin density in a nitronyl nitroxide free radical. Polarized neutron diffraction investigation and ab initio calculation, J. Am. Chem. Soc., 116, 2019-2027. 

We report herein a single crystal polarized neutron investigation of the spin density of two purely organic nitronyl nitroxide free radicals which present ferromagnetic interactions the 2-(6-ethynyl-2-pyridyl)-4,4,5,5-tetramethylimidazoline-l-oxyl-... 

Method involves measuring the change in rotational correlation time for a free radical probe (e.g., nitroxide spin probes) introduced into a sample being studied. The temperature associated with a decrease in the rotational correlation time of a spin probe is assigned as Tg... 

Diradical species 35a and 35b, in which two imino nitroxide 35a or nitronyl nitroxide 35b radical centers are attached to thieno[2,3-. ]thiophene, were prepared and their intramolecular exchange interactions were investigated in frozen solutions by means of electron spin resonance (ESR) spectroscopy and magnetic susceptibility measurements at cryogenic temperature <1996T6893>. 

Organic nitroxides (136a, 5), nitroparafins, and nitrolic acids (107) react with aqueous solutions of pentacyanocobaltate(II) salts to give alkyl pentacyanocobalt(III) nitroxide anion radicals, spin traps involving the formation of an N—Co bond (5). 

There have been many investigations of phosphotransferases by NMR and EPR methods.681,682 One approach is to use paramagnetic ions such as Mn2+, Cu2+, or Cr3+ to induce nuclear relaxation in substrate and coenzyme molecules at active sites of enzymes. Flavin radicals and specifically introduced nitroxide spin labels can serve as well. Paramagnetic ions greatly increase the rate of magnetic relaxation of nearby nuclei (Chapter 3, Section I). Thus, small amounts of... 

Belkin S, Mehlhorn RJ, Hideg K, Hankovsky O, Packer L (1987) Reduction and destruction rates of nitroxide spin probes. Arch Biochem Biophys 256 232-243 Berdnikov VM, Bazhin NM, Fedorov VK, Polyakov OV (1972) Isomerization of the ethoxyl radical to thea-hydroxyethyl radical in aqueous solution. KinetCatal (English translation) 13 986-987 BerkowitzJ, Ellison GB, Gutman D (1994) Three methods to measure RH bond energies. J Phys Chem 98 2744-2765... 

The reaction of cyclic nitrones with free radicals takes place as in Scheme 8.21 with 5,5-dimethyl-l-pyrroline IV-oxide (DMPO) as an example.119,120 The resulting stable nitroxide spin adducts can be detected and quantified by ESR. 

Electron spin resonance (ESR) is the most direct method for detection of free radicals. Direct ESR, however, is not very sensitive, and radicals with short lifetimes do not build up to detectable levels of steady-state concentrations. The technique of spin trapping is a very effective approach to increase the lifetime of free radicals such as OH, O -, etc. [68], Spin trapping is a kinetic technique in which the transient radical reacts with a nitrone or nitroso spin trap to produce a more stable nitroxide spin adduct. 

Spin traps are diamagnetic organic molecules containing either nitroso (-N=0) or nitrone (>C=N—>0) functional groups. The spin-trapping reaction is a kinetic method in which the transient radical (R ) reacts with a nitrone or nitroso spin trap forming a more stable nitroxide spin adduct (Eqs. 1 and 2), which is readily detectable by ESR. 

The other, very important role played by nitroxides in radical chemistry is via the use of spin-traps which are converted into nitroxides on reaction with reactive radicals (e.g., reactions [1.15] and [1-16]). 

It has been demonstrated that it is possible to insert paramagnetic probes into nucleic acids in a site- and type-specific manner. To this end, the 2,2,6,6-tet-ramethyl-l-piperidinyloxy free radical-labelled phosphoramidites of 2-amino-2 -deoxyadenosine, 2 -deoxyadenosine, 2 -deoxycytosine and 5-methyl-2 -deoxycytosine have been prepared and used for the automatic synthesis of mono-labeled oligodeoxynucleotides which proved active by EPR. The phosphoramidites carry a nitroxide spin-label directly linked to the exocyclic amino group. Starting from appropriately protected 2 -deoxyguanosine or 2 -deoxyinosine, the precursors to (66a) and (66b) were obtained by nucleophilic... 

Hindered nitroxides are a class of free radicals that are unusually long-lived. The presence of methyl groups y to the radical center decreases the rate of self reaction to such an extent that the radical becomes kinetically stable. Hindered nitroxides can be obtained as crystalline solids or pure liquids, which in the absence of other materials are very stable. These materials have found extensive use as spin labels and spin probes. Indeed, almost all spin-label studies have employed nitroxides spin-probe studies have used either nitroxides or paramagnetic metal ions. 

Wien, Morrisett, and McConnell23 have employed nitroxide free-radical spin labels instead of paramagnetic metal ions. They found that lysozyme covalently spin-labeled at histidine-15, which is not at the active site, broadens the proton resonances of bound N-acetyl-a-D-glucosamine, enabling a distance (of the order of 20 A) to be estimated, in agreement with the x-ray structure. 

Block equations can be constructed that describe the macroscopic magnetization of the spin system including the relaxation mechanism involving spin exchange caused by molecular collisions. These equations were solved as a function of the exchange frequency, wex, for the nitroxide free radical case. Spectra were computed by assuming the degrees of motional freedom observed with the labeled androstane or cholestane molecules localized at the air-water interface. 

For such short-lived radicals, the technique of spin trapping may be employed (Janzen, 1980). This technique involves the addition of the radical (R ) to an organic diamagnetic nitrone or nitroso compound to form a longer lived nitroxide free radical. The structure of the parent free radical may then be determined from the hyperfine coupling of the ESR spectrum of the resultant spin adduct. Nitrones are very reactive and their adducts are stable, even though they do not provide much structural information. On the other hand, nitroso adducts have unique ESR spectra but they are photolytically unstable. 

Other quenchers that have been used in the benzophenone-sensitized cis trans isomerization of stilbene are /9-carotene, oxygen, and di-tert-butyl nitroxide. /9-Carotene shifts the photostationary state to the trans side similar to the azulene effect [237]. In the presence of di-tert-butyl nitroxide, a radical quencher, the photostationary state is slightly shifted to the cis side Caldwell and Schwerzel [226] have suggested the involvement of the twisted triplet state and a quenching mechanism other than energy transfer, probably vibrational relaxation to the ground state caused by spin exchange. 

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