Dinitroxides

A second class of molecules with two unpaired electrons has the two electrons localized, to a first approximation, in different parts of the molecule. We refer to such molecules as biradicals (or, with three or more unpaired electrons, triradicals, etc.). Examples are the dinitroxides (e.g. 1), and certain binuclear vanadium(iv) and copper(n) complexes. 

Figure 6.1 Stick spectra for a dinitroxide biradical with a= 13 G for various values of the exchange coupling constant J. (Several very small, widely spaced resonances have been omitted for J=2A and J=5A.)...

Maliakal AJ, Turro NJ, Bosman AW, Cornel J, Meijer EW. Relaxivity studies on dinitroxide and polynitroxyl functionalized dendrimers effect of electron exchange and structure on paramagnetic relaxation enhancement. J Phys Chem A 2003 107 8467-8475. 

The dinitroxide (20) is one of several nitroxide biradicals used to test the point-dipole approximation for electron-electron dipolar interactions/ ... 

Novel dinitroxide derivatives of ketones, of type (19), have been obtained from the corresponding imidazolidines (20) and offer prospects as spin labels. ... 

Maliakal et aV have studied the proton relaxivity produced by dinitroxide spin labels attached to poly(propylene imine) dendrimers as potential MRI contrast agents. They find that proton relaxivity, which is substantially higher than for mononitroxide spin labels, is not seriously affected by electron selfexchange. 

Some dinitroxide radicals, e.g. (360), have been made which fulfil the... 

The synthesis and e.s.r. spectrum of the dinitroxide radical (505) have been reported. ... 

Reduction of the radical center to the hydroxylamino group, its shielding and subsequent oxidation of the amino group with m-chloroper-benzoic acid, and deshielding produced dinitroxide 18, in which the nitroxide groups are separated from each other by only one carbon atom. 

The interaction of 64a with sodium amide in liquid ammonia gives dinitroxide 51, and the interaction of 64b with 1,5-diazabicyclo- [5,4,0] undecene-5 (DBU) in THF yields 4-vinyl-3-imidazoline-3-oxide-l-oxyl (73). This vinyl nitroxide (73) easily adds a molecule of bromine, giving dibromo derivative 74, and it regenerates 6b on treatment with NaBH4. Dehydrobromination of 75 under similar conditions leads to nitroxide 76 (Grigor ev et al, 1979a). 

The interaction of 4-phenyl-3-imidazoline-l-oxyl (13c) with PdCl2 led to cyclometalation product 116, a dinitroxide with halo gen-bridging atoms (Ovcharenko et al, 1979). The reaction of 13c with mercury salts... 

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