Nitroxide biradicals

To illustrate the potential of the method, we present in Figure 7 the 2D ESRI perspective plot of a phantom sample consisting of nitroxide biradicals derived from Tinuvin 770 (obtained by oxidation of the amine) in two different environments in a plaque of heterophasic propylene-ethylene copolymers (HPEC). One side of the plaque was doped with the biradical by contact with a biradical solution in toluene. The same biradical solution was placed on transparent tape and the solvent was evaporated the tape was then folded and attached to the other side of the plaque. The corresponding spectral slices are... 

Figure 7 (a) 2D spectral-spatial perspective plot of nitroxide biradicals derived from HAS doped in HPEC on one side of the plaque (left), and in a transparent film attached to the plaque on the opposite side (right), (b) Spectral slices determined nondestructively from 2D plot ESR spectra of biradicals located in two amorphous phases in the polymer (upper), and dispersed in a film (bottom). 

Now consider a concrete example. Suppose we have a nitroxide biradical with aN = 13 G. In the strong exchange limit, we expect a five-line spectrum with a spacing of 6.5 G and the usual 1 2 3 2 1 intensity ratios for two equivalent spin-1 nuclei. In the weak exchange limit, we expect a three-line spectrum with a spacing of 13 G and intensity ratios 1 1 1. In intermediate cases, up to 15 lines are expected, as shown in Figure 6.1. 

The nitroxide biradical 35 has approximate Z)2 symmetry. The electron spin resonance (ESR) spectrum has been obtained in ethanol and computer simulated in frozen ethanol <1995MRCS129>. 

Pulse Measurements. Comparison of 3-pulse PELDOR data for nitroxide biradicals with interspin distances of 15.4 to 24.0 A at X-band and S-band demonstrated the separation of nuclear hyperfine and dipolar interactions and the separation of contributions from dipolar and exchange interactions.20 However, the deeper nuclear modulation at S-band limits the options for interpulse spacings. The inability to observe dipolar modulation in a PELDOR experiment... 

The four-pulse DEER experiment permits dead-time free measurements.21 For a nitroxide biradical with an interspin distance of 19.4 A there was good agreement between results obtained by 3-pulse and 4-pulse DEER. For a second biradical the interspin distance of 28.3 A obtained by 4-pulse DEER was in good agreement with the X-ray crystal structure. The accessible distance range was estimated to be 15 to 80 A. 

Margraf, D., Bode, B. E., Marko, A., Schiemann, O., and Prisner, T. F. (2007). Conformational flexibility of nitroxide biradicals determined by X-band PELDOR experiments. Mol. Phys. 105, 2153-2160. 

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

Formation of Stable Radicals. The oxidation of A(,A( -di-hydroxyimidazolidines (7) with sodium periodate or silver(I) oxide affords several stable oligopyridine-based nitronyl-nitroxide biradicals (8) in moderate to good yields, under phase transfer conditions (dichloromethane-water medium) at 0 °C (eq 36). Also, the related bis-iV-hydroxyimidazolidines react in a similar way furnishing the corresponding imino-nitroxide biradicals albeit in modest yields. ... 

The exchange coupling can in certain cases be determined by an analysis of the hyperfine pattern due to nitrogen observed for instance in nitroxide biradicals in liquid solution. A procedure to extract the value of J was developed many years ago [43]. This procedure does not depend on the microwave frequency and measurements were usually made at X-band. Three cases may appear, (1) / [Pg.195]

Figure 23.6 (a) Two-dimensional (2D) spectral-spatial perspective plot of nitroxide biradicals derived from hindered amine stabilizer (HAS) doped in HPEC (copolymer of ethylene and propylene [39c,b]) on one side of the... 

Perhaps the first studies of nitroxide pairs with HF EPR were carried out by Lebedev and co-workers. The authors noted that although the number of spectral components of the rigid-limit 140 GHz EPR spectra of a nitroxide biradical is greater than at conventional X-band, the spectrum is still easier to interpret because of an excellent angular resolution which allows the spectral components and the corresponding dipolar coupling to be distinguished. Basically, for well-resolved spectra, the components of the dipolar tensor can be estimated directly from the spectrum. 

Aussenac, F., Yulikov, M., Jeschke, G., Copdret C., Lesage, A., Tordo, P., and Emsley, L. (2013) Large molecular weight nitroxide biradicals providing efficient dynamic nuclear polarization at temperatures up to 200 K. / Am. Chem. Soc., 135,12790-12797. 

We would like to stress that in all these publications the authors investigated peculiarities of the rotational and translational diffusion of spin-probe molecules in various room temperature ionic liquids, comp)ared them with molecular dynamics in common organic solvents. Correlations with Stokes-Debye-Einstein or Stokes-Enstein laws were foimd. Areas in RTILs (polar, non-polar), in which spin probes (hydropElic, charged, hydrophobic) are localized were determined. Just recently, attention of the scientists was attracted to another type of molecular motions in the ionic liquids (Tran et al., 2007a, 2009). Such processes as well as solvent effects on them can be examined in detail by EPR sp>ectroscopy with the use of stable nitroxide biradicals (Parmon et al., 1977a, 1980). 

Kokorin, A. I. (2004). Regularities of the spin-exchange coupling through a bridge in nitroxide biradicals. Appl. Magn. Reson., Vol. 26, No. 1-2, 253-274, ISSN 0937-9347... 

Parmon, V. N. Kokorin, A. I. Zhidomirov, G. M. (1977a). Conformational structure of nitroxide biradicals. Use of biradicals as spin probes. Russ.. Struct Chem., Vol. 18, No. 1,104-147, ISSN 0022-4766... 

Tran, V. A. Kokorin, A. L Grampp, G. Rasmussen, K. (2009). Peculiarities of spin exchange in nitroxide biradicals in the ionic liquid bmimPFg. Appl, Magn. Reson, Vol. 35, No. 3, 389-398, ISSN 0937-9347... 

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