Powder ENDOR spectra

ENDOR measurements of glassy, polycrystalline or amorphous samples usually aim at deducing structural properties from an analysis of hyperfine- and nuclear quadrupole interactions that are too small to be resolved by ESR. The subject has been summarized in recent textbooks [3,4,12], in reviews about radicals on surfaces [38], about radical ions in frozen matrices [39], and about paramagnetic species in biological systems [40 4]. 

Several factors unique for ENDOR affect the intensities, i.e. magnetic relaxation, hyperfine enhancement, and angular selection. The two first effects also affect spectra of liquid and crystalline samples, while the third is typical for powder spectra of species with anisotropic g-values. Methods that take the two latter effects into account have been developed and are usually incorporated in software developed for the simulation of ENDOR spectra in the solid state. Simulations that take magnetic relaxation effects into account have been employed only to analyse ENDOR spectra in the liquid state [2]. It is possible that the commonly observed poor agreement between experimental and simulated intensities in the solid state is at least in part due to relaxation effects that are not taken into account in any software we are aware of. 

Powder ENDOR lines are usually broadened by the anisotropy of the hyperfine couplings. The parameters of well resolved spectra can be extracted by a visual analysis analogous to that applied in ESR. The principle is indicated in Fig. 3.25 for an 5 = V2 species with anisotropic H hyperfine structure, where the hyperfine coupling tensor of axial symmetry is analysed under the assumption that 0 A Aj. 2 vh- The lines for electronic quantum numbers ms = V2 and -Vi, centered at the nuclear frequency vh 14.4 MHz at X-band, are separated by distances equal to the principal values of the hyperfine coupling tensor as indicated in the figure. Absorption-like peaks separated by A in the 1st derivative spectrum occur due to the step-wise increase of the amplitude in the absorption spectrum, like in powder ESR spectra (Section 3.4.1). The difference in amplitude commonly observed between the ms = /2 branches is caused by the hyperfine enhancement effect on the ENDOR intensities first explained by Whiffen [45a]. The effect of hyperfine enhancement is apparent in Figs. 3.25 and 3.26. 

Anisotropic hyperfine couplings of rhombic symmetry give rise to powder ENDOR spectra of the type shown in Exercise E3.20. Absorption-like peaks in the ENDOR spectrum occur also in this case. 

Angular selection can affect the powder spectrum shape as schematically shown in Eig. 3.26. 

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CW ENDOR spectrum measurements carried out at 120 K (the optimum temperature for measuring resolved CW ENDOR powder spectra of carotenoid radicals) shows resolved lines from the P-methyl hfc (Piekara-Sady et al. 1991,1995, Wu et al. 1991, Jeevarajan et al. 1993b) (see Figure 9.5). The lines above 19 MHz are due to neutral radicals according to DFT calculations (Gao et al. 2006). 

Principal values of hyperfine coupling tensors for H (and other I = V2 nuclei) can often be obtained by analysis of the ENDOR powder spectra. Visual analysis is sometimes sufficient, in other cases computer simulation is required to refine the analysis. 

Nuclear quadrupole couplings due to nuclei with I > Y2 are obtained by fitting simulated ENDOR powder spectra to experimental. The procedure can be quite... 

FIGURE 9.5 CW ENDOR spectrum of 1-carotene radicals, (a) Experimental spectrum of Figure 9.4. (Reported in Wu, Y. et al., Chem. Phys. Lett., 180, 573, 1991.) (b) Simulated ENDOR powder pattern (using linewidth of 0.6MHz) for the sum of radical cation and neutral radicals in 5 3 1 1 ratio. (Reported in Gao, Y. et al., J. Phys. Chem. B, 110, 24750, 2006. With permission.)... 

Nitrogen ENDOR spectra of frozen solutions of the copper-containing analogue of vitamin BJ2 have been used to verify the corresponding EPR data obtained from powder simulations2331. The hf values from the very poorly resolved ENDOR spectra agree with the EPR results within experimental error. Since the errors estimated from the ENDOR spectra are only insignificantly smaller than those from the EPR powder spectrum, the ENDOR measurements will not alter the interpretation of the data. 

Proton ENDOR and DOUBLE ENDOR data on VO(acac)2 in frozen solutions have been reported by van Willigen2895. Since the CH-protons exchange with deuterons in CD3OD they may easily be discriminated from the methyl protons. From the single crystal-like ENDOR spectrum for B0 along g and the two-dimensional powder spectrum at gi, the hf coupling constants Ap 5 = 0.7 MHz and AfHj = 0.7 MHz, A 3 = 1.5 MHz, respectively, have been found. The relative signs of the principal... 

Fig. 9.5 X-band ESR spectra of L-a-alanine powder at room temperature. Exp. experimental powder spectrum X-irradiated to 60 kGy, Sim. reconstructed powder spectrum due to three radicals, Rl, R2, R3, with parameters determined by single crystal ENDOR measurements and with relative intensities 0.589 0.335 0.076, see text for assignment. Spectra are adapted from [M.Z. Heydari et al. J. Phys. Chem. A 106, 8971 (2002)] with permission from the American Chemical Society...

Furthermore, the method of orientation selection can only be applied to systems with an electron spin-spin cross relaxation time Tx much larger than the electron spin-lattice relaxation time Tle77. In this case, energy exchange between the spin packets of the polycrystalline EPR spectrum by spin-spin interaction cannot take place. If on the other hand Tx < Tle, the spin packets are coupled by cross relaxation, and a powder-like ENDOR signal will be observed77. Since T 1 is normally the dominant relaxation rate in transition metal complexes, the orientation selection technique could widely be applied in polycrystalline and frozen solution samples of such systems (Sect. 6). 

In many planar metal complexes it is not possible to record an ENDOR spectrum which only contains contributions from Bo orientations in the complex plane. This is due to the fact that in the powder EPR spectrum the high- or low-field turning points may arise from extra absorption peakssl which do not correspond to directions of the principal axes. ENDOR spectra observed near the in-plane region of such a powder EPR spectrum are due to molecules oriented along a large number of B0 directions (in- and out-of-plane), so that the orientation selection technique is no longer effective. 

Fig. 12 a, b. Orientation selection in ENDOR. a) Powder EPR spectrum of Co(salen)py. Arrow indicates EPR observer b) Single crystal-like ENDOR spectrum of the pyridine nitrogen with B0 along g . (From Ref. 80)... 

A number of poorly resolved proton ENDOR peaks have been observed between 10-20 MHz269-271. Only small changes of the overall shape of the ENDOR spectrum were detected when the sample was freeze-dried and redissolved in D20, i.e. no strongly coupled exchangeable protons were present. From comparison with the proton ENDOR spectrum in an anhydrous powder, it was assumed that the signals arose from the methylene protons of the cysteine ligands and that the iron-sulfur chromophore was not exposed to solvent water270/. 

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