Anisotropic ENDOR spectra

Carotenoid neutral radicals are also formed under irradiation of carotenoids inside molecular sieves. Davies and Mims ENDOR spectra of lutein (Lut) radicals in Cu-MCM-41 were recorded and then compared with the simulated spectra using the isotropic and anisotropic hfcs predicted by DFT. The simulation of lutein radical cation, Lut +, generated the Mims ENDOR spectrum in Figure 9.7a. Its features at B through E could not account for the experimental spectrum by themselves, so contribution from different neutral radicals whose features coincided with those of the experimental... 

Thus, as a consequence of the selection of orientation subsets by choice of field value, the ENDOR spectrum changes as a function of the external field position or g value at which it is measured. A series of ENDOR spectra collected at fields across the EPR envelope samples different sets of molecular orientations. An example of this is shown in Fig. 6B. It can be seen that at the extreme g values (g, and g ) the ENDOR spectrum is the least complex, whereas at intervening g values the ENDOR spectrum shows multiple frequencies. The analysis of such spectra taken at several magnetic field positions gives the full tensor of the hyperfine interaction A, from which the isotropic and anisotropic components can be deduced. Procedures for this analysis have been described in detail elsewhere. 

For an arbitrary orientation of Bq, will no longer be parallel or antiparallel to Bq. The intensity ratio of transitions induced by I.h. and r.h. rotating fields is then not only determined by the anisotropic enhancement factor but also by the noncoincidence of Ben and Bq. For proton hfs with Af < 15 MHz the residual lines induced by a r.h. rotating field will be small, i.e. again an ENDOR spectrum with a reduced number of lines will be observed. In most metal complexes the dipolar part of the proton hfs tensors have been found to be below 15 MHz. 

Fig. 3.25 Schematic X-band powder ENDOR spectrum of an. S = Vi species with anisotropic H hyperfine structure. The hyperfine couphng tensor of axial symmetry is analysed under the assumption 0 < A < Aj. < 2-vh The lines for electronic quantum numbers ms = Vi and -Vi are centred at the nuclear frequency, vh 14.4 MHz, and are separated by distances equal to the principal values of the hyperfine coupling tensor as indicated in the figure. The difference in intensity of the ms = Vi and -Vi branches is due to hyperfine enhancement. Absorption-like peaks separated by A in the 1 st 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)...

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. 

E3.20 The procedure to obtain the anisotropic hyperfine couplings with rhombic symmetry is indicated in the idealized ENDOR spectrum below obtained at X-band (v = 9.5 GHz). 

The anisotropy of the line width is clearly manifested. When the external magnetic field is applied parallel to the stretching direction of the film, the signal intensity has a higher intensity in most fi equency regions than that in the case of the field being perpendicular to the direction. The spectra seem to be composed of more than two components. The ENDOR features at low temperatures can be interpreted as the direct evidence of the soliton like spin density by the simulation of the anisotropic spectrum. The maximum frequency of the ENDOR spectrum is related to the spin density, p(0) at the central carbon of the soliton as indicated in Fig. 7.41. 

As mentioned above, in an ENDOR experiment the rf field is swept while the static magnetic field is held at a constant position in the EPR spectrum. For slow sweep rates and narrow EPR lines a device would be desirable which is able to stabilize the ratio of the microwave frequency to the static magnetic field. The applicaiton of a commercially available field/frequency lock system is restricted to a region of 6 mT about the DPPH resonance field33). In metal complexes with strongly anisotropic EPR spectra, however,... 

From these results one can see the incredible power of the combined EPR/ENDOR experiment. While the EPR spectrum of irradiate adenosine had rather narrow lines, the spectrum was unresolved due to the overlap of several radicals. The ENDOR spectra were easy to follow for complete rotations about all three crystallographic axes. Analysis of the ENDOR data yielded accurate anisotropic hyperfine tensors that could be related to two different free radicals. From these results one can confidently say that Radical I is the N3 protonated adenine anion A(N3+H) and Radical II is the N6 deprotonated adenine cation A(N6-H) With ENDOR data one is able to determine the protonation state of a radical, and if care is taken in the analysis, to even discern slight deviations from planarity of radicals. 

Superhyperfine interactions are rarely observed in the ESR spectrum of the VO + ion because the unpaired electron interacts only weakly with the hgand nuclei, so that often the size of the coupling is less than the ESR bandwidth. This problem has been surmounted through the use of ENDOR spectroscopy. In ENDOR spectroscopy, molecules with their V=0 axes either parallel or perpendicular to the direction of the static magnetic field are selectively irradiated. In this way, the anisotropic superhyperfine coupling constants of H and " N and the " N quadrupolar coupling constants can be obtained. 

The hyperfine couplings can be obtained by direct analysis of the modulations on the echo decay curve. The analysis must then generally be made by fitting a simulated curve to the experimental. This procedure was usually employed in early work. Analysis of frequency domain spectta obtained by Fourier tfansformation (FT) is more common in recent studies. The resulting FT or frequency domain spectrum has lines with the same frequencies as in ENDOR. Like in ENDOR visual analyses of FT spectra are often followed by simulation to obtain accurate values for the anisotropic hyperfine coupling. 

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