ENDOR orientation selection

In powder samples with broad EPR lines, large Zeeman modulation amplitudes have to be applied to improve the sensitivity. Such amplitudes often produce microphonic noise in the cavity and cause an uncertainty in the orientation selection in single crystal-like ENDOR spectra (Sect. 4.1). A modulation technique which avoids these problems in powder ENDOR studies has been proposed by Hyde et al.32). In this scheme the Zeeman modulation is replaced by a 180° modulation of the phase of the microwave signal. 

Orientation selection 4.1 Determination of hf and quadrupole coupling constants from single crystal-like ENDOR spectra... 

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)... 

ENDOR on VO(II) complexes is facilitated for several reasons289 (a) the EPR transitions can easily be saturated (ENDOR signals are often observed at 100 K), (b) the large anisotropy of the Av tensor allows for a high orientation selectivity in powder samples, and (c) the V=0 bond may be used as internal reference axis. Moreover, the g tensor is nearly isotropic so that contributions to the hf interactions from an unquenched orbital moment may be neglected (Sect. 5.1). 

ENDOR experiments can be performed in liquid solution, in which only the isotropic hfc s (Ajso) are detected. They are proportional to the spin density at the respective nucleus. Erom the assigned isotropic hfc s a map of the spin density distribution over the molecule can be obtained. In frozen solutions and powders the anisotropic hf interactions can also be determined. Eurthermore, the method allows the detection of nuclear quadrupole couplings for nuclei with 1 1. For dominant g anisotropy as found in many metal complexes the external magnetic field can be set to several specific g values in the EPR, thereby selecting only those molecules that have their g tensor axis along the chosen field direction. In such orientation-selected spectra only those hf components are selected that correspond to this molecular orientation ( single crystal-like ENDOR ). 

Gessner, C., Stein, M., Albracht, S. P. and Lubitz, W. (1999) Orientation-selected ENDOR of the active center in Chromatium vinosum [NiFe] hydrogenase in the oxidized ready state. /. Biol. Inorg. Chem., 4, 379-89. 

Foerster S, van Gastel M, Brecht M, Lubitz W. An orientation-selected ENDOR and HYSCORE study of the Ni-C active state of Desulfovibrio vulgaris Miyazaki F hydrogenase. J Biol Inorg Chem. 2005 10(1) 51 62. 

The orientation selection that results as gobs values are varied across the EPR powder pattern envelope would be of little interest were it not for the fact that this procedure reveals anisotropy in the hyperfine coupling. This is shown in cartoon fashion in Figme 3. The analysis of such a field-dependent 2-D ENDOR pattern can be challenging, even when there are no overlapping signals from different nuclei. The quantitative analysis of such complex patterns is a hallmark of many recent ENDOR studies of bioinorganic systems. " " ... 

So far, no single-crystal EPR studies of flavin radicals have been reported. However, the orientations of the g-principal axes relative to the molecular frame of the flavin s isoalloxazine moiety have been derived from orientation-selection effects of the quite anisotropic hyperfine coupling of H5 (or D5 in an isotope-exchange experiment) of the neutral flavin radical, both with EPR [28] and ENDOR... 

When the -anisotropy is small the analysis of the orientation-selective ENDOR experiments benefits from the improved resolution of the different -components at high field, e.g. at W-band. This procedure is well illustrated by work to clarify the structures of the adsorption complexes of nitric oxide (NO) interacting with metal ions in zeolites [50]. These structures are of interest from an applied view to elucidate the catalytic decomposition of NO into N2 and O2 over transition metal... 

Powder ENDOR Hyperfine couplings obtained by the angular selection method with the field set at anisotropic gx, gy and gz features can give single-crystal-like ENDOR spectra from randomly oriented samples. The enhanced resolution of g-anisotropy at high magnetic field increases orientation selectivity of ENDOR spectra in amorphous systems. 

The principal values and even the orientation of the principal axes of the Na hyperfine coupling tensor with respect to axes of the g tensor could be determined from Mims and Davies pulsed ENDOR spectra, refer to Section 2.3.3 in Chapter 2. The values Axx( Na) = Ayy( Na) = 6.3 and Azz( Na) = 10.9 MHz were obtained by simulation taking angular selection into account. The so-called hyperfine enhancement of ENDOR intensities due to the interaction between the radio frequency field and the electron spin could lead to pronounced differences in the ENDOR intensities between signals from different rris electron spin states in experiments at conventional MW frequencies such as in X-band, but also at the W-band. The Na (I = 3/2) nuclear quadrupole tensor is almost coaxial to the A tensor, 2zz = 0.48 MHz, Qyy = -0.07 MHz, and Qxx = -0.41 MHz. Simulation of orientation-selective ENDOR spectra as described in [26, 33] serves to refine the principal values of the hyperfine coupling tensors estimated from experiment. In... 

More advanced experiments, such as ENDOR, electron spin echo envelope modulation (ESEEM), or relaxation measurements by pulsed ESR rely on a selective excitation of spins close to the resonance field. Usually, the powder ESR spectrum is much broader than the excitation bandwidth of the pulses, which is in the range between 2 and 10 G. In cases where one anisotropic interaction dominates the spectrum, the experiments thus select contributions only from certain orientations of the molecule with respect to the external magnetic field. Such orientation selection is more efficient and easier to interpret at a field that is high enough for the g anisotropy to dominate. Finally, the size of mw resonators scales with wavelength and thus scales inversely with frequency. At higher frequency, spectra can thus be measured with much smaller sample volumes, yet the concentration does not need to be significantly increased. 

High-field ENDOR also improves orientation selection, which allows for determination of the relative orientation of the hyperfme tensor with respect to the g tensor and also leads to resolntion enhancement. For observation at the extreme positions (edges) of the ESR spectrum, only molecules with the g or g principal axes along the magnetic field contribute, and single-crystal like spectra are observed. 

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