High-frequency EPR

Nevertheless many recent studies simply could not have been carried out without the use of high frequency EPR. One of these is described below. In this,... 

Note that mosaic artifacts can also occur physically in real spectra when a real powder sample of a model compound exhibits microcrystallinity and thus contains too few different molecular orientations. This phenomenon is rare in X-band EPR and is usually easily solved by grinding the sample in a mortar it is, however, not at all uncommon even for extensively ground samples in high-frequency EPR with single-mode resonators where the sample size is orders of magnitude less than that of an X-band sample. 

Here is an additional checklist to prepare for high-frequency EPR experiments ... 

A rather different type of advanced EPR concerns the detection of transient intermediates, which was briefly discussed in Chapter 13, Section 13.4. Here, the advanced (i.e., instrumentally complex and with its own price tag) refers to the sample preparation, not to the spectroscopy. The common denominator of the rapidfreezing (> 5 ms) or even hyperquenching ( 1 ms) kinetic equipment with double resonance and high frequency EPR is the requirement for a specialized laboratory and knowledge infrastructure. This makes detailed planning of experiments, including expert consultancy, equally important. 

In the resonance Raman spectra of GO0X (125), vibrational modes have been assigned to both the tyrosinate ligand (Tyr 495) as well as the tyrosyl radical (Tyr 272). The spectrum does not provide evidence for the speculation that the tyrosyl radical is delocalized onto the jr-stacked tryptophan residue (Trp 290) (126, 127). Recent results of high-frequency EPR measurement (30) on the apogalactose oxidase radical are also consistent with the radical spin density being localized on the modified Tyr 272 moiety only. 

A review has been given on paramagnetic resonance spectroscopy of metal centers. It comprises both paramagnetic NMR as well as high frequency EPR methods applied mostly to copper proteins of the blue type but also to iron and manganese containing proteins.42... 

The g-tensor components are usually not resolved for a triplet state of a porphyrin or chlorin molecule at X and Q band frequencies and only the zero field splitting (ZFS) parameters are obtained. This situation changed with the advent of high field/high frequency EPR techniques. The first data of 3P700 and 3Chl a were published by Poluektov et a/. 194 the spectra obtained at D-band are shown in Fig. 5. 

The semiquinone biradical produced in the photocycle of bacterial photosynthetic reaction centres was trapped at 77 K and examined at 9.6, 35, and 94 GHz.16 Simulations of spectra at the multiple resonant frequencies using the simulated annealing method revealed the spatial and electronic structure of the biradical. The value of r was found to be 17.2 + 0.2 A, which is in good agreement with the value of 17.4 0.2 A obtained in an X-ray crystal structure. This study shows the power of high-frequency EPR combined with data obtained at lower frequencies. 

Multi-frequency EPR and High-Frequency EPR Tie, T2e (ts) geffective. Through analysis of the field/frequency dependence of electronic relaxation and geffective zero field splitting (ZFS, D, E, A2), rv. Also details of BPCA-protein interactions, including BPCA fraction bound to protein. 

In this work we have attempted to model the available experimental data for CaO Cu2+ using a variety of parameter sets taken from the literature. We conclude that only O Brien s multimode model [7], for which hw 216 cm 1 and jX/hoy 8, can satisfactorily account for all the data. These systems are particularly attractive to both the theoretician and experimentalist fascinated by Jahn-Teller funny business in that the states pertaining to j = 1/2 and 3/2 span just a few wave numbers. We have demonstrated how a number of modem physical techniques can be employed to elucidate further the low-lying vibronic structure. In particular it should be possible to obtain a direct measure of the tunnelling splitting from a high-held, high-frequency EPR experiment. 

Recently high-field/high-frequency EPR spectra of [Mn(taa)] were recorded for a powder sample at 1017.6 GHz. Analysis of the spectra showed the presence of a rhombic zero-field splitting in the HS molecule [30], which is consistent with the symmetry lowering by EQ c Jahn-Teller distortions and the dynamic reorientation of deformation dipoles slower than the time scale of 10-12 s. 

Teutloff, C., Hofbauer, W., Zech, S. G., Stein, M., Bittl, R., and Lubitz, W. (2001), High-frequency EPR studies on cofactor radicals in photosystem I, Applied Magnetic Resonance 21, 363-379. 

Conventional X- and Q-band EPR spectra are generally complex and often difficult to interpret. The recent use of high-field and high-frequency EPR spectroscopy (HF-EPR) has led to simplified spectra with remarkable resolution and provides an accurate determination of the electronic parameters of the studied compounds. Therefore, with this technique, structural information can be afforded both on solid state and in solution. 

To understand this discrepancy, three different theories have been proposed such as Chudnovsky et al. s dislocation-induced tunneling [85], Cornia et al. s solvent-disorder model [86], and Pederson et al. s spin-vibron coupling for isotopically impure samples [87], Magnetic tunneling measurements [88,89] and high-frequency EPR spectroscopy [84,90]... 

Bendix J, Gray HB, Golubkov G, Gross Z (2000) High-field (high-frequency) EPR spectroscopy and structural characterization of a novel manganese(III) corrole. Chem Commun 19 1957-1958... 

Reijerse E J, van Dam P J, Klaassen A A K, Hagen W, van Bentum P J M and Smith G 1998 Concepts in high-frequency EPR—applications to bio-inorganic systems Appl. Magn. Reson. 14 153-67... 

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