Electron nuclear double resonance hyperfine interactions

An exception to this rule arises in the ESR spectra of radicals with small hyperfine parameters in solids. In that case the interplay between the Zeeman and anisotropic hyperfine interaction may give rise to satellite peaks for some radical orientations (S. M. Blinder, J. Chem. Phys., 1960, 33, 748 H. Sternlicht,./. Chem. Phys., 1960, 33, 1128). Such effects have been observed in organic free radicals (H. M. McConnell, C. Heller, T. Cole and R. W. Fessenden, J. Am. Chem. Soc., 1959, 82, 766) but are assumed to be negligible for the analysis of powder spectra (see Chapter 4) where A is often large or the resolution is insufficient to reveal subtle spectral features. The nuclear Zeeman interaction does, however, play a central role in electron-nuclear double resonance experiments and related methods [Appendix 2 and Section 2.6 (Chapter 2)]. 

Electron-nuclear double resonance (ENDOR) spectroscopy A magnetic resonance spectroscopic technique for the determination of hyperfine interactions between electrons and nuclear spins. There are two principal techniques. In continuous-wave ENDOR the intensity of an electron paramagnetic resonance signal, partially saturated with microwave power, is measured as radio frequency is applied. In pulsed ENDOR the radio frequency is applied as pulses and the EPR signal is detected as a spin-echo. In each case an enhancement of the EPR signal is observed when the radiofrequency is in resonance with the coupled nuclei. 

Electron nuclear double resonance (ENDOR) and electron spin-echo envelope modulation (ESEEM) are two of a variety of pulsed EPR techniques that are used to study paramagnetic metal centers in metalloenzymes. The techniques are discussed in Chapter 4 of reference la and will not be discussed in any detail here. The techniques can define electron-nuclear hyperfine interactions too small to be resolved within the natural width of the EPR line. For instance, as a paramagnetic transition metal center in a metalloprotein interacts with magnetic nuclei such as H, H, P, or these... 

Electron spin resonance spectra of coals usually consist of a single line with no resolvable fine structure however, the electron nuclear double resonance (ENDOR) technique can show hyperfine interactions not easily observable in conventional electron spin resonance spectra. Recently, this technique has been applied to coal, and it is claimed that the very observation of an ENDOR signal shows interaction between the electron and nearby protons and that the results indicate that the interacting protons are twice removed from the aromatic rings on which, it is assumed, the unpaired electron is stabilized. 

In the form in which it has so far been applied to the study of carbenes, EPR spectroscopy is unable to investigate the hyperfine interactions of the unpaired spins with the constituent atomic nuclei because of the broad lines which are observed. However, the technique of electron nuclear double resonance ( endor ) promises to permit such investigations to be made, so providing even more detailed information about the electronic structure of carbenes (Hutchison, 1967). 

DPPH = 2,2-diphenyl-1-picrylhydrazyl ENDOR= electron-nuclear double resonance EPR = electron paramagnetic resonance ESE = electron spin echoes ESEEM = electron spin echo envelope modulation EFT = fast fourier transformations FWHM = fidl width at half maximum HYSCORE = hyperfine sublevel correlation nqi = nuclear quadrupole interaction TauD = taurme/aKG dioxygenase TWTA = traveling wave tube amphfier ZFS = zero field sphtting. 

Electron-nuclear double resonance (ENDOR) studies of PFL-AE complexed to specifically isotopically labeled AdoMets has revealed the details of the interaction between AdoMet and the cluster in this enzyme. Deuterium ENDOR spectra of PFL-AE in the [4Fe-4S]" state complexed with methyl-D2-AdoMet showed a pair of peaks centered at the deuteron Larmor frequency and split by the hyperfine coupling to the spin of the cluster. Examination of the field-dependence of the coupling showed that it was dipolar in nature, and gave an estimation of the... 

Lunsford [3b] and Hoffman and Nelson [23] first reported the ESR spectra for adsorbed NO molecules. Then, Kasai [4b] revealed that ESR spectra of NO probe molecules are very sensitive to the interaction with metal ions and Lewis acid sites in zeolites. The earlier ESR studies of the NO/zeolite system have been summarized in several review papers [3a, 4a, 8]. A number of ESR studies have been also carried out for NO adsorbed on metal oxides such as MgO and ZnO as reviewed by Che and Giamello [5]. Modern ESR techniques such as pulsed ESR [25-27], ENDOR (Electron Nuclear Double Resonance) [26], and multi-frequency (X-, Q-, and W-band) ESR [28] are especially useful for an unambiguous identification of the ESR magnetic parameters (g, hyperfine A, and quadrupole tensors, etc.) and, consequently, for a detailed characterization of structural changes and motional dynamics involved. Some recent advancements in ESR studies on NO adsorbed on zeolites are presented in this section. 

Electron nuclear double resonance is a powerful tool for the study of the electronic structure of triplet states because of its high precision. ENDOR linewidths can be as narrow as 10 kHz, which represents an increase in resolution of better than six orders of magnitude over that which can be obtained optically. The technique is particularly useful when combined with hf methods owing to the first-order nature of the hyperfine interaction in the presence of a field. Although such experiments are difficult, the information obtained is unique. Accordingly, the hf EPR (or ODMR) spectrometer has been modified for ENDOR operation in several laboratories. In order to illustrate the power of the method, we discuss here some recent optically detected hf ENDOR experiments on (njr ) benzophenone and its iso-topically labeled derivatives (Brode and Pratt, 1977, 1978a,b). The results, although incomplete, show considerable promise for the ultimate determination of the complete spin distribution in this prototype triplet state. 

Hyperfine splitting due to interaction with ligand nuclei with 7 > 0 reflects the extent of spin delocalization onto neighboring atoms and can be used to characterize the types and numbers of such nuclei. In cases where these couplings are too small to be resolved in the EPR spectra, electron nuclear double resonance (ENDOR) or electron spin echo envelope modulation (ESEEM) can be used to measure the couplings as discussed in Chapter 2.3. Modern calculational tools are approaching the capabilities required to calculate g and A values from electronic wave functions. However, much of the spectroscopy that has been performed to date has used empirical correlations to interpret g and A values. 

Not all the information can be obtained by the basic CW experiment that is considered by many chemists as all there is to EPR. Elucidating geometric structure or small spin densities requires the separation of small hyperfine couplings or dipole-dipole couplings between electron spins from larger interactions. This can be achieved by double resonance experiments, such as electron nuclear double resonance (ENDOR) [8,9] and electron electron double resonance (ELDOR) spectroscopy and further pulse-EPR techniques [10] such as electron spin echo envelope modulation (ESEEM). Pulse-EPR techniques may also provide more information on dynamic processes than simple CW experiments and may access longer time scales. 

Later [38, 39], oxygen vacancies (Fig. 2.2) and E point defects present in glassy Si02 could be studied in great detail, including also full ab-initio calculations of the hyperfine parameters experimentally detected by electron-nuclear double resonance (ENDOR) experiments. Indeed, these types of measurements are nowadays routinely done to identify this class of paramagnetic defects. In the ENDOR technique, some Si atoms are substituted with their isotopes Si. This confers anon-zero nuclear spin I to the atomic nucleus that couples to the electron spin S via a tensor A. On the theoretical front, the calculation from first principles DFT approaches does not pose particular problems since the hyperfine interaction is still a ground state property which can be expressed in terms of the electronic density p x). The interaction between an electron spin (S) and a nuclear (I) spin is in fact described by the Hamiltonian... 

The pulse EPR methods discussed here for measuring nuclear transition frequencies can be classified into two categories. The first involves using electron nuclear double resonance (ENDOR) techniques where flie signal arises from the excitation of EPR and NMR transitions by microwave (m.w.) and radiofrequency (r.f) irradiation, respectively. In the second class of experiments, based on flic electron spin echo envelope modulation (ESEEM) effect, flic nuclear transition frequencies are indirectly measured by the creation and detection of electron or nuclear coherences using only m.w. pulses. No r.f irradiation is required. ENDOR and ESEEM spectra often give complementary information. ENDOR experiments are especially suited for measuring nuclear frequencies above approximately 5 MHz, and are often most sensitive when the hyperfine interaction in not very anisotropic. Conversely, anisotropic interactions are required for an ESEEM effect, and the technique can easily measure low nuclear frequencies. 

Sammet A, Hubrich M, Spiess HW. 1995. Nature and dynamics of radicals in polyara-mide as studied by pulsed electron nuclear double resonance. Adv Mater 7 747-750. Bennebroek MT, Schmidt J. 1997. Pulsed ENDOR spectroscopy at large thermal spin populations and the absolute sign of the hyperfine interaction. J Magn Reson 128 199-206. 

The importance of accurate estimates for the cfi parameters is twofold. As discussed above, the cfi affects the electron spin relaxation, and flius contributes to the relaxivity of MRI contrast agents. Second, flic cfi affects the direction of the electron spin quantization axis, which leads to certain effects in nuclear transition spectra (e.g., electron-nuclear double resonance, ENDOR) that are necessary to take into account in order to accurately determine the electron-nuclear hyperfine interaction ihfi) and the distance from the Gd(III) ion to the ligand protons. 

The hyperfine interaction. A, of an unpaired electron with nearby nuclei can be obtained by the electron-nuclear double resonance (ENDOR) technique. The ENDOR resonance condition is given by... 

The hyperfine interactions with nuclei adjacent to the paramagnetic centre are often too small to be observed because they are smaller than the EPR line width. Electron-nuclear double resonance is a technique which detects these interactions. In addition to the microwave field a radiofrequency field, B2, is applied at right angles. A commercial design uses a coil wound around the sample tube in a special cavity. 

IspG is a protein that carries out an essential reduction step in isoprenoid biosynthesis. Using electron-nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopies on labeled samples, Oldfield et al. have specifically assigned the hyperfine interactions in a reaction intermediate (144), which was created as a result of unusual 4Fe-4S cluster containing protein IspG catalysed reduction of 2-C-methyl-erythritol-cyc/o-2,4-diphosphate (143) to ( )-l-hydroxy-2-methyl-but-2-enyl-4-diphosphate (145) and then to dimethylallyl diphosphate (146) and isopentenyl diphosphate (147) (Scheme 39). " ... 

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