Magnetic ELDOR

With help of the four-level diagram of the =I= system (see figure BL15.8 two conniion ways for recording ELDOR spectra will be illnstrated. In freqnency-swept ELDOR the magnetic field is set at a value that satisfies the resonance condition for one of the two EPR transitions, e.g. 4<- 2, at the fixed observe klystron frequency, The pump klystron is then turned on and its frequency, is swept. When the pump... 

H. Mino and T. Ono, Applications of pulsed ELDOR-detected NMR measurements to studies of photosystem II Magnetic characterization of Yd tyrosine radical and Mn2+ bound to the high-affinity site, Appl. Magn. Reson., 2003, 23, 571. 

Abstract Multi-resonance involves ENDOR, TRIPLE and ELDOR in continuous-wave (CW) and pulsed modes. ENDOR is mainly used to increase the spectral resolution of weak hyperfine couplings (hfc). TRIPLE provides a method to determine the signs of the hfc. The ELDOR method uses two microwave (MW) frequencies to obtain distances between specific spin-labeled sites in pulsed experiments, PELDOR or DEER. The electron-spin-echo (ESE) technique involves radiation with two or more MW pulses. The electron-spin-echo-envelope-modulation (ESEEM) method is particularly used to resolve weak anisotropic hfc in disordered solids. HYSCORE (Hyperfine Sublevel Correlation Spectroscopy) is the most common two-dimensional ESEEM method to measure weak hfc after Fourier transformation of the echo decay signal. The ESEEM and HYSCORE methods are not applicable to liquid samples, in which case the FID (free induction decay) method finds some use. Pulsed ESR is also used to measure magnetic relaxation in a more direct way than with CW ESR. 

Hyde, J. S., Sneed, R. C., and Rist, G. H. 1969, J. Chem. Phys. 51, 1404. In addition to the pioneering ELDOR work, this paper presents an excellent summary of magnetic resonance studies of OPPH up to 1969. 

There are a variety of techniques for the determination of the various parameters of the spin-Hamiltonian. Often applied are Electron Paramagnetic or Spin Resonance (EPR, ESR), Electron Nuclear Double Resonance (ENDOR), Electron Electron Double Resonance (ELDOR), Nuclear Magnetic Resonance (NMR), occassionally utilizing effects of Chemically Induced Dynamic Nuclear Polarization (CIDNP), Optical Detection of Magnetic Resonance (ODMR), Atomic Beam Spectroscopy and Optical Spectroscopy. The extraction of the magnetic parameters from the spectra obtained by application of these and related techniques follows procedures which may in detail depend on the technique, the state of the sample (gaseous, liquid, unordered solid, ordered solid) and on spectral resolution. For particulars, the reader is referred to the general references (D). 

Knowledge of the magnetic (and optical) properties of triplet states has been greatly enhanced by the development of zero-field (zf) resonance techniques, especially those employing optical detection. In what follows, we review the selection rules which govern the transitions in the zf experiment. We then present recent results from this laboratory on the lowest (nTc ) states of 1-halonaphthalenes and discuss in some detail the analysis of these spectra and their significance with respect to the intramolecular heavy-atom effect on the properties of the parent molecule. Next, we survey some representative results from other laboratories, including zf EPR, ODMR, ENDOR, and ELDOR experiments, and close with a brief description of other zf applications. 

Pulsed ELDOR. Distances between electron spins can be measured by double electron-electron resonance (DEER) experiments such as the four-pulse experiment illustrated in Figure 6 (34). Similar to measurements of electron-nucleus distances, this technique is based on the r dependence of the magnetic dipole interaction between electron spins and can determine larger distances, in the range 1.5-5 nm. One of two spins (color-coded green, observer) is observed by a refocused primary echo with fixed interpulse delays ti and t2, so that relaxation does not induce variations in the echo amplitude during the experiment. The second spin (color-coded red, pumped) imposes a local dipole field at the site of the first spin, with a magnitude that depends on the distance. At a variable delay t with respect... 

Electron Nuclear Double Resonance (ENDOR), Electron-Electron Double Resonance (ELDOR), and magnetization [93Kahl, 94Rajl]. 

A large variety of hyperfine spectroscopy methods exist that allow the detection of hyperfine and nuclear quadrupole interactions electron spin-echo envelope modulation (ESEEM), ENDOR, and ELDOR-detected NMR (electron-electron doubleresonance detected nuclear magnetic resonance) [13]. Although there are cases in which ESEEM and ENDOR perform equally well, ESEEM-like methods tend to be... 

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