Electron-spin-echo

Electron Spin Resonance Spectroscopy. Several ESR studies have been reported for adsorption systems [85-90]. ESR signals are strong enough to allow the detection of quite small amounts of unpaired electrons, and the shape of the signal can, in the case of adsorbed transition metal ions, give an indication of the geometry of the adsorption site. Ref. 91 provides a contemporary example of the use of ESR and of electron spin echo modulation (ESEM) to locate the environment of Cu(II) relative to in a microporous aluminophosphate molecular sieve. 

Figure Bl.15.11. Fomiation of electron spin echoes. (A) Magnetization of spin packets i,j, /rand / during a two-pulse experiment (rotating frame representation). (B) The pulse sequence used to produce a stimulated echo. In addition to this echo, which appears at r after the third pulse, all possible pairs of the tluee pulses produce primary echoes. These occur at times 2x, 2(x+T) and (x+2T).

In electron-spin-echo-detected EPR spectroscopy, spectral infomiation may, in principle, be obtained from a Fourier transfomiation of the second half of the echo shape, since it represents the FID of the refocused magnetizations, however, now recorded with much reduced deadtime problems. For the inhomogeneously broadened EPR lines considered here, however, the FID and therefore also the spin echo, show little structure. For this reason, the amplitude of tire echo is used as the main source of infomiation in ESE experiments. Recording the intensity of the two-pulse or tliree-pulse echo amplitude as a function of the external magnetic field defines electron-spm-echo- (ESE-)... 

In electron spin echo relaxation studies, the two-pulse echo amplitude, as a fiinction of tire pulse separation time T, gives a measure of the phase memory relaxation time from which can be extracted if Jj-effects are taken into consideration. Problems may arise from spectral diflfrision due to incomplete excitation of the EPR spectrum. In this case some of the transverse magnetization may leak into adjacent parts of the spectrum that have not been excited by the MW pulses. Spectral diflfrision effects can be suppressed by using the Carr-Purcell-Meiboom-Gill pulse sequence, which is also well known in NMR. The experiment involves using a sequence of n-pulses separated by 2r and can be denoted as [7i/2-(x-7i-T-echo) J. A series of echoes separated by lx is generated and the decay in their amplitudes is characterized by Ty. ... 

Rowan L G, Hahn E L and Mims W B 1965 Electron-spin echo envelope modulation Phys. Rev. 137 A61-A71... 

Merks R P J and de Beer R 1979 Two-dimensional Fourier transform of electron spin-echo envelope modulation. An alternative for ENDOR J. Phys. Chem. 83 3319-22... 

Thurnauer M C and Norris J R 1980 An electron spin echo phase shift observed in photosynthetic algae. Possible evidence for dynamic radical pair interactions Chem. Phys. Lett. 76 557-61... 

Riedel A, S Fetzner, M Rampp, F Lingens, U Liebl, J-L Zrmmermann, W Nitschke (1995) EPR, electron spin echo envelope modulation, and electron nuclear double resonance studies of the 2Ee-2S centers of the 2-halobenzoate 1,2-dioxygenase from Burkholderia (Pseudomonas) cepacia 2CBS. J Biol Chem 270 30869-30873. 

Advanced EPR techniques such as CW and pulsed ENDOR, electron spin-echo envelope modulation (ESEEM), and two-dimensional (2D)-hyperfine sublevel correlation spectroscopy (HYSCORE) have been successfully used to examine complexation and electron transfer between carotenoids and the surrounding media in which the carotenoid is located. 

G.R. Eaton and S.S. Eaton, Electron-nuclear double resonance spectroscopy and electron spin echo envelope modulation spectroscopy, Comprehensive Coordination Chemistry II, Elsevier, Boston, 2004, 49. 

Y. Deligiannakis, M. Louloudi and N. Hadjiliadis, Electron spin echo envelope modulation (ESEEM) spectroscopy as a tool to investigate the coordination environment of metal centers, Coord. Chem. Rev., 2000, 204, 1. 

J. McCracken, Electron spin echo modulation, in Handbook of Electron Spin Resonance, ed. C.P. Poole and H.A. Farach, Springer-Verlag, New York, 1999, vol. 2. 

K. Fukui, Y. Fujisawa, H. Ohya-Nishiguchi, H. Kamada and H. Sakurai, In vivo coordination structural changes of a potent insulin-mimetic agent, bis(picolinato)oxovanadium(IV), studied by electron spin-echo modulation spectroscopy, J. Inorg. Biochem., 1999, 77, 215. 

Double-resonance spectroscopy involves the use of two different sources of radiation. In the context of EPR, these usually are a microwave and a radiowave or (less common) a microwave and another microwave. The two combinations were originally called ENDOR (electron nuclear double resonance) and ELDOR (electron electron double resonance), but the development of many variations on this theme has led to a wide spectrum of derived techniques and associated acronyms, such as ESEEM (electron spin echo envelope modulation), which is a pulsed variant of ENDOR, or DEER (double electron electron spin resonance), which is a pulsed variant of ELDOR. The basic principle involves the saturation (partially or wholly) of an EPR absorption and the subsequent transfer of spin energy to a different absorption by means of the second radiation, leading to the detection of the difference signal. The requirement of saturability implies operation at close to liquid helium, or even lower, temperatures, which, combined with long experimentation times, produces a... 

In order to obtain kinetic parameters for the electron transfer of [11] /K +, the dephasing time tm of the electron-spin echo near the phase-transition temperature Tt was measured. These experiments gave a correlation time tc of 100 ns for the electron transfer at Tg = 170 K. From the assumption of an exponential decrease of c in solution, a value of 100 ps was estimated for tc at room temperature (Rautter, 1989 Rautter et al., 1992). 

Since the phenoxyls possess an S = ground state, they have been carefully studied by electron paramagnetic spectroscopy (EPR) and related techniques such as electron nuclear double resonance (ENDOR), and electron spin-echo envelope modulation (ESEEM). These powerful and very sensitive techniques are ideally suited to study the occurrence of tyrosyl radicals in a protein matrix (1, 27-30). Careful analysis of the experimental data (hyperfine coupling constants) provides experimental spin densities at a high level of precision and, in addition, the positions of these tyrosyls relative to other neighboring groups in the protein matrix. 

ESEEM Electron spin-echo envelope modulation... 

Electron spin echo 4.8 ESE pulsed MW field Measurement of hf coupling constants... 

Electron Spin Echoes and Electron Spin Echo ENDOR... 

Electron spin echoes (ESE) were first observed in 1958 by Blume1071. In the last few years this time domain technique has found several interesting applications. For comprehensive summaries on the subject, the reader is referred to the review articles of Mims108) and Norris et al.109) and to the monograph of Kevan and Schwartz1101. 

ESE-ENDOR. In ESE-ENDOR, introduced by Mims1181, a rf pulse in a three-pulse electron spin echo experiment is applied during the time interval between the second and third microwave pulse. The ENDOR spectrum is obtained by monitoring the decrease... 

Fig. 27a-c. Electron spin echo envelope modulation of Co(acacen), temperature 4K. a) Nuclear modulation pattern of Co(acacen) diluted into a Ni(acacen) 1/2 H20 single crystal. Crystal setting rotation axis I,

Fourier transform of the nuclear modulation pattern (From R. de Beer1 4)) c) Stick spectrum ENDOR frequencies (AmN = 1, 2) calculated from the hfs and quadruple tensors in Ref. 59 dashed lines ms = - 1/2, full lines ms = 1/2... 

Mims, W. B. Electron spin echoes in Electron Paramagnetic Resonance, ed. Geschwind, S., New York Plenum Press (1972), chapt. 4... 

During the last few years the versatility of ENDOR spectroscopy has been improved by a number of new techniques which make use either of special types of pumping fields (CP-ENDOR, PM-ENDOR), of more than one rf field (DOUBLE ENDOR, multiple quantum transitions, nuclear spin decoupling) or a different display of the spectrum (EI-EPR). In addition to these techniques, alternative methods have been developed (electron spin echo and electron spin echo ENDOR) which are able to supplement or to replace the ENDOR experiment under certain conditions. The utility of all these various advanced techniques, particularly in studies of transition metal compounds, has recently been demonstrated. 

---