Echo modulation

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. 

Three-pulse ESEEM spectrum of perdeuterated P-carotene imbedded in Cu-MCM-41 exhibits an echo decay with an echo modulation due to deuterons. The three-pulse ESEEM is plotted as a function of time, and curves are drawn through the maximum and minima. From ratio analysis of these curves, a best nonlinear least-squares lit determines the number of interacting deuterons, the distance (3.3 0.2A), and the isotopic coupling (0.06 0.2MHz). This analysis made it possible to explain the observed reversible forward and backward electron transfer between the carotenoid and Cu2+ as the temperature was cycled (77-300 K). 

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. 

Performing the CPMG experiment on a spin system with homonuclear coupling introduces the complication of echo modulation by the scalar couplings [4, 5, 7, 36]. This makes transverse proton relaxation measurements... 

The optical absorption spectra depend on the solvent. They are red-shifted with the decreasing polarity of the solvents as in EDA, liquid NH3, where they appear at a longer wavelength than in water [49], and blue-shifted in methanol [38]. Moreover, the maximum in NH3 is red-shifted with the increase of temperature. In water, the Ag band is almost unchanged in the range 20-200 °C, while that of Ag2 is markedly shifted to the red [50]. Electron spin echo modulation analysis of Ag in ice or methanol glasses has concluded to a charge transfer character to solvent (CTTS) of the absorption band [51]. 

A Cu(II) complex containing two spin-labelled ligands was prepared with a rigid linkage between the terpyridine chelator and a nitroxyl ring.26 Due to the large anisotropy of the Cu(II) spectrum only a small fraction of the Cu(II) spins are excited by the pulses of a DEER experiment. Analysis of the echo modulation... 

Figure 19 Schematic representation of [VO(H20) +] and average intemuclear distances (A) from electron spin echo modulation (ESEM), electron nuclear double resonance (ENDOR) and X-ray diffraction studies4 ...

Using electron spin echo modulation spectroscopy, Narayana et al.33 showed that in 0.25% V205 on Si02 a vanadyl species with two co-ordinated water molecules, giving octahedral co-ordination, is present. This species loses the two water molecules even at 100°C leaving a tetrahedral species. 

DeRose, V.J. C.-H., Kim, W.E., Newton, D. R.D., and Hoffman B. M. (1995) Electron Spin Echo Modulation Spectroscopic Analysis of Altered Nitrogenase MoFe Proteins of Azotobacter vinelandii, Biochemistry 34, 2809-2814... 

The fundamentals and analysis of electron spin echo modulation have been described in some detail (9). Here the essential details will be sumnarized to aid in understanding the structural data to be presented. Electron spin echoes are generated in pulsed electron spin resonance experiments. The most comnon type... 

Cu isotopes both with nuclear spin I-3/2. The nucle r g-factors of these two isotopes are sufficiently close that no resolution of the two isotopes is typically seen in zeolite matrices. No Jahn-Teller effects have been observed for Cu2+ in zeolites. The spin-lattice relaxation time of cupric ion is sufficiently long that it can be easily observed by GSR at room temperature and below. Thus cupric ion exchanged zeolites have been extensively studied (5,17-26) by ESR, but ESR alone has not typically given unambiguous information about the water coordination of cupric ion or the specific location of cupric ion in the zeolite lattice. This situation can be substantially improved by using electron spin echo modulation spectrometry. The modulation analysis is carried out as described in the previous sections. The number of coordinated deuterated water molecules is determined from deuterium modulation in three pulse electron spin echo spectra. The location in the zeolite lattice is determined partly from aluminum modulation and more quantitatively from cesium modulation. The symmetry of the various copper species is determined from the water coordination number and the characteristics of the ESR spectra. 

For a two-pulse (90° - t - 180°), or primary echo experiment, the integrated intensity of the spin echo, which occurs at time t after the 180° pulse, is measured as a fimction of increasing t from the probe s dead-time ( 100 ns) to a time where the echo amplitude has decayed to a few percent of its initial amplitude (2-8 ps for most powder samples). A two-pulse ESE decay envelope for the type-1 Cu(II) site of a multi-copper oxidase, Fet3p, is shown in Figure 1(a). The data show an overall decay characterized by a phase memory time, Tm or T, of < 1.0 ps. Superimposed on this decay are echo modulations that arise ft om hyperfine coupling to the N nuclei of two histidyl imidazole ligands and the protons of the snrronnding matrix. 

Analytical expressions for the primary echo modulation function of an B = 1 /2, / = 1/2 system were worked out for some of the earliest ESEEM studies that appeared in the literature. Perhaps the most general of these theoretical treatments is that of Mims where the two-pulse ESEEM function is given by... 

Direct information about the local solvation structure for the solvated electron in condensed media is scarce, although having an accurate picture of, at least, the ground state of the solvated electron, is important to interpret its properties. Steady-state electron paramagnetic resonance (EPR) and electron spin-echo modulation (ESEM) experiments as well... 

Narayana M, Kevan L. (1980) Electron spin echo modulation study of the geometry of solvated electrons in ethanol glass An example of a molecular dipole oriented solvation shell. J Chem Phys 72 2891-2892. 

Narayana M, Kevan L, Samskog PO, Lund A, Kispert LD. (1984) Electron spin echo modulation studies of the structure of solvated electrons in ethylene glycol glass. J Chem Phys 81 2297-2299. 

Long range dipolar interactions between an unpaired electron and nuclear spins on adjacent atoms will not normally be resolved in conventional powder EPR spectra.The pulse technique of electron spin echo modulation (ESEM) is in favourable cases able to detect very weak hyperfine interactions not seen in CW EPR. The method measures modulation of the electron spin echo signal by dipolar hyperfine coupling in the time domain at fixed magnetic field. Until recently,... 

The electron spin echo modulation technique detects directly the coordination environment around a paramagnetic ion by observing the dipolar coupling to nuclei of weakly coordinated ligands. This technique has been used extensively by Kevan, for example, to examine transition metal ion exchanged and substituted zeolite materials [19]. 

A. Poppl and L. Kevan, Study of Adsorbate Interactions of Cu(II) Ion-exchanged into Siliceous MCM-41 Silica Tube Material by Electron Spin Resonance and Electron Spin Echo Modulation Spectroscopy. Langmuir, 1995, 11, 4486 1490. 

A. Poppl, M. Newhouse, and L.E. Kevan, Electron Spin Resonance and Electron Spin Echo Modulation Studies of Cupric Ion Ion-exchanged into Siliceous MCM-41. J. Phys. Chem., 1995, 99, 10019-10023. 

RD Britt, K Sauer, MP Klein, DB Knaff, A Kriauciunas, C-A Yu, L Yu and R Malkin (1991) Electron spin echo modulation spectroscopy supports the suggested coordination of two histidine ligands to the Rieske Fe-S centers of the cytochrome b f complex of spinach and cytochrome bc complexes of Rhodospirillum rubrum, Rhodobacter sphaeroides R26, and bovine heart mitochondria. Biochemistry 30 1892-1901 R Gurbiel, T Ohnishi, DE Robertson, F Daldal and BM Hoffman (1991) Q-band ENDOR spectra of the Rieske protein from Rhodobacter capsulatus ubiquinol-cytochrome c oxidoreductase show two histidines coordinated to the [2Fe2S] cluster. Biochemistry 30 11579-11584... 

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