Spectroscopy ENDOR

Pulsed ENDOR offers several distinct advantages over conventional CW ENDOR spectroscopy. Since there is no MW power during the observation of the ESE, klystron noise is largely eliminated. Furthemiore, there is an additional advantage in that, unlike the case in conventional CW ENDOR spectroscopy, the detection of ENDOR spin echoes does not depend on a critical balance of the RE and MW powers and the various relaxation times. Consequently, the temperature is not such a critical parameter in pulsed ENDOR spectroscopy. Additionally the pulsed teclmique pemiits a study of transient radicals. 

Figure Bl.15.13. Pulsed ENDOR spectroscopy. (A) Top energy level diagram of an. S-/=i spin system (see also figure Bl,15,8(A)). The size of the filled circles represents the relative population of the four levels at different times during the (3+1) Davies ENDOR sequence (bottom). (B) The Mims ENDOR sequence.

Piekara-Sady L and Kispert L D 1994 ENDOR spectroscopy Handbook of Electron Spin Resonance ed C P Poole and H A Farach (New York American Institute of Physics) ch 5, pp 311-57... 

Grupp A and Mehring M 1990 Pulsed ENDOR spectroscopy in solids Modern Pulsed and Continuous-Wave Electron Spin Resonance ed L Kevan and M K Bowman (New York Wiley) ch 4, pp 195-229... 

Gerson, F. Radical Ions of Phanes as Studied by ESR and ENDOR Spectroscopy. 115, 57-105 (1983). 

Geometries, hyperfme structure, and relative stabilities of the different positional isomers of monodeuterated benzene cations have been studied theoretically by density functional theory, using the B3-LYP functional, and experimentally by ESR and ENDOR spectroscopy. A comparison between theoretical and experimental results at 30 K gives acceptable agreement, but further experiments on multiply deuterated species should improve the analysis by making the effects of deuteration larger. 

Bar, G., M. Bennati et al. (2001). High-frequency (140-GHz) time domain EPR and ENDOR spectroscopy The tyrosyl radical-diiron cofactor in ribonucleotide reductase from yeast. J. Am. Chem. Soc. 123 3569-3576. 

Kispert, L. D. and L. Piekara-Sady (2006). ENDOR Spectroscopy. New York Springer Science Publishers. 

Piekara-Sady, L. and L. D. Kispert (1994). ENDOR Spectroscopy. In Poole, C. P. and Farach, H. A. (eds.), Handbook of Electron Spin Resonance Data Sources, Computer Technology, Relaxation and ENDOR. New York AIP Press. 

N.D. Chasteen and P.A. Snetsinger, ESEEM and ENDOR spectroscopy, in Physical Methods in Bioinorganic Chemistry, Spectroscopy and Magnetism, ed. L. Que, Jr, University Science Books, Sausalito, CA, 2000. 

The inactive form GOin, which displays a typical Cu(II) EPR signal, yields upon one-electron oxidation the EPR silent active form GO0X. For many years the presence of a Cu(III) ion (ct,. S = 0) in the active site (121) of the fully oxidized state GO0X was assumed. The Whittakers (122) showed in 1990 that one-electron oxidation of the copper depleted apoenzyme of GO produced an EPR active, remarkably stable Tyr radical that was studied by UV-vis, EPR, and ENDOR spectroscopy. From these studies, they concluded that the thioether modified Tyr 272 was oxidized and, consequently, they proposed that GOcx contains a Tyr 272 radical coordinated to a Cu(II) ion. 

The application of circularly polarized rf fields in ENDOR spectroscopy (CP-ENDOR) will be described in Sect. 4.6. Circularly polarized rf fields may be obtained by the vector... 

In polarization modulated ENDOR spectroscopy (PM-ENDOR)45, discussed in Sect. 4.7, the linearly polarized rf field B2 rotates in the laboratory xy-plane at a frequency fr fm, where fm denotes the modulation frequency of the rf carrier. In a PM-ENDOR experiment the same type of cavity, with two rf fields perpendicular to each other, and the same rf level and phase control units used in CP-ENDOR can be utilized. To obtain a rotating, linearly polarized rf field with a constant magnitude B2 and a constant angular velocity Q = 2 fr (fr typically 30-100 Hz), double sideband modulation with a suppressed carrier is applied to both rf signals. With this kind of modulation the phase of the carrier in each channel is switched by 180° for sinQt = 0. In addition, the phases of the two low-frequency envelopes have to be shifted by 90° with respect to each other. The coding of the two rf signals is shown in Fig. 8. 

The phenomenon of asymmetric hfs tensors was first discussed by McConnell134). Later, Kneubiihl135,136) proved the existence of asymmetric g and A tensors in paramagnetic systems with low symmetry. Evaluation of the asymmetry of A using EPR and ENDOR spectroscopy has been treated by several authors132,137 141). Recently, low-symmetry effects in EPR have been covered in a comprehensive review article by Pilbrow and Lowrey142). 

This section covers the literature of ENDOR on transition metal complexes with organic ligands through the end of 1980. The discussion also includes unpublished results and papers submitted for publication which came to our knowledge. Only contributions for which ENDOR spectroscopy yields either new or significantly improved results compared with the data obtained from EPR measurements will be discussed in more detail. 

Cu(II) impurity complexes in amino acid single crystals have been the subject of several EPR studies181-183. Since nitrogen and proton hf structures are only partially resolved in the EPR spectra, no detailed information about the electronic properties of the complex in the neighborhood of the metal ion can be evaluated. ENDOR spectroscopy has therefore been applied58,63 to draw detailed pictures of the positions and the molecular environment of Cu(II) impurities in amino acid crystals. 

A single crystal ENDOR study comprising 1H-, 13C-, 14N- and 59Co-ENDOR investigations of the Schiffbase compound Co(acacen) (Fig. 42) doped into Ni(acacen) 1/2H20 was therefore undertaken by Rudin et al.12,59,61,219). As illustrated in Fig. 2b, ENDOR spectroscopy allows hf and quadrupole interactions of all the magnetic ligand... 

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