Electron nuclear double resonance spectroscopy envelope modulation

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. 

Electron-Nuclear Double Resonance Spectroscopy and Electron Spin Echo Envelope Modulation Spectroscopy... 

Electron Paramagnetic Resonance Spectroscopy (S.S. Eaton, G.R. Eaton). Electron-Nuclear Double Resonance Spectroscopy and Electron Spin Echo Envelope Modulation Spectroscopy (S.S. Eaton, G.R. Eaton). 

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... 

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. 

Valuable spectroscopic studies on the dithiolene chelated to Mo in various enzymes have been enhanced by the knowledge of the structure from X-ray diffraction. Plagued by interference of prosthetic groups—heme, flavin, iron-sulfur clusters—the majority of information has been gleaned from the DMSO reductase system. The spectroscopic tools of X-ray absorption spectroscopy (XAS), electronic ultraviolet/visible (UV/vis) spectroscopy, resonance Raman (RR), MCD, and various electron paramagnetic resonance techniques [EPR, electron spin echo envelope modulation (ESEEM), and electron nuclear double resonance (ENDOR)] have been particularly effective probes of the metal site. Of these, only MCD and RR have detected features attributable to the dithiolene unit. Selected results from a variety of studies are presented below, chosen because their focus is the Mo-dithiolene unit and organized according to method rather than to enzyme or type of active site. 

Electron Paramagnetic Resonance (EPR) Spectroscopy, Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy Electron-Nuclear Double Resonance (ENDOR) Spectroscopy Nuclear Magnetic Resonance (NMR) Spectroscopy of Inorganic/Organometallic Molecules. 

The intent of this chapter is not to survey noninvasive surface spectroscopy but to illustrate briefly how it is applied to resolve the Stummian issue of whether inner-sphere surface complexes form. For this purpose, the application of electron spin resonance (ESR), electron nuclear double resonance (ENDOR), and electron spin echo envelope modulation (ESEEM) spectroscopies to elucidate metal cation speciation and the use of extended X-ray absorption fine structure (EXAFS) spectroscopy to detect surface anion species will be described. Emphasis will be on the interpretation of spectra. Sample preparation and instrumentation details were reviewed in recent volumes edited by Hawthorne (55) and Perry (27). Because the constant capacitance model was developed in the context of adsorption by hydrous oxides, these... 

ESR-related spectroscopies that hold the potential to overcome some resolution limitations and yield more information than the classical ESR approach about the chemical environment of paramagnetic metal ions are the electron—nuclear double resonance (ENDOR) (Kevan and Kispert, 1976) and electron-spin echo envelope modulation (ESEEM) (Kevan and Schwartz, 1979) spectroscopies. Either ENDOR or ESEEM represents by principle a useful tool in extending resolution of the ESR experiment. However, the sensitivity of ENDOR and ESEEM is much lower than that of ESR, and interpretation of ENDOR and ESEEM spectra is not a simple matter, especially if ligands are not well characterized, as is the case for HSs. Both ENDOR and ESEEM techniques have not yet been applied to strictly metal-HS complexes, but the sensitivity and ease of carrying out experiments are improving rapidly, so major scientific activity may be anticipated to occur in this area of ESR spectroscopy. 

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