Electron-nuclear resonance techniques

Here it is worthwhile to present a concise summary of the advantages and disadvantages of the various electron-nuclear resonance techniques and Table 1 contains this information. 

J. A. Pople (Cambridge) The electron density at the proton is not measured in a direct manner by the nuclear resonance technique, and it is quite possible that there may be an increase of electron density at the proton in spite of the fact that the shielding is reduced when a hydrogen bond is formed. 

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. 

In an early review, Hyde [272] discussed ENDOR in proteins, including flavo-proteins, copper proteins, hemeproteins, two-iron ferrodoxin and bacteriochloro-phyll. Kevan and Kispert s book [19] is an introductory text on ENDOR (electron nuclear double resonance) and ELDOR (electron electron double resonance) techniques. Poole [11] includes a chapter on double resonance techniques in his text. Schweiger [21] has covered ENDOR of transition metal complexes, including a section on biological applications. Recent reviews of ENDOR spectroscopy of chlorophylls [273], heme and heme proteins [274] and iron sulfur proteins [275] demonstrate how additional detail can be obtained from ENDOR data. 

MoOL2(Tp)] (L = OPh, SPh, or Cl) and related nitrosyl complexes, as also the dinuclear B-B linked complex reported in Fig. 2.26, have been investigated by electron magnetic resonance techniques, electron nuclear double resonance, and hyperfme sublevel correlation spectroscopy 11B hyperfme and quadrupolar couplings have been measured.120... 

In this account we will attempt to trace some of the developments in the application of nuclear and electron magnetic resonance techniques to the study of catalysts and catalytic processes from the pioneering studies up to the middle 1960 s. 

A variety of spectroscopic techniques, however, are of value to determine the local bonding and, occasionally, oxidation states of various ions. Frequently, they can perform satisfactory quantitative analysis or estimates as well. Adsorption, emission, and Raman spectroscopy operating from the UV through the IR region of the spectrum can provide such information. These optical spectroscopies can be performed in either a transmission or surface-scattering mode based on the thickness and absorption properties of the specific sample. Nuclear magnetic resonance (NMR), Mossbauer spectroscopy, and electron spin resonance techniques are some other forms of spectroscopy frequently used to determine local bonding and oxidation states of specific species, primarily in the bulk rather than on the surface. These methods are limited to particular atoms or ions and are not universally applicable. 

In Part I- F the magnetic properties of metal-ammonia solutions were listed. As we have seen, the obseiwed magnetic properties consisted of results of total susceptibility measurements, spin susceptibility measurements using electron spin resonance techniques, dynamic features of electron spin resonance involving measurements on the relaxation times, and nuclear resonance studies. We shall first take up the explanation of the susceptibility data using the cavity, cluster, and unified models and subsequently consider the interpretation of the results of resonance studies. 

This comprehensive review of theoretical models and techniques will be invaluable to theorists and experimentalists in the fields of infrared and Raman spectroscopy, nuclear magnetic resonance, electron spin resonance and flame thermometry. It will also be useful to graduate students of molecular dynamics and spectroscopy. 

In this chapter we have limited ourselves to the most common techniques in catalyst characterization. Of course, there are several other methods available, such as nuclear magnetic resonance (NMR), which is very useful in the study of zeolites, electron spin resonance (ESR) and Raman spectroscopy, which may be of interest for certain oxide catalysts. Also, all of the more generic tools from analytical chemistry, such as elemental analysis, UV-vis spectroscopy, atomic absorption, calorimetry, thermogravimetry, etc. are often used on a routine basis. 

Exchange reactions can be sometimes investigated by the techniques of polari-metry, nuclear magnetic resonance and electron spin resonance. The optical activity method requires polarimetric measurements on the rate of racemization in mixtures of d-X (or /-X) and /-Y (or d-Y). 

In general, several spectroscopic techniques have been applied to the study of NO, removal. X-ray photoelectron spectroscopy (XPS), electron paramagnetic resonance (EPR), nuclear magnetic resonance (NMR), extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) are currently used to determine the surface composition of the catalysts, with the aim to identify the cationic active sites, as well as their coordinative environment. 

This chapter concludes with a brief description of one advanced technique, Electron Nuclear Double Resonance (ENDOR), the capabilities for which, unlike pulsed methods, may be added as a relatively minor modification to commercial CW ESR spectrometers. 

As discussed in Chapter 6, in systems with more than one unpaired electron the ESR spectrum contains features that involve electron-electron coupling parameters analogous to the nuclear hyperfine parameters. In those types of samples the advantages of double resonance are carried out by employing the use of two different microwave frequencies, one fixed and saturating, and one variable frequency that searches for transitions. This technique is known as ELDOR (electron-electron double resonance).38,40,41,44 It has been used much less than ENDOR and usually requires custom-built equipment. 

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... 

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