Related Techniques ENDOR

The instrumental method described in Chapter 1 and illustrated in this chapter proves to be adequate for studying many of the samples to which chemists and biologists wish to apply ESR. Indeed, even if more advanced techniques turn out to be required, CW, field-swept X-band ESR still provides the most convenient, and commonly used, method for preliminary examination of samples that are known, or suspected, to possess interesting paramagnetic properties. Nevertheless, for those who may need to extend their studies to more advanced methods Appendix 2 lists several of the most useful techniques and gives references to recent reviews and relevant papers that should serve as an entry into the still developing literature on advanced ESR. 

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. 

An early example of an application of ENDOR to extract hyperfine couplings from a very complex, poorly resolved ESR spectrum is that of Allendoefer and Englemann19 who studied a solution of bis(p-methoxyphenyl)nitroxide and obtained the results shown in Table 2.4. The figures in parentheses indicate the level of precision. 

Electron Spin Relaxation in Liquids, ed. L.T. Muus and P.W. Atkins, Plenum Press, New York, 1972. 

Most of the information content from ESR spectra of organometallic radicals and coordination complexes comes from dilute single-crystal spectra or frozen solution spectra. Nonetheless, there are some bits of information and applications that come uniquely from isotropic spectra, and we discuss those aspects in this chapter. 

---

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. 

Information on the nature of the chemical environment of trace metal Ions adsorbed on clay minerals can be obtained by a number of spectroscopic methods, but the principal applications have used either XPS or ESR spectroscopy, or one of its related techniques, such as ENDOR and ESEM spectroscopy. 

There are a variety of techniques for the determination of the various parameters of the spin-Hamiltonian. Often applied are Electron Paramagnetic or Spin Resonance (EPR, ESR), Electron Nuclear Double Resonance (ENDOR), Electron Electron Double Resonance (ELDOR), Nuclear Magnetic Resonance (NMR), occassionally utilizing effects of Chemically Induced Dynamic Nuclear Polarization (CIDNP), Optical Detection of Magnetic Resonance (ODMR), Atomic Beam Spectroscopy and Optical Spectroscopy. The extraction of the magnetic parameters from the spectra obtained by application of these and related techniques follows procedures which may in detail depend on the technique, the state of the sample (gaseous, liquid, unordered solid, ordered solid) and on spectral resolution. For particulars, the reader is referred to the general references (D). 

NMR and EPR techniques provide unique information on the microscopic properties of solids, such as symmetry of atomic sites, covalent character of bonds, strength of exchange interactions, and rates of atomic and molecular motion. The recent developments of nuclear double resonance, the Overhauser effect, and ENDOR will allow further elucidation of these properties. Since the catalytic characteristics of solids are presumably related to the detailed electronic and geometric structure of solids, a correlation between the results of magnetic resonance studies and cata lytic properties can occur. The limitation of NMR lies in the fact that only certain nuclei are suitable for study in polycrystalline or amorphous solids while EPR is limited in that only paramagnetic species may be observed. These limitations, however, are counter-balanced by the wealth of information that can be obtained when the techniques are applicable. 

A technique related to EPR, electron nuclear double resonance (ENDOR), allows the assignment of the individual hfcs to particular nuclei and, with reasonable assumptions, will also identify the sign of the interaction. The only obvious drawback of this technique lies in the fact that it requires sophisticated instrumentation, which is, so far, available in only a few laboratories. Applications to strained ring systems, viz., cyclobutene, bicyclobutane, or a tricyclic derivative, have been reported. Howcvct, applications to simple cyclopropane systems have not been reported to date. 

A similar connection between copper coordination to peptides and medical consequences exists for the Alzheimer disease.79 The amyloid-p peptide in senile plaque is the site of copper binding and as before there is interest to study details of the coordination and the stability. The neurotoxicity seems to be related to free radical damage and Cu2+ chelators are probed as therapy. So far, there seems to be no investigation at a resolution possible to pinpoint copper ligands as derived from ENDOR or related pulse techniques. We mention some relevant EPR studies as introductory sources into the topic.80 81,82... 

A prototypical example of a molecular probe used extensively to study the mineral adsorbent-solution interface is the ESR spin-probe, Cu2+ (Sposito, 1993), whose spectroscopic properties are sensitive to changes in coordination environment. Since water does not interfere significantly with Cu11 ESR spectra, they may be recorded in situ for colloidal suspensions. Detailed, molecular-level information about coordination and orientation of both inner- and outer-sphere Cu2+ surface complexes has resulted from ESR studies of both phyllosilicates and metal oxyhydroxides. In addition, ESR techniques have been combined with closely related spectroscopic methods, like electron-spin-echo envelope modulation (ESEEM) and electron-nuclear double resonance (ENDOR), to provide complementary information about transition metal ion behaviour at mineral surfaces (Sposito, 1993). The level of sophistication and sensitivity of these kinds of surface speciation studies is increasing continually, such that the heterogeneous colloidal particles in soils can be investigated ever more accurately. 

Contents Introduction. - ENDOR-Instrumentation. - Analysis of ENDOR Spectra. - Advances ENDOR Techniques. - Interpretation of Hyperfine and Quadrupole Data. - Discussion of the Literature. - Concluding Remarks. - Appendix A Abbreviations Used in this Paper. - Appendix B Second Order ENDOR Frequencies. - Appendix C Relations Between Nuclear Quadrupole Coupling Constants in Different Expressions of Hq (Sect.5.2). - References. - Subject Index. 

The above point is related to the Larmor-centered frequencies at which Mims ENDOR signals appear. It must be made clear, however, that this technique is not applicable irrespective of hyperfine coupling. Mims ENDOR is not... 

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. 

The frequencies and the amplitude of the modulation measured in an ESEEM experiment can both be simulated to derive information about the hyperfine interaction. The ESEEM-derived spectrum in the frequency domain is related to, but not necessarily identical to, an ENDOR spectrum. Thorough reviews of this technique and its analysis are found elsewhere" here we give a brief comparison between the information derived from ESEEM and that directly determined by ENDOR. In an ESEEM experi-... 

Electron paramagnetic resonance (EPR) spectroscopy [1-3] is the most selective, best resolved, and a highly sensitive spectroscopy for the characterization of species that contain unpaired electrons. After the first experiments by Zavoisky in 1944 [4] mainly continuous-wave (CW) techniques in the X-band frequency range (9-10 GHz) were developed and applied to organic free radicals, transition metal complexes, and rare earth ions. Many of these applications were related to reaction mechanisms and catalysis, as species with unpaired electrons are inherently unstable and thus reactive. This period culminated in the 1970s, when CW EPR had become a routine technique in these fields. The best resolution for the hyperfine couplings between the unaired electron and nuclei in the vicinity was obtained with CW electron nuclear double resonance (ENDOR) techniques [5]. 

---