Analysis of ENDOR Spectra

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. 

Thus, as a consequence of the selection of orientation subsets by choice of field value, the ENDOR spectrum changes as a function of the external field position or g value at which it is measured. A series of ENDOR spectra collected at fields across the EPR envelope samples different sets of molecular orientations. An example of this is shown in Fig. 6B. It can be seen that at the extreme g values (g, and g ) the ENDOR spectrum is the least complex, whereas at intervening g values the ENDOR spectrum shows multiple frequencies. The analysis of such spectra taken at several magnetic field positions gives the full tensor of the hyperfine interaction A, from which the isotropic and anisotropic components can be deduced. Procedures for this analysis have been described in detail elsewhere. 

Abstract The theoretical principles of the ESR technique and its application in the field of molecular sieve science are reviewed. The first part of this chapter focuses on the basic principles and instrumentation of the ESR, ENDOR, ESE and ESEEM techniques. Special attention will be given to spectral simulation and quantitative analysis of ESR spectra. In the second part, the general features of the ESR spectra of transition metal ions and paramagnetic clusters in molecular sieves are presented and discussed. In addition, some remarks will be made about the use of paramagnetic molecules, such as NO. 

ENDOR and FT ESEEM spectra differ mainly in the intensities of the lines, which in ESEEM are given by a factor related to the ESR transition probabilities. A necessary prerequisite for modulations in the time domain spectrum is that the allowed Ami = 0 and forbidden Ami = 1 hyperfine lines have appreciable intensities in ESR. The zero ESEEM amplitude thus predicted with the field along the principal axes of the hyperfine coupling tensor is of relevance for the analysis of powder spectra. Analytical expressions describing the modulations have been obtained for nuclear spins I = V2 and / = 1 [54, 57] by quantum mechanical treatments that take into account the mixing of nuclear states under those conditions. Formulae are reproduced in Appendix A3.4. 

The first three terms are usually the ones of relevance for the ESR analysis, where D and A are the zero-field (or fine structure) and hyperfine coupling tensors. They are represented by 3-3 symmetric matrices and specified by three principal values and three principal directions as for the -tensor. The remaining nuclear Zeeman and quadrupole (/ > Vi) terms do not affect the ESR spectra, unless they are of comparable magnitude to the hyperfine coupling, but must be taken into account in the analysis of ENDOR and ESEEM spectra. The spin Hamiltonian formalism introduced by M.H.L. Pryce and A. Abragam [79] is used explicitly or implicitly in the ESR literature as a convenient way to summarise resonance parameters. 

Knowledge of the magnetic (and optical) properties of triplet states has been greatly enhanced by the development of zero-field (zf) resonance techniques, especially those employing optical detection. In what follows, we review the selection rules which govern the transitions in the zf experiment. We then present recent results from this laboratory on the lowest (nTc ) states of 1-halonaphthalenes and discuss in some detail the analysis of these spectra and their significance with respect to the intramolecular heavy-atom effect on the properties of the parent molecule. Next, we survey some representative results from other laboratories, including zf EPR, ODMR, ENDOR, and ELDOR experiments, and close with a brief description of other zf applications. 

Figure 2. Component analysis of an X-irradiated polycrystalline frozen solution (c a 1 mol%) of C6H5D in CFCI3. a) simulated components 1 (dashed) and 2 (solid) using hyperfine coupling data from ENDOR (Table 4). b) experimental (solid) and fitted (dashed) ESR spectra of CgH5D...

Nuclear spin decoupling in ENDOR may be used to facilitate the analysis of complicated ENDOR spectra. Some of the applications are40 ... 

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