General Features of ESR Spectra

In the presence of a magnetic field H the energy of a magnetic moment (jl is given by the equation 

For an electron, the magnetic moment resulting from the intrinsic spin momentum is customarily written 

The unpaired d electrons in an isolated transition metal ion or atom possess orbital angular momentum in addition to the spin momentum, and these couple together to give a total angular momentum J. The Zeeman interaction for these ions can be expressed by Eqs. (1) and (2) if S is replaced by J and g by the Lande g factor  

The presence of nuclear moments in either the transition-metal atom or in the surrounding ligand atoms can contribute local magnetic fields at the 

A typical hyperfine spectrum is shown in Fig. 1 in which the spectrum of an aqueous solution of K3Mo(CN)8 is given. This spectrum is a plot of the first derivative of the absorption curve as a function of the magnetic field, the frequency being held constant. (Most ESR spectra are displayed as the first derivative of the absorption, although some workers are now plotting the second derivative instead.) Seventy-five per cent of the molybdenum atoms are isotopes with no nuclear spin, whereas 25 per cent are the isotopes 

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This review deals with both the theoretical and practical aspects of the use of ESR spectroscopy in molecular sieve science. No attempt has been made to compile an exhaustive list of references to all the work that has been published so far. Rather, we have selected what, we feel, are the most important developments and also, where possible, we have drawn examples from our own work. In a first part, the ESR technique and its extensions such as ENDOR and ESEEM, will be highlighted with special emphasis on the principles of the techniques and the instrumental requirements. Attention will also be given to spectral simulation and to quantitative analysis. In a second part, the general features of ESR spectra of transition metal ions in molecular sieves are analyzed. In addition, some remarks will be made on paramagnetic metal clusters and on paramagnetic molecules. The chapter closes with general conclusions and an outlook into the future. For detailed explanations and discussions-in-depth, we refer to several excellent text books [1-9] and review papers [10-14]. 

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