Multi-frequency ESR

Table 4.1 Notation for frequency bands in multi-frequency ESR...

The Z-component is by convention taken as the numerically largest. The system is axially symmetric when E = 0, and deviates the maximum amount from axial when E = l/3- H, therefore 0 < E <1/3 H. The sign of D can be determined by employing multi-frequency ESR measurements described later in this chapter. Occasionally, measurements at lower fields can be employed to obtain the relative signs of the zfs and an anisotropic hfc (Section 4.4.2). The sign of the s is of interest for zero or numerically small values of E, and becomes immaterial in the limit of maximum asymmetry. 

The work by Fournel et al. [41] provides an instructional example of the use of multi-frequency ESR to obtain detailed data for a complex biochemical system (a bacterial enzyme), in particular the exchange interaction between a transition metal ion and a radical and the determination of the magnitude and sign of the zero-field coupling. Procedures are described for the less complex biradical systems in the following section. 

An important advantage of using multi-frequency ESR spectroscopies is to separate the g and hyperfine (hf) A tensor components and to resolve the weak deviation (gxx-gyy) from the axially symmetry of theg tensor so as to evaluate accurate values of E, A and / according to Eqs. (6.1), (6.2) and (6.3). 

Notes ESR g and A( N) principal values in (a) and (b) were obtained from the simulation of CW ESR spectra observed at three different MW frequencies (multi-frequency ESR spectra). The E, A, I, SE, SA and SI values were evaluated from theg values and their distribution widths (5g). The computational DFT results correspond to the B3LYP/6-31+G(d) optimized geometry of the Na-NO complex in model 3A in [32]. 

The interaction of nitric oxide (NO) with metal ions in zeolites has been one of the major subjects in catalysis and environmental science and the first topic was concerned with NO adsorbed on zeolites. NO is an odd-electron molecule with one unpaired electron and can be used here as a paramagnetic probe to characterize the catalytic activity. In the first topic focus was on a mono NO-Na" complex formed in a Na -LTA type zeolite. The experimental ESR spectrum was characterized by a large -tensor anisotropy. By means of multi-frequency ESR spectroscopies the g tensor components could be well resolved. The N and Na hyperfine tensor components were accurately evaluated by ENDOR spectroscopy. Based on these experimentally obtained ESR parameters the electronic and geometrical structures of the NO-Na complex were discussed. In addition to the mono NO-Na complex the triplet state (NO)2 bi-radical is formed in the zeolite and dominates the ESR spectrum at higher NO concentration. The structure of the bi-radicai was discussed based on the ESR parameters derived from the X- and Q-band spectra. Furthermore the dynamical ESR studies on nitrogen dioxides (NO2) on various zeolites were briefly presented. 

The second topic is an extension of the first one and was concerned with ESR studies of the Cu(I)-NO complexes. Copper ion exchanged high siliceous zeolites such as ZSM-5 and MCM-22 have been considered as a promising environmental catalyst for the NO decomposition. The Cu(I)-NO complex has attracted special interest because of its important intermediate in the catalytic NO decomposition. Poppl and other scientists have extensively applied multi frequency ESR, pulsed ENDOR and HYSCORE methods to clarify the local structure of Cu(I)-NO adsorption complexes. 

NLSL.SRLS Performs fitting for multi-frequency ESR spectra using the Slowly Relaxing Local Structure (SRLS) model (Ref 26). 

The advances in technology in the last years led to the development and optimization of new ESR methods such as high field/multi-frequency ESR [29-31], double resonance [32, 33], and pulse methods [34-36]. The improvements in resolution brought by these methods have made possible analysis and interpretation of more complex systems and detection and characterization of transient paramagnetic intermediates inaccessible before. 

Multi-frequency and high field studies are at present mostly carried out in CW-mode. The chapter is concerned with examples of CW ESR applications, mostly in rigid amorphous matrices, e.g. frozen solutions, multiphase systems and in polycrystalline samples where the additional information from multifrequency and/or high field measurements are the most crucial for accurate characterisation. 

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