Resonance condition anisotropy

Before we develop the resonance conditions for systems with hyperhne and with zero-held interactions, we return to the electronic Zeeman term S B as an example interaction to discuss a hitherto ignored complexity that is key to the usefulness of EPR spectroscopy in (bio)chemistry, namely anisotropy the fact that all interactions... 

The EMR spectra of polymer films prepared without external field demonstrate quite broad lines (Figure 3). Center of these lines is strongly dependent on the sample orientation in MF of spectrometer. This results from the anisotropy of demagnetizing fields, which determine resonance conditions for the thin magnetic film ... 

The factor (1-tr) converts the general resonance condition for the nucleus P to the specific resonance condition for the individual phosphorus atom in the actual compound that we want to measure. The parameter phosphorus atom is subject to. These are normally limited to diamagnetic factors, such as electronegativity of substituents, and diamagnetic anisotropy due to participation in double bonds or shielding cones of adjacent functional groups. 

Fig. 21. (a) H-decoupled NMR spectrum of polycrystalline N-labelled N-methyldiphenylphosphoiamidate, without sample rotation. The P-shielding anisotropy and asymmetry parameters are -21.24 kHz and 0.19, respectively, (b) With ma c-angle sample rotation at 1.5 kHz. (c)-(e) indicate when an r.f. irradiation is on at the N-15 isotropic shift frequency. The irradiation intensity fulfils the rotary resonance conditions [Pg.82]

For an actual fine-particle system, with volume and easy axis distribution, the dependence of the resonance condition upon the direction of anisotropy axes provides an asymmetric lineshape at low temperature, accompanied by a shift of the resonant field. With increasing temperature, the width reduces and the line transforms to a Lorentzian shape with an isotropic resonance field thermal fluctuations (with t t, ... 

The above described temperature evolution of the lineshape is the result of three different mechanisms, having different temperature dependences. In addition to the spreadout of anisotropy axes relative to the external field, another mechanism of inhomogeneous broadening does exist, due to the fact that the instantaneous resonance condition depends on the relative orientation (described by the angle 0) of the magnetic moment of the particle and its anisotropy axis ... 

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