Orientation signals principal

Between the centers of adjacent principal signals secondary maxima of similar form can be distinguished. As J increases from J = 10 to 100, Fig. 6.8, principal (/ ) signals become relatively sharper (see Fig. 6.9), and secondary peaks, whilst increasing in number, become negligible in their effect. Such behavior of the orientation reminds one of a diffraction-grating-signal characteristic pattern. In this case the k(0) dependence,... 

In organic radicals in solution, the y-factor anisotropy cannot be detected one needs oriented samples. In crystals of free radicals, this anisotropy is easily measured—for example, in crystals of sodium formate (Na+ HCOO-) the principal-axis components are gxx = 2.0032, gyy = 1.9975, and gzz = 2.0014. If there is some spin-orbit interaction in an organic molecule (e.g., if a compound contains S or Cl), then y-values as high as 2.0080 are encountered. In disordered powders with narrow EPR lineshapes, the y-factor anisotropy can produce considerable distortion in the overall signal, due to averaging of the y-tensor. 

Lastly, we consider the diffusive contribution to the signal. Since this portion of the signal arises from molecular reorientation, it should be completely depolarized unless these diffusive reorientational dynamics also have a significant DID component. The orientational decay will be made up of exponential components, the number of which depends on the molecular symmetry and the relationship between the principal axes of the diffusion and polarizability tensors of the molecules (8). If these tensors share no axes, the orientational decay will be composed of a sum of five exponentials. If the tensors share one axis, the decay will be composed of three exponentials. If the tensors share all three axes, the decay will be composed of two exponentials. If the molecule is further a symmetric top, then reorientation about the axis of symmetry cannot be observed, and the decay will be composed of a single exponential. In principle, considerably more information is available when the principal axes of the diffusion and polarizability tensors are not shared however, in practice it is virtually impossible to find a unique fit to the sum of five exponentials, some of which may have very small amplitudes. In the remainder of this chapter we will therefore concentrate on symmetric-top liquids. 

EPR signal within the = 1/2 doublet of this S = 3/2 spin system with the free electron g-value, 2.00. Examination of the Ci H-ESEEM data (Figure 10—solid lines) shows that a pair of frequencies separated by 0.4 and 0.5 MHz is resolved at both g and gy, respectively. Because the effective g-value doubles as you go from gy to gx, the experimental results lead one immediately to the conclusion that the deuteron giving rise to the ESEEM is oriented close to gy or the principal axis of the ZFS tensor. 

A simulation of such a superposition, using the fine-structure constants of the naphthalene molecule (D/hc = 0.100 cm , E/hc = -0.015 cm , and g = ge), and with a linewidth of AB = 1.2 mT for each component is shown in Fig. 7.13a. The discontinuities in the broad Am = 1 ESR spectrum correspond to the principal-axis orientations Bo u u = x,y,z). They occur because of the high density of resonance-field values in the neighbourhood of the principal-axis directions (compare Fig. 7.4). For B = 0, two of these discontinuities occur at the same place. The Am = 2 spectrum is narrow because the anisotropy of the Am = 2 transitions is to first approximation zero. The ESR signal (Fig. 7.13b) is - as usual in ESR due to the method of measurement - the first derivative of the spectrum (absorption vs. frequency or field strength. Fig. 7.13a). Even with a powder or a glass sample. 

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