Anisotropy of the spectrum

Fig. 9. Dependence of the outer-extrema splitting 2A in nitroxide spectra on temperature (simulation for an activated process with activation energy of 40 kJ mol— ). At the temperature 50G> where 2A = 50 G, the correlation time matches the inverse anisotropy of the spectrum.

With h 6) - 1/sin 0)5(0 — Oq), one obtains the same result as given by (4.58), which implies that the anisotropy of the/factor cannot be derived from the intensity ratio of the two hyperfine components in the case of a single crystal. It can, however, be evaluated from the absolute/value of each hyperfine component. However, for a poly-crystalline absorber (0(0) = 1), (4.66) leads to an asymmetry in the quadrupole split Mossbauer spectrum. The ratio of l-Jh, as a function of the difference of the mean square amplitudes of the atomic vibration parallel and perpendicular to the y-ray propagation, is given in Fig. 4.19. 

It is quite common to observe that the lines in a nitroxide spectrum show variations in width. One feature which contributes to this is the anisotropy of the nitroxide grouping, as a consequence of which the high field component of the nitrogen triplet may be perceptibly broadened. This effect is particularly noticeable when free tumbling of the nitroxide molecule is restricted either by the viscosity of the solvent or when the radical is incorporated into a very large (e.g. polymer) molecule. This selective line broadening is, of course, one of the principal sources of information in spin-labelling experiments (Berliner, 1976). 

Unfortunately the optical spectrum (159) of VEPI in the visible and ultraviolet appears to be dominated by electronic transitions of the porphyrin group such that the spectrum of the 3d electron is masked, and direct comparison of the experimental gi-factors with the theoretical ones (Equation 40) cannot be made. The value of k necessary to account for the anisotropy of the hfs is 0.71 with the result that... 

A Cu(II) complex containing two spin-labelled ligands was prepared with a rigid linkage between the terpyridine chelator and a nitroxyl ring.26 Due to the large anisotropy of the Cu(II) spectrum only a small fraction of the Cu(II) spins are excited by the pulses of a DEER experiment. Analysis of the echo modulation... 

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