Orthorhombic g-tensor

A more direct method of studying rotation is to look for variation in the g or hyperfine (Section III,A,2) tensors as the temperature is changed. For certain peroxy radicals in polymers, considerable changes in the g tensor occur which can be correlated with both the onset of rotation of the polymer chain and also rotation about the C—O bond with increasing temperature (58, 59). Similar changes in the g tensor have been observed for ions formed by irradiation in frozen alcohols (60). Kazusaka et al. (61) have observed changes from an orthorhombic g tensor gt = 2.0266, g2 = 2.0097, g3 = 2.0042 at 77 K to an axial g tensor with gl = 2.007 and g = 2.018 at... 

The classic model of a side-on adsorbed 02 radical predicts an orthorhombic g tensor, so at first glance the assignment of the signal at = 2.026, = 2.012... 

Vedrine et al. (102a) have reported a different type of motion for 02 in a Ce-X zeolite which is characterized by unusual parameters gt = 2.0242, g2 = 2.0208, g3 = 2.0112, and At = 24, A2 = 66, A3 = 12 G. However, inspection of the published spectra indicates that A, and A3 are difficult to estimate. The two oxygens are apparently equivalent and the spectrum is tentatively interpreted in terms of a rotation of 02 about an axis perpendicular to the internuclear axis at 77 K, i.e., the y axis in Fig. 4. This is difficult to reconcile with the observed orthorhombic g and A tensors, which are not averaged. A rotation of 02 around the internuclear axis has also been reported by Breysse et al. (62) for 02 with equivalent nuclei adsorbed on Th02. The gxx and gyy components, which are distinct at 77 K, are averaged to gL = (gxx + gyy)/ 2 at 298 K and this is explained in terms of a rotation around the internuclear z axis. The l70 hyperfine tensor, however, is not completely averaged, ff one assumes that Ayy is zero at 77 K, as for most oxides (Appendix B), then Ayy and Axx should average to A = (Axx + Ayy)f2 = 37.5 G at 298 K. This value is substantially different from the experimental one of 65 G reported by the authors (62). 

A theoretical analysis of the g tensor for O trapped in alkali-halide single crystals has been given to second order in the case of axial (7) and orthorhombic symmetry (8). Recently, refined theoretical treatments have been... 

Here D denotes the axial fine structure parameter, whereas E describes the orthorhombic fine structure parameter [5]. The influence of an axial (i.e. D O and E=0) and an orthorhombic fine structure splitting (i.e. D O and E O) on a powder spectrum is shown in Fig. 3 for an isotropic g-tensor and for S equal to (e.g. Cr +). 

Fig. 2 Raman spectra of a single-crystal of orthorhombic Sg at three different polarizations in which the off-diagonal elements of the Raman scattering tensor are non-zero big, b2g, b g), after [105]. However, Raman intensities of other polarizations like flg components ( 54 cm ) penetrate in the spectra due to optical anisotropy in the crystal...

Figure 2 shows the Q band (35 GHz) powder spectrum of Co(acacen) doped into Ni(acacen) as a typical example of the EPR spectra of planar, four-fold coordinated, low-spin Co(II) Schiff base complexes. The structure, due to the three principal values of the g and A tensors is nicely resolved. Both tensors are clearly orthorhombic, with one large and two similar, smaller principal values. Powder spectra yield however no information on the orientation of the tensors with respect to the molecular frame. This information is extremely important in complexes having low molecu-... 

Note that for molecules of lower than orthorhombic symmetry, the off-diagonal components of the tensors g, and g, are not zero by symmetry. 

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