G tensor values

FIGURE 9.8 HF-EPR spectra of canthaxanthin radical cation adsorbed on silica-alumina (solid line)—experimental spectrarecorded at5 K (dotted line)—simu lated spectra using g-tensor values g = 2.0032 and g =gyy = 2.0023 and linewidth of 13.6G. (From Konovalova, T.A., J. Phys. Chem. B, 103, 5782, 1999. With permission.)... 

EPR. The frozen solution EPR spectrum of 75 shown in Fig. 13 exhibits rhombic symmetry typical of a Ni(III) bis(dithiolene) complex (126), with g-tensor values of gx = 2.13, gy = 2.04, g, = 1.99. The nickel dimer can thus be viewed as a classical bis(dithiolene) moiety with a 3B3g ground state where the odd electron orbital composed primarily of a metal dyZ orbital and sulfur 2pz orbitals. The Ni(II) in the pz, as expected, is EPR silent (19, 122). 

Comparing spin-Hamiltonian parameters measured for NO centers in different inorganic matrixes (Table 8.5) one can conclude that for all of them Ay = 30 2.5 G, Az < 10 G and A < 5 G although g-tensor values are varying in a rather wide range depending on the local crystal fields in the lattice. 

Here, k is the dilution factor. The g-tensor values were determined experimentally for all complexes in low-temperature solid sulphuric acid/acetonitrile matrices. It turned out that these values are essentially invariant ( gj = 2.6 and g, =1.0) through the series of compounds. They correspond to a mixing angle of x=67.5°. 

The 72 ground state of [Fe(CN)g] is split by spin-orbit coupling into an E" ground state and an U excited state at (3/2)g. g-tensor value expressions show significant orbital contribution to the low-temperature magnetic behavior (Table 2). Similarly, the Ti ground state of [M(CN)6] (M = Mn Cr ) splits into an Ai... 

Sample Radical generation Sample temperature g tensor values Radical type Reference... 

The accuracy of DK KS wave functions for the description of g values can be illustrated by switching off the relativistic form of the Hartree interaction [19,35]. The commonly used DKnuc restriction (neglecting the two-electron contribution to the spin-orbit interaction) [14,16] notably overestimates the spin-orbit splitting (Section 3.1) [33]. For molecules without very heavy atoms, this approximation does not significantly change common observables [19]. However, g tensor values are much more sensitive to the proper relativistic form of... 

The observed interval of the g-factor values at various a is characteristic of ARs. Therefore, in analyzing ESR spectra, the anisotropic magnetic parameters determined for low-molecular ARs were used [47]. It is commonly recognised for low-molecular ARs that the z axis is aligned with the 2p-orbital of the unpaired electron of the N atom, and that this direction determines the maximum value of the A -tensor and g-tensor value close to that for free electrons. The x axis is aligned with the N-O bond characterised by the maximum value of the g-tensor (2.0088-2.0104) and small (0.5-0.7 mT) A valne. The y axis determines a g-tensor value of 2.006-2.007, and the hyperfine coupling (HFC)-tensor value approximately corresponds to [55]. 

This system serves as the prototype for aryl thiyl radicals. Measurements of the high g tensor values and anisotropy observed in ESR [36] was interpreted to indicate a high degree of localization of the unpaired electron on the sulfur atom. This conclusion has been strongly reinforced [37] by experimental observations of a high rate constant for second order decay implying rapid spin relaxation, the formation of diphenyldisulfide as dominant combination product rather than any ring-bonded products and the close similarity of vibrational frequencies in the radical with those of its parent thiophenolate anion and thiophenol species. Further support comes from measurements [38] of the pXg, reduction potential, and electron transfer kinetics. 

In many instances observation of an ESR signal and measurement of the principal values of the g-tensor suffices to determine the symmetry of the lanthanide ion site in the crystal lattice. In addition, in many cases estimates of the B" parameters which characterize the CEF (or at least the ratios of these parameters) may be inferred from the measured g-tensor values. 

---