G-Tensor principal value

The g-tensor principal values of radical cations were shown to be sensitive to the presence or absence of dimer- and multimer-stacked structures (Petrenko et al. 2005). If face-to-face dimer structures occur (see Scheme 9.7), then a large change occurs in the gyy component compared to the monomer structure. DFT calculations confirm this behavior and permitted an interpretation of the EPR measurements of the principal g-tensor components of the chlorophyll dimers with stacked structures like the P 00 special dimer pair cation radical and the P700 special dimer pair triplet radical in photosystem I. Thus dimers that occur for radical cations can be deduced by monitoring the gyy component. 

ENDOR on transition metal complexes in solutions is only attainable if no other nuclei possessing a much larger hf anisotropy than the protons are present. Moreover, the deviation of the g tensor principal values from g should be small, so that Tr(g - gel)2 < 3 10-3. Solvent and temperature, however, appear to have minor influence on optimum ENDOR detection conditions. 

For characterizing supramolecular interactions, it is of interest that g tensor principal values are sensitive to the polarity of the environment and are influenced by hydrogen bonding to an atom with significant spin density in a p orbital.For nitroxide spin probes, it was demonstrated that the two effects can be separated by correlating g,v.i to... 

Figure 6.4 Dynamic Jahn-Teller effect in Cu -doped Tutton s salt (data taken from Ref. 42). (a) Schematic plot of the part of the potential energy surface relevant at temperatures below 310 K. (b) Dependence of the experimental g tensor principal values on temperature (full circles) and fit by the Silver-Getz model (solid lines).

One of the first attempts, in the eontext of EPR spectroscopy, to describe spec-troseopie features of iron-sulfur proteins in terms of theoretical models was intro-dueed for the [2Fe2S] eluster in spinaeh ferredoxin [34]. Gibson and coworkers presented a formula for the g-tensor principal values that eoupled a high spin ferric and a high spin ferrous ion in an antiferromagnetic fashion. Each iron carried an individual g-value (gi and g2, respectively) eoupled to total g by... 

On the basis of the Q-band spectrum giving better resolution than that the X- and S-bands, we concluded that the dominant line-broadening mechanism at Q-band EPR was g-strain and that there was broadening in the X- and S- band spectra. From spectra simulation in the Q-band spectrum, we obtained the g principal values of gxx = 5.20, gyy = 4.75, and gzz = 2.24, and the g-strain tensor principal values <7gxx =1.11 (0.2), dgyy = 1.11 (0.2), and dgzz = 0.19(0.01). 

The value of g-factor is a measure of the coupling between the spin of an unpaired electron and an external magnetic held. It is not only dependent on the spin species but also on its environment. A single numerical value of g is applicable only to systems that behave isotropically. With anisotropic systems, a modihed term that accommodates the variability of g with orientations relative to the external held is introduced as g-tensor. Three values, gx, gy, and gz, which represent principal gxx, gyy> and gzz values of the g-matrix, are important EPR parameters. 

Determination of g-tensor components from resolved 327-670 GHz EPR spectra allows differentiation between carotenoid radical cations and other C-H jt-radicals which possess different symmetry. The principal components of the g-tensor for Car"1 differ from those of other photosynthetic RC primary donor radical cations, which are practically identical within experimental error (Table 9.2) (Robinson et al. 1985, Kispert et al. 1987, Burghaus et al. 1991, Klette et al. 1993, Bratt et al. 1997) and exhibit large differences between gxx and gyy values. 

Petrenko, A., K. Redding et al. (2005). The influence of the structure of the radical cation dimer pair of aromatic molecules on the principal values of a g-tensor DFT predictions. Chem. Phys. Lett. 406 327-331. 

To a limited extent the degree of motion of an adsorbed paramagnetic molecule may be deduced from the g tensor. Rapid tumbling of the paramagnetic species relative to a time scale of 10"9 sec results in an averaging of the principal g values so that... 

The polyerystalline spectrum of N02 on MgO is somewhat complex, but it yields an unambiguous g and hyperfine tensor which can be checked by comparison with data for NO2 in single crystals. For N02 on MgO, principal values of the hyperfine tensor are m = 53.0, 21 = 49.0, and a31 = 67.0 G (29). It should be noted here that neither the signs of the coupling constants nor their directions relative to the molecular coordinates... 

It is apparent from either set of equations that the spectrum of O- will be fairly anisotropic for reasonable values of X and AE. Furthermore, two principal components of the g tensor will be greater than ge] whereas, the third component will nearly equal ge. 

The form of the g tensor depends upon the choice of orthogonal axes. If the axis system is chosen along the molecular symmetry axes, the tensor contains only diagonal terms with the off-diagonal terms being zero. Conveniently, the principal g values derived from the polycrystalline spectrum are just those of a diagonalized matrix, i.e.,... 

From this final equation it may be observed that large deviations from the free electron g value occur when (a) the integral (0O Lz2 0O) is nonzero and (b) excited electronic states are rather close in energy to the ground state. The other principal values of the g tensor have the same form as shown in Eq. (18C). 

We know that a model in which the principal values of the g tensor are random variables, leading to Equation 9.1, falls short of describing experimental data in detail. Therefore, we now expand the model as follows the random variables are the principal values of a physical entity that is characterized by a tensor in 3-D space, but... 

Throughout this paper the principal values of magnetic coupling tensors are denoted by lower indices x, y, z if the corresponding principal axes coincide with the g tensor axes. In all other cases, indices 1, 2, 3 are used... 

EI-EPR has also been applied in powder samples, where single crystal-like EPR spectra can be obtained for B0 observer fields which correspond to the orientations of principal values of ligand hfs tensors37. This is illustrated in Fig. 15 for a powder sample of dibenzene vanadium diluted into polycrystalline ferrocene. The hfs tensors of the twelve geometrically equivalent benzene protons are not coaxial with the g and the... 

Reaction of nitric oxide with ferrohemoproteins produces paramagnetic NO-ligated heme proteins (S = 1/2, rhombic g tensors with principal values in the range 1.96-2.08). In many compounds studied so far by EPR the hf interaction of the NO nitrogen and of a second axial nitrogen is clearly resolved in the intermediate g-value region near... 

Furthermore, room-temperature CW EPR only picks up the isotropic g value that represents the average of the three principal g values. Two complexes with entirely different surroundings, and hence entirely different g tensors, may accidentally have the same isotropic g value. For instance, the example in Fig. 1 may stem from a complex with gx = 1.9825, gy = gz= 1.9792 or from a complex with gr =1.9825, gy= 1.9801, gz= 1.9784. The first concerns a molecule where the unpaired electron senses a local axial symmetry, the latter is due to a Cr(V) site with rhombic surroundings. From the room-temperature CW-EPR spectrum, both cases would be considered identical if no additional proton hyperfine value is resolved. The full g tensor can only be obtained from low-temperature CW-EPR, preferably at higher microwave frequencies.42,48... 

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