Superhyperfine tensor

A list of reported g values for O2 species on different supports is given in Appendix A and has been used to construct Fig. 3. This can be a useful guide for determining the site of adsorption of the O2 provided that the simple ionic model of Eq. (6) is appropriate. The superhyperfine tensor (Section III,A,3), where available, also confirms the nature of the adsorption site. 

The superhyperfine tensor is primarily used to confirm the nature of the adsorption site. This is clearly seen for 02 on enriched 95Mo03/A1203... 

The superhyperfine tensor has also been used to derive the amount of spin delocalization on the cation leading to the superhyperfine structure. In view of what has been said above on the origin of the superhyperfine interaction, the result must be handled with caution. Thus, the unpaired electron... 

In addition to the nature of the cation at the adsorption site, the superhyperfine tensor can also give information on neighboring atoms further away. For example, OJ adsorbed on MgO exhibits a superhyperfine tensor ascribed to the presence of a nearby proton, presumably as a hydroxyl group (68) and this has been confirmed by isotopic labeling with deuterium (see Section IV,A). Superhyperfine tensors indicating the presence of nearby protons have also been reported for O J adsorbed on ferrocene deposited on porous Vycor glass (PVG) (120) and for alkylperoxy radicals supported on Ti02 (90). 

The information which is obtained from the superhyperfine tensor is important and much effort has been aimed at obtaining this parameter. 

The g- and superhyperfine tensors, in general, are eharacterized on the basis of symmetry, and their relative orientation is fixed in the moleeular referenee frame (usually defined by molecular symmetry axes). The Hamiltonian that describes the electronic Zeeman interaction in a principal axis frame (no terms of the form gij, where i J) is... 

Similar results have been obtained by Ismailov et al. (240) in Y-type zeolites containing iron impurities (Section IV,C,3). The formation of 02 on the ferrocene/porous Vycor glass system has been observed by Vanderspurt et al. (120) with a g tensor of 2.0300, 2.0100, 2.0020 and a superhyperfine sextet centered on each g component. These results were interpreted in terms of 02 adsorbed on the cyclopentadienyl ring of ferrocene and, assuming an ionic model, the g2Z value of 2.0300 is broadly consistent with a +3 charge at the adsorption site. 

However, it must be borne in mind that in previous work, H2 did not react with a triangular array of O ions to form OH" ions (354). If such a reaction with H2 occurred, then the O" ions would no longer be available for Oj formation. Moreover, the reaction of pairs of O ions with oxygen should lead to pairs of O J ions which would have an abnormal EPR spectrum if they can be seen at all. In fact, the g tensor is as expected for isolated OJ ions. The CoO-MgO system behaves as CaO for the formation of Oj, i.e., via invisible O ions. The ozonide ions characterized by a three-g-value EPR signal (2.0025, 2.012, 2.017) do not exhibit any superhyperfine interaction with cobalt nuclei, suggesting that they are adsorbed on Mg2+ ions (110). Depending on the system (MgO, CaO, CoO-MgO) and the experimental conditions, the ozonide ion Oj disappears irreversibly between 25° and 130°C. In the case of MgO (333,334), OJ ions are formed when O J ions are destroyed, whereas for CaO (158) and CoO-MgO (110) the evidence is not clear. 

A wide range of five- and six-coordinate porphyrin complexes of the types [MoO(por)]+ and trara-[MoO(por)X]0/+ ((191), Scheme 8 X including OH, Cl, Br, NCSA OR, 02CMe, 03SPh, N03, H20, dmso) have been reported. These exhibit axial EPR spectra, with g ca. 1.965-1.977 and AMo ca. 55 x 10 4 and 46 x 10 4 cm 1 for five- and six-coordinate species, respectively theoretical calculation of the g tensors has advanced considerably.595 In six-coordinate species, superhyperfine coupling to the porphyrin nitrogens (dN 2x 10 4cm ) leads to characteristic nine-line resonances. The chemistry, electrochemistry, photochemistry, and catalytic properties of oxo-Mo porphyrins have been reviewed (see Scheme 8). The structures of... 

Aa and AB are the local hyperfine tensors and IA and IB the nuclear spin operators. In Eq. (3), superhyperfine and transferred hyperfine interactions are neglected. 

Table 3.4 Representative superhyperfine and quadrupolar parameters of various nuclei magnetically coupled enzymes from XO and SO families. Coupling constants are given in MHz, Euler angles are in degrees and to the principal axis system of the g-tensor.

Incubation of recombinant human SO with arsenate leads to a Mo(v) species that has similar g-values to the phosphate species but with additional resolved structure due to ligand superhyperfine coupling. Simulations of the CWX- and Q-band EPR spectra led to Aj 2,3 ( As) = [15.9,24.9,12.4] MHz, Pj 2,3 ( As) = [4.5, 3.3, -7.8] MHz, with collinear A- and P-tensors. Comparison of these data with those of the Pi-inhibited form led to the suggestion that the arsenate and the phosphate-inhibited species share a similar structure, consistent with the interpretation of the EXAFS results that lead to a Mo...As distance of 3.2 A. 

In the case of transition metal complexes with large g anisotropy in disordered matrices, mw frequencies <9.4 GHz are sometimes preferable, because local heterogeneities (strain) of the matrix lead to a distribution of the principal values of the g- and A-tensors g- and A-strain) and thus to field-dependent line broadening. Such a situation is illustrated in Fig. 11 for Cu(II) in Nation perfluorinated ionomers swollen by acetonitrile the line width of the parallel components was measured at four mw frequencies in the range 1.2-9.4 GHz, and the narrowest line widths were detected for the two low-field lines of the parallel quartet at C band (4.7 GHz) and L band (1.2 GHz). In this way, clear superhyperfine splittings from nuclei were resolved, in addition of course to the hyperfine splittings from Cu(ll). 

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