Superhyperfine interactions

Combines sensitivity of EPR and high resolution of NMR to probe ligand superhyperfine interactions For paramagnetic proteins enhanced chemical shift resolution, contact and dipolar shifts, spin delocalization, magnetic coupling from temperature dependence of shifts Identification of ligands coordinated to a metal centre... 

An alternative model (Venable 1967) proposes that the main cause of inhomogeneous broadening is unresolved superhyperfine interactions and, therefore, that the linewidth expression should be equivalent to the Equation 5.12 for the angular dependence of first-order hyperfine splitting ... 

The main application of double resonance is the resolution of hyperfine and particularly superhyperfine interactions that are not extractable from regular EPR spectra because they are lost in the inhomogeneous line. The biological relevance is in otherwise unavailable detailed information on the electronic structure and the coordination of active sites and their interaction with reactants, such as enzyme substrates. To be well prepared, check off the items in the following list. 

Besides the hyperfine constants for the muonium impurity itself, one can also investigate the so-called superhyperfine interaction for the neighboring 29Si atoms. These values have also been accurately measured (Kiefl et al., 1988) with level-crossing resonance.For the anisotropic parameters, it is customary to compare b with Aj,ree, which is an average of r 3 determined for the valence p-orbital. The results are given in Table II. Both... 

The superhyperfine interaction is observed for metal complexes in cases where the metal ligands have a nuclear moment. For instance, the nitrosyl (NO) complexes of iron(II) heme proteins have two inequivalent axial nitrogen ligands. The 14N(/ = 1) NO couples strongly to the unpaired electron, yielding a widely split triplet with each component of equal intensity and separated by 2.1 mT. The second... 

Halogen-donor Ligands. The a- and p-modifications of LiVF have been obtained by reaction of LiF and VF3 (3 1) and quenching or slowly cooling the products, respectively." Hyperfme and superhyperfine interactions of V " ions in MFj crystals (M = Ca. Sr, or Cd) have been studied at 4.2 K. " ... 

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... 

Where naturally occurring isotopes of the cation are not suitable, then enrichment of the surface cations with nonzero-nuclear-spin isotopes is a powerful technique, e.g., 95Mo03 on A1203, Si02, or MgO (108). Often, the presence of a superhyperfine interaction leads to spectra of low resolution and it is useful to increase both the intensity of the spectra and the resolution. Second or even higher derivative spectra can be used to enhance the resolution and in some cases secondary reactions (121) have been shown to increase the intensity. 

Because of the superhyperfine interaction which arises when the OJ ion is formed on a cation with nonzero nuclear spin (see Section III,A,3) vanadium pentoxide, with 100% naturally abundant5 V isotope (I = ), has been of considerable interest. However, the presence of a superhyperfine splitting has created some difficulty in the assignment of the signals. V205 cannot be prepared with large surface area and most of the data refer to supported V205 systems. 

Since the gzz value can only give an indication of the charge at the adsorption site, it is more informative to study the superhyperfine interaction from the cation (see Section III,A,3). For this purpose, 95Mo-enriched catalysts... 

The range of the gzz values is shown clearly by a comparison of the results for the NaY and NaX zeolites. Since the migration of Na+ ions is related to the presence of water (76), it is likely that the type of precursor (Na4)4+ -(H20)x complex formed after a proper degree of dehydration (278) will be strongly dependent on the pretreatment conditions. This will be reflected in the gzz values of the OJ produced during y irradiation by electron transfer from the precursor (278). It is also likely that the OJ can migrate after its formation as shown by Kasai and Bishop (264). These authors (272) have detected a superhyperfine interaction from Na nuclei (I = ) in the EPR spectrum of OJ formed in Na-reduced NaY zeolite and characterized by gzz = 2.113. This value is very close to those observed for alkalisuperoxides trapped in krypton matrices (Ref. 44, Appendix A). 

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. 

Extensive work has been done on the hyperfine interaction of the hexafluoride complexes of several transition ions (29, 39-45). To illustrate the approach used in interpreting the superhyperfine interaction, we shall consider the case of the ds configuration of Mn2+ and Fe3+. For this case we have an 6S-state ion with each d orbital singly occupied. The equations for the five antibonding MO s are... 

Superhyperfine interactions are not easily observed in dy, because MO s involving the t orbitals cannot include s orbitals from the ligand, and it is the s orbital that contributes the major portion of the superhyperfine interaction. The F19 hyperfine has been detected (42, 45), and it was found that the isotropic contact term was indeed small as expected. Kuska and... 

The EPR spectrum of VO(Et2Dtc)2, enriched with 13C at the CS2 group of the E2Dtc ligand, shows13C superhyperfine interactions. 

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