Superhyperfine interaction, ligand

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... 

Electron-nuclear double resonance (ENDOR) Combines sensitivity of EPR and high resolution of NMR to probe ligand superhyperfine interactions... 

The ESR signal is highly dependent on the nature of the local environment about the absorbing electron that is, the position of the ESR signal and the overall ESR spectral pattern depend on the environment conditions in the vicinity of the electron. The most important types of interactions in the spin system that affect the position and pattern of the ESR spectrum are the electron Zeeman, nuclear hyperfine, and ligand superhyperfine interactions (Wertz and Bolton, 1972). 

A ligand superhyperfine interaction can occur if the ligand atoms have nuclear spin, such as N (7 = 1). A number of components may thus result for each ligand nucleus, which leads to very complex ESR patterns, particularly if tlie ligands are not identical. 

We restrict ourselves to the following terms in the interaction Hamiltonian of the R ion with nuclei of ligands (superhyperfine interaction) ... 

Transannular interaction via the electron-delocalization mechanism was found, but lessened by 10-15% for the ligand superhyperfine splitting and 30-35% for the hyperfine splitting (62) in the epr spectrum. The crystal structure of [VOS2CNEt2)2] shows that the molecular core has the expected C2V symmetry [V-0 = 159.1(4), V-S = 138.7(2)-241.0(2) pm] (63). Magnetic and spectral data provided evidence for a tetragonal, pyramidal structure (VII) for these complexes. Like many other coordinatively unsaturated, metal... 

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. " ... 

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. 

Superhyperfine interactions are rarely observed in the ESR spectrum of the VO + ion because the unpaired electron interacts only weakly with the ligand nuclei, so that often the size of the coupling is less than the ESR bandwidth. This problem has been surmounted through the use of ENDOR spectroscopy. In ENDOR spectroscopy, molecules with their V=0 axes either parallel or perpendicular to the direction of the static magnetic field are selectively irradiated. In this way, the anisotropic superhyperfine coupling constants of and N and the N quadrupolar coupling constants can be obtained. 

Hyperfine interactions will also arise if there is delocalization of the unpaired electron from the paramagnetie metal centre onto ligands in the first coordination sphere, atoms in which possess / > 0 (e.g. F, 100%, I = ). In this case, the spectrum is more complex because of the presence of the transitions arising from this so-called superhyperfine interaction. 

Oxovanadium(IV) complexes with dithiophosphate ligands have been extensively examined <8,121.161,252,386) x typical ESR spectrum is shown in Fig. 7. In addition to the eight vanadium 1=112 hyperfine lines phosphorus (/ = 1/2) superhyperfine splitting is also observed. The phosphorus superhyper-fine splitting can be considered a bit unusual since the phosphorus is located about 3 A or more away from the metal ion. P and As superhyperfine splitting has been observed in the ESR spectra of ill-defined vanadium phosphine 388) and arsine 389) complexes but in those cases, presumably, direct V-P and V—As interactions occur. ESR parameters have been tabulated for a large number of dithiophosphate 121,252) dithiophosphinate 121.252) complexes. Evaluation 3i) of the fractional 3s character of unpaired electron in dithiophosphate complexes yielded a value of 1.35%. The vanadyl(IV) complexes possess approximate C2V symmetry. The unpaired d electron resides... 

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