Hyper-fine coupling

The envelope curve depicted in Fig. 116 also applies to anisotropies caused by nuclei other than protons. As has been stressed already, such anisotropy generally arises from dipole-dipole interactions between electrons in p- or d-levels on the atom concerned. In that case, the hyper-fine coupling tensor should have axial symmetry, so that two of the three elements of the hyperfine tensor will be equal, and components similar to that in Fig. 106 will be detected. 

A molecular modeling program to calculate electron paramagnetic resonance hyper-fine couplings in semiquinone anion radicals was offered [173b]. 

The ESR spectra of the tris-bipyridyl complexes of Cr(I), V(0), and Ti(—I) have been reported (183, 233, 427). Isotropic g values and hyper-fine coupling constants are given in Table XIII. Konig (427) shows that... 

TABLE 4 D and AZ)Ar Values of the Triplet Diradicals 12, Calculated a. Spin Densities for the Monoradicals 16, and a Hyper fine Coupling Constants of the Benzyl-Type Monoradicals 15... 

Because of the relationship between polarization intensities and hyper-fine coupling constants (Section III.C), the intermediates can be identified as well, from what is called the polarization pattern (the relative polarization intensities of the different protons). This use of... 

An alternative expression for the variation of the anisotropic hyper-fine coupling with the angle is... 

TABLE 5. Temperature dependence of the a-CH proton hyper-fine couplings in RSOjNPr1 ... 

Table 12 Isotropic hyper fine coupling constants (in G) for various small systems as obtained with LDA calculations and different types of basis functions (GTO or STO for Gaussians or Slater-type orbitals, respectively) together with experimental values. In some cases more values are reported corresponding to different sizes of the basis sets. From ref. 74...

The appearance of an ENDOR spectrum depends on the relative size of the hyper-fine coupling (hfc) and the NMR frequency vn. For a nucleus with nuclear g-factor = 5.586 a frequency of vh = 15 MHz is characteristic for... 

Fig. 2.23 Schematic HYSCORE spectrum showing (a) the HYSCORE sequence, (b) the 2D time-domain modulation signal, (c) the 2D HYSCORE spectrum and (d) the contour plot of a single crystal sample for an 5 = Vi species containing a H nucleus with an axitilly symmetric hyper-fine coupling. The magnetic field is at an angle 0 = 10° with the A axis. The nuclear Zeeman frequency vh 15 MHz is larger than the hyperflne coupling, i.e. Ai I < A < 2 vh...

Abstract The analysis of ESR, ENDOR, and ESEEM data to extract the resonance parameters is treated. Free radicals in solution are mainly identified by their hyper-fine couplings (hfc). The analysis of ESR and ENDOR spectra by visual inspection and by computer simulation is discussed. The Schonland method to obtain the principal values and directions of the anisotropic g- and hfc- tensors from single crystal ESR and ENDOR data is presented. The modifications needed when 5 > A or / > A to obtain zero-field splitting ( i) or nuclear quadrupole coupling (nqc) tensors are considered. Examples of simulations to extract g-, hfc-, Tfs-, and n c-tensors from ESR and ENDOR spectra of disordered systems are presented. Simulation methods in pulsed ESR (1- and 2-dimensional ESEEM) studies are exemplified. Internet addresses for down-loading software for the simulation of ESR, ENDOR, and ESEEM spectra are provided. Software for the analysis of single crystal data by the Schonland method is also available. 

Separation of interactions allows for precise measurements of the small interactions of the observed electron spin with remote spins in the presence of line broadening due to larger contributions. Such techniques are therefore most useful for solid materials or soft matter, where ESR spectra are usually poorly resolved. The most selective techniques for isolating one type of interaction from all the others are pulsed double resonance experiments, such as ENDOR and electron-electron double resonance (ELDOR), which are discussed in more detail in Chapter 2. If the hyper-fine couplings are of the same order of magnitude as the nuclear Zeeman frequency, ESEEM techniques may provide higher sensitivity than ENDOR techniques. In particular, the two-dimensional hyperfme sublevel correlation (HYSCORE) experiment provides additional information that aids in the assignment of ESEEM spectra. These experiments are also discussed in Chapter 2. 

Tyiyshkin AM, Dikanov SA, Reijerse EJ, Burgard C, Hilttermann. 1999. Characterization of bimodal coordination structure in nitrosyl heme complexes through hyper-fine couplings with pyrrole and protein nitrogens. JAm Chem Soc 121(14) 3396-3406. 

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