Isotropic hyperfine coupling anisotropic

The and operators determine the isotropic and anisotropic parts of the hyperfine coupling constant (eq. (10.11)), respectively. The latter contribution averages out for rapidly tumbling molecules (solution or gas phase), and the (isotropic) hyperfine coupling constant is therefore determined by the Fermi-Contact contribution, i.e. the electron density at the nucleus. 

HypB protein, 47 289 HypC protein, 47 289 Hyperfine coupling, 13 149-178 anisotropic, 13 150-161 Hyperfine coupling anisotropic dipolar, 13 150-154 nuclear Zeeman interaction, 13 155 quadrupole interaction, 13 154, 155 factors affecting magnitude of metal influence of charge on metal, 13 169-170 isotropic and anisotropic, 13 166-170 libration, 13 170... 

These workers (Adrian et al., 1962) also studied the spin resonance spectrum of DCO radicals and obtained remarkably narrow lines and shoulders which gave sufficient detail that the anisotropic hyperfine tensor could be deduced. This result then enabled them to extract the data tabulated from the spectrum of HCO. In particular, it is pointed out that as the g- and hyperfine-anisotropies have different principal axes, there has to be an extra term (Ayz) where the hyperfine tensor is expressed in terms of the axes of the gr-tensor. A careful analysis of all the data led these authors to the conclusion, based entirely upon experiment, that the large isotropic hyperfine coupling must be positive. 

The goal is to make comparisons of calculated and experimental isotropic and anisotropic hyperfine couplings a useful guide in identifying radiation induced free radicals. The basic problem here is that the calculation of accurate hyperfine coupling constants is rather difficult. Two factors are involved the isotropic component (Aiso) (see Eq. 18-2) and the anisotropic component (Axx, Ayy, Az/ ) (See Eq. 18-3). One must have a good description of electron correlation and a well defined basis set in order to calculate accurate isotropic hyperfine couplings. This is not easy to do with molecules the size of the DNA bases. Even when the computational demands are met, the theoretical calculations may deviate more than 20% from the experimental results. 

The contribution of the hyperfine interactions to the relaxation rates of the radical depends on whether the dominant contribution comes from the anisotropic (dipolar) or the isotropic (scalar) part of the hyperfine interaction. Usually, the anisotropic contribution predominates because this interaction can be readily modulated by the tumbling motion of the molecule. However, in radicals and radical anions such as the trifluoroaceto-phenone (115,116), the rotation of the CF3 group may modulate the isotropic part of the hyperfine interaction and the scalar relaxation W0 could dominate the dipolar transition W2. In such a case, the authors have pointed out that the sign of the resulting CIDNP will be independent of the sign of the isotropic hyperfine coupling constants. 

The presence of a radical having an isotropic hyperfine coupling to a single proton in the 7 to 10 gauss region and an anisotropic g-tensor seems to be indicated, but several features of the results, especially on annealing, are puzzling. 

In the above description, it was assumed that the nucleus lines up with the applied field in all orientations of the orbital, i.e., the applied field Hq is much stronger than the field due to the electron at the nucleus. In practice, however, this strong field approximation is only a good one when (a) the applied field Po is large (e.g., at Q-band frequencies) (b) the anisotropic hyperfine coupling is small (and so the field at the nucleus is still relatively small) and (c) the isotropic hyperfine coupling is large (since the field due to the electron at the nucleus reinforces the applied field). Often when electrons are confined to p or d orbitals, the... 

Unlike the isotropic hyperfine coupling constants, the anisotropic ones are mainly determined by dipole magnetic interactions. The contribution of the nearest atoms is expected to be suppressed by a distance factor. The values of AHC tensor are presented in Fig. 3. The results show that as in the previous case the tensor diagonal elements have the greatest values for the Ni atom varying from 70 to 150 MHz. But unlike to the previous situation the values for the nitrogen atoms are one order of magnitude lower as compared to the values for Ni atom. This is due to the fact that in this case the AHC constants are mainly... 

The value is very close to that estimated from the anisotropic hyperfine coupling of the nitrogen nuclei. There is, however, also a certain probability for the unpaired electron to be located in s-orbitals at the nitrogen and hydrogen atoms, giving rise to the isotropic hyperfine couplings on = 32.2 MHz, an = 30.8 MHz. With use of Table 1.1 one obtains ... 

This type of coupling is called isotropic hyperfine coupling, since it does not depend on the orientation of the unpaired electron spin with the external magnetic field.There is also an anisotropic hyperfine coupling that depends on the orientation of the electron spin and the external magnetic field, but it is much weaker than the isotropic hyperfine coupling and is not observed for freely moving radicals in solution. 

The MesSn" radical appears to couple to only two of the methyl groups. Moreover, the isotropic and anisotropic tin hyperfine coupling constants indicate that the Sn 5s and 5p orbital contributions are roughly 0.03 and 0.32, respectively (Table 6). Thus, compared... 

Several recent papers have reported Density Functional Theory (DFT) calculations on the primary oxidation and reduction products observed in irradiated single crystals of the common nucleobases thymine [53], cytosine [54], guanine [55], and adenine [56]. The theoretical calculations include estimates of spin densities and isotropic and anisotropic hyperfine couplings which can be compared with experimental results (obtained from detailed EPR/ENDOR experiments). 

Compound Isotropic and Anisotropic Hyperfine Coupling Constants (Mc/sec) ... 

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