Isotropic coupling constant

From the isotropic coupling constant one may calculate C12, the fractional occupancy of the nitrogen s orbital, which is equal to AUo/Ao. The term Ao is the Fermi contact interaction for an unpaired electron in a pure nitrogen 2s orbital. For NO2 the value of ci2 = 56.5/550 = 0.103. The fraction of the unpaired electron associated with the N nucleus is then... 

The EPR spectra have always been interpreted2994 using an effective S = 2 spin Hamiltonian including the Zeeman term, /iBB g-S, and the hyperfine term, ICa-A-S, which describes the interaction of the unpaired electrons with the copper nucleus (7Cu = I). The spectra are very sensitive to the ratio between the isotropic coupling constant J and the local zero field splitting of nickel(II), Z)Ni.2982 In the limit J DNi it can easily be shown that the following relations hold ... 

Beveridge and Miller B1> in an INDO study, have calculated the vibronic effects in substituted methyl redicals and have satisfactorily reproduced the trends of isotope effects on isotropic coupling constants (Table 37). 

The only atomic wave-functions that do not have a node at the nucleus are s-functions. The isotropic coupling constant is thus a measure of the s-character of the wave-function of the unpaired electron at the nucleus in question. The coupling constant for an atomic s-electron can be either measured experimentally or calculated from Hartree-Fock atomic wave-functions so that, to a first approximation, the s-electron density may be calculated from the ratio of the experimental and atomic coupling constants. Should the first-order s-character of the wave-function of the unpaired electron be zero, as for example in the planar methyl radical, then a small isotropic coupling usually arises from second-order spin-polarization effects. The ESR spectra of solutions show only isotropic hyperfine coupling. 

Significant changes are also observed for the proton and halogen constants. Thus, the 11 isotropic coupling constant which is negative in the planar methyl radical (spin polarization, Aiso = -23 G), becomes positive when protons are substituted by fluorines133 (Table 15) and a monotonic increase in the 19F isotropic coupling constant is also caused by the... 

TABLE 15. Isotropic coupling constants (gauss) for some H3 wX C and Me3 X C radicals... 

The isotropic coupling constants for some cyclopropyl radicals containing halogen atoms are given in Table 17. 

Irradiation at 4 K gives a silver species designated as Ag°(B) in all of the hydrated A-zeolites studied. This particular silver species is characterized by a silver isotropic hyperfine coupling of about 1480 MHz and a g-factor of 1.991. For comparison the isotropic coupling constant of the silver atom in the gas phase is 1979 MHz, so species Ag°(B) shows considerable interaction with its environment. If the A-zeolites are dehydrated by heating to 400° C no silver species are formed by irradiation. Presumably the silver atoms that are formed are more mobile in the absence of water and form dimers or multimers. In addition, more prominent spectra of Ag2+ are seen upon irradiation of the dehydrated A-zeolites. Thus it seems that species Ag°(B) is probably associated with some adjacent water molecules. 

The fractional occupancy of the nitrogen s orbital, may be determined from the isotropic coupling constant, which is equal to (where... 

The large isotropic component is due to the unpaired electron spin density in the carbon 2s orbital, and this value (544MHz) can be used to derive an estimate of the carbon 2 s orbital contribution to the molecular orbital. Since the theoretical isotropic coupling constant for is 3777MHz, then C2s = 544/3777 = 0.144. The anisotropic dipolar part of the hyperfine arises from unpaired spin density in the 2p orbital. However because the dipolar contribution in Equation 1.52 cannot be reduced to zero, this implies that a fraction of the spin density is allocated to the 2p orbital perpendicular to the molecular plane. Therefore, the dipolar component of Equation 1.52 must be further decomposed into two symmetrical tensors oriented along the z and x axes ... 

To determine the shape of the hydrophobic barrier of bilayer membranes, fatty acids and PC molecules spin labeled with nitroxides at various positions along the lipid chains were diffused into vesicles and their solvent-sensitive isotropic coupling constants were measured [54]. Results are plotted in Figure 5 in terms of distance of the probe from the bilayer center. Also shown is the profile of the dielectric constant along the membrane normal evaluated from the fluorescence lifetime distribution of fluorescence probes in PC liposomes [55]. These data correlate well with results from neutron diffraction studies that map the positional distribution of water and lipid moieties along the bilayer normal [56]. 

Results for NH2 (21) are in accord with expectation, although only the isotropic coupling constants are known. Thus the isotropic coupling to nitrogen is close to that for N02 and that to the protons is similar to the coupling in NH3+, indicating that the structure is such that the unpaired electron is still largely in the p, level. 

TABLE 3. or As isotropic coupling constants for some phosphinyl and arsinyl radicals... 

Hyperfine interaction with j3-protons is mainly isotropic. The isotropic coupling constant takes the form... 

Isotropic Coupling Constants of the Histidine Proton Signals in the ENDOR Spectra of Bovine SOD ... 

Isotropic coupling constants are in agreement with those obtained from the NMR contact shifts within experimental error except for three coupling constants, for which the corresponding values from the NMR contact shifts are given in parentheses for comparison. 

In addition to this possibility of determining isotropic /-coupling constants, the -echo concept has been extended to address two further aspects of general interest in the context of 2 experiments experimental determination of zero-quantum relaxation characteristics, and excitation of doublequantum coherence through dipolar coupling. ... 

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