Hyperfme splitting

Reductant Radical trapped by MNP Hyperfme splitting constant (mT) a N a ... 

Table 6 Hyperfme Splitting Constants of CAN-reductant- PBN Systems [38] ...

Table 3 presents the hyperfme splitting constants of some sulfur-containing aromatic radical anions. The series studied included the monoxides and dioxides of dibenzothio-phene 1, thioxanthene 2, thioxanthone 3, dibenzo[b,/] thiepin 4 and dithienothiophene dioxide 5. 

Table 3. Spin densities (p) and calculated and experimental hyperfme splittings (lo l in G)...

The ESR spectra of a large variety of sulfonyl radicals have been obtained photolytically in liquid phase over a wide range of temperature. Some selected data are summarized in Table 2. The magnitudes of hyperfme splittings and the observations of line broadening resulting from restricted rotation about the C—S bond have been used successfully in conjunction with INDO SCF MO calculations to elucidate both structure and conformational properties. Thus the spin distribution in these species is typical of er-radicals with a pyramidal center at sulfur and in accord with the solid-state ESR data. 

For instance, nitration of naphthalene, azulene, biphenylene, and triphenylene proceeds preferentially in positions with the greatest constant of hyperfme splitting at the hydrogen atom in ESR spectra of the corresponding cation radicals. The constant is known to be proportional to the spin density on the carbon atom bearing the mentioned hydrogen. It is important, however, that the same orientation is also observed in the classical mechanism of nitration in the cases of naphthalene, azulene, and biphenylene but not of triphenylene (see Todres 1985). 

Figure 4.19 The partial-wave singlet (full curves) and triplet (broken curves) absorption cross sections in e+ + H(1s) collisions, plotted versus the incident positron energy measured from the threshold energy for positronium formation. Results of hyperspherical closecoupling calculations including the absorption potential —iVabs in the Hamiltonian. Note that the thresholds Etu for the full and broken curves are different by 0.841 meV, the hyperfme splitting. Figure from Ref. [16].

The dominant line-broading mechanism of Co-deuterolysin was unresolved hyperfme splitting of cobalt nucleus and g-strain. The line width from a g-strain mechanism increases with increasing microwave frequency, but the contribution from hyperfme splitting is independent of the microwave frequency. 

For the molecular case, the essential conclusion is that the orbital must have some s (or cr) character for the impaired electron to interact with a magnetic nucleus. Consider however the case of the benzene radical anion, in which the electron is usually described as being in a tt orbital with a node in the molecular plane. As a consequence no coupling with the proton nuclei is expected, a prediction clearly in conflict with the hyperfme splitting of 3.75 gauss seen in the ESR spectrum of this species as shown in Fig. 2. Flow, then, does the unpaired tt electron density appear at the Ft nucleus ... 

Molecular-orbital calculations have also been made for the methyl- and t-butyl-substituted benzosemiquinones, and the resultant spin densities are listed in Table 3. In these cases, the transfer of unpaired spin density from the tt system to the proton is explained in terms of hyperconjugation. Use these theoretical results as an aid in assigning your experimental hyperfme splitting constants to specific protons. Can you rationalize the charge distributions in these species and the changes from the distributions in the parent benzosemiquinones ... 

Table VII. Geometrical parameters (A and degrees) and isotropic hyperfme splittings (hfs, G) for HjNO, computed by different methods.

The hyperfme splitting arises from H nuclear hyperfmeinteraction. The coupling scheme is J = N + S,F=J+ /h- >) Authors estimate of the experimental uncertainty, in units of the last quoted decimal place. 

Additional hyperfme splitting observed at selected orientations and attributed to three nuclei of the aromatic ring. ) Assigned to H-addition radical at position C(4) this assignment has been questioned by [83Thel]. 

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