Hydrogen hyperfine splitting constants

It can be noticed there are two types of hydrogen atoms atoms 1-5 lying in the molecular plane and atoms 6 and 7 lying out of the plane. Thus, the hyperfine splitting constants (an) cannot be interpreted by means of one relationship, but the expressions... 

The reported ESR spectra of polyisobutylene, irradiated with high energy radiation and measured at low temperatures (31, 35, 47, 48, 49, 52) are similar in form, showing a broad doublet with a hyperfine splitting constant of 20 gauss. These spectra have been interpreted as caused by radicals (XIII) formed by hydrogen abstraction from the main chain methylene groups ... 

The hyperfine splitting constants for all protons of the two benzo[c/ / ]bis-l,3-dioxoles (3) and (4) and benzo[d experimental splitting constants of the hydrogen atoms were explained by applying perturbation theory to interaction between the aromatic and heterocyclic ring n MOs <82ZN(A)680>. 

The unusually large (22.49 G) bridgehead proton hyperfine splitting constant for the radical (53) is accounted for (INDO calculations) by through-space as well as through-bond spin-transfer from C-1 to the hydrogen atom at The dicyclopentadien-... 

Fig. 5a-c. Effect of exchange rate on spectrum for two equivalent sets of two hydrogens with hyperfine splitting constants a, and a (a) slow, (b) intermediate, and (c) fast exchange rates... 

In a u radical system, the amount of unpaired down-spin density (pn) ori a hydrogen atom, and hence the hyperfine splitting constant (oh) of the hydrogen atom in a ESR spectrum of the compound, is proportional to the amount of unpaired up-spin density (pc) the carbon atom to which the hydrogen atom is attached [30]. In other words. 

For instance, nitration of naphthalene, azulene, biphenylene, and triphenylene proceeds preferentially in positions with the greatest constant of hyperfine splitting at the hydrogen atom in ESR spectra of 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 at classical mechanism of nitration in cases of naphthalene, azulene, and biphenylene, but not triphenylene (see Todres 1985). 

This discussion of the hyperfine splitting of the hydrogen isotopes was still affected by some uncertainty originating in part from the inaccuracy in the value of the electrodynamic coupling constant a. 

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