Hyperfine splitting constant, electron interactions

The hyperfine splitting constant ay has units of energy and is a characteristic parameter for the interaction of the unpaired electron with a nucleus of type /. It is the analog of the NMR J spin-spin constant describing the magnetic coupling between two nuclei. 

Figure 3 Solution spectrum of a typical nitroxide radical where the unpaired electron is mainly interacting (i.e., hyperfine interaction) with the nitrogen nucleus. Since nitrogen is predominantly the isotope, the line is equally split (hyperfine splitting constant) into three (ra/ = - -1, 0, — 1) lines...

When hydroxyl radicals are generated in the flow cell in the presence of diallyl-malonic acid (R = H in 5-1), the ESR spectrum shown in Fig. 35 was recorded, which consists of a doublet of triplets the hyperfine splitting constants for the doublet and the triplets are 22.0 and 24.5 G, respectively. This multiplicity requires that the radical formed has a structure in which the unpaired electron interacts with three protons, two of which are equivalent. Of the following possible species (5-lV, -V, -VI, -VII, and -VIII), which can be formed initially, only two (3-V and -VII) fulfil this requirement. 

Eight lines of this structure derives from the contact interaction of the unpaired electron with the nuclear spin = 7/2 of one cobalt ion Co. These spectra are the evidence that the density of the unpaired electron is located mainly on the oxygen molecule as supported by the low value of the hyperfine splitting constant... 

If the unpaired electron is a tt electron, HMO theory can be applied to the value of the ESR splitting constant. The interaction between an unpaired n electron and a proton (or other nucleus with nonzero nuclear spin) falls off rapidly with increasing separation. Therefore, the hyperfine structure is generally ascribed to interactions involving protons directly bonded to carbons in the tt system a protons) or else separated from the n system by two a bonds protons). The equilibrium position of an a proton is in the nodal plane of the it system, so it is clear that any net spin density at the proton must be only indirectly due to the presence of an unpaired it electron. This indirect effect arises because the unpaired it electron interacts slightly differently with a- and P-spin a electrons on carbon, and so the spatial distributions of these become slightly different in the a MOs, ultimately producing net spin density at the proton the a electrons are said to be spin polarized by the it electron (see Fig. 8-17). 

This EPR spectrum shows a considerable broadening of linewidth when comparing with an EPR spectrum of free TAM nitric oxide. An EPR characterization of nitroxyl radicals is based on the measurement of the signal intensity of an EPR spectrum. This measurement can provide fundamental information of free radicals, such as linewidth that bears a relationship to the tumbling motion of free radicals, g-value which largely depends on the immediate environments of the free radicals, and hyperfine splitting constants which describe the classical multiplicity of EPR spectrum due to the interaction of the unpaired electron spins with nuclear spins. 

In principle, the polarity profile could be determined by measuring the three-hyperfine splitting constants T, Tyy and for the -N -0 group at various positions along the lipid chain. In practice, none of these parameters can be measured directly because of the partial motion average of the electron-nuclear dipolar interaction. An indirect method of obtaining the polarity profile is to estimate and Tj with the spin labels in randomly oriented samples. The -N -0 group at the C-5 position in the lipid matrix has been reported to be in an environment that is more polar than the C-12 and C-16 positions which are in a more hydrocarbon-like environment (Scheme 1). 

Figure 2.98 The hyperfine splitting patterns resulting from the interaction of an electron spin with two protons with (a) different hyperfine coupling constants and (b) the same hyperfine...

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