Electron spin resonance hyperfine interactions

Electron Spin Resonance Data for Hyperfine Interaction... 

Electron spin resonance, nuclear magnetic resonance, and neutron diffraction methods allow a quantitative determination of the degree of covalence. The reasonance methods utilize the hyperfine interaction between the spin of the transferred electrons and the nuclear spin of the ligands (Stevens, 1953), whereas the neutron diffraction methods use the reduction of spin of the metallic ion as well as the expansion of the form factor [Hubbard and Marshall, 1965). The Mossbauer isomer shift which depends on the total electron density of the nucleus (Walker et al., 1961 Danon, 1966) can be used in the case of Fe. It will be particularly influenced by transfer to the empty 4 s orbitals, but transfer to 3 d orbitals will indirectly influence the 1 s, 2 s, and 3 s electron density at the nucleus. 

Electron spin resonance spectra of coals usually consist of a single line with no resolvable fine structure however, the electron nuclear double resonance (ENDOR) technique can show hyperfine interactions not easily observable in conventional electron spin resonance spectra. Recently, this technique has been applied to coal, and it is claimed that the very observation of an ENDOR signal shows interaction between the electron and nearby protons and that the results indicate that the interacting protons are twice removed from the aromatic rings on which, it is assumed, the unpaired electron is stabilized. 

The electron spin resonance (E.S.R.) spectra of a paramagnetic organic molecule, e.g. free radical, radical cation or radical anion, is directly related to its unpaired electron distribution (spin density). In the region of a magnetic nucleus the hyperfine interaction between the magnetic moments of the nucleus and the electron is a function of the spin density. It has been shown that, for an atom N, a direct correlation exists between its observed hyperfine coupling constant, and [pa—pP), the unpaired electron population of its atomic orbitals 1). 

Fig. 3. First derivative electron spin resonance spectra. (A) ESR spectrum of an unpaired electron. (B) ESR spectrum of an unpaired electron interacting with a nitroxide resulting in a nitrogen hyperfine coupling constant aN. (C) ESR spectrum of an unpaired electron interacting with a H nucleus and a l4N nucleus as is typical for PBN radical adducts. (D) ESR spectrum of an unpaired electron interacting with the l3C nucleus, the H nucleus and the 14N nucleus of the trichloromethyl radical adduct of PBN, where the carbon tetrachloride was labeled with 13C.

The above form of the spin Hamiltonian is common in electron spin resonance. In magnetochemistry the hyperfine interaction is neglected and the temperature-independent paramagnetic term can be omitted (this can be included in the empirical correction of experimental data, together with the diamagnetic term). 

Electron Spin. - With electron spin resonance (ESR) experiments one studies the spin properties of the electrons. The hyperfine coupling constants describe the interactions between the total spin of the electrons and those of the nuclei and they are only then non-vanishing when none of the two spins vanishes. This means that most often such systems have an odd number of electrons, although deviations from this exist. These coupling constants depend essentially on the difference in the spin-up and spin-down electron densities at... 

Making use of electron-spin resonance in the radical-ion salts, in particular the diffusion constant D and the hyperfine interaction constant A of the electron spins can be measured. One thus obtains independent information on the dynamics and the spatial distribution of the conduction electrons within the stacks. 

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