Free radicals magnetic interactions

The electron spin resonance spectrum of a free radical or coordination complex with one unpaired electron is the simplest of all forms of spectroscopy. The degeneracy of the electron spin states characterized by the quantum number, ms = 1/2, is lifted by the application of a magnetic field, and transitions between the spin levels are induced by radiation of the appropriate frequency (Figure 1.1). If unpaired electrons in radicals were indistinguishable from free electrons, the only information content of an ESR spectrum would be the integrated intensity, proportional to the radical concentration. Fortunately, an unpaired electron interacts with its environment, and the details of ESR spectra depend on the nature of those interactions. The arrow in Figure 1.1 shows the transitions induced by 0.315 cm-1 radiation. 

Two kinds of environmental interactions are commonly important in the ESR spectrum of a free radical (i) To the extent that the unpaired electron has residual, or unquenched, orbital angular momentum, the total magnetic moment is different from the spin-only moment (either larger or smaller,... 

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). 

Interactions with an applied magnetic field are particularly important for open shell free radicals, many with 2 n ground states having been studied by magnetic resonance methods. The Zeeman Hamiltonian may be written as the sum of four terms ... 

A free radical in a typical ESR spectrometer s magnetic field will result in the unpaired electron occupying one of two energy states (Fig. 2). These states occur due to the interaction of the free radical s unpaired electron with the magnetic field, and thus exist only when the sample experiences... 

This chapter will show that only atoms with partially filled shells (i.e. atoms with unpaired electrons) can possess a net magnetic moment in the absence of an external field. Since main group p block) elements have atoms with filled d subshells and tend to form compounds with other p-block elements that result in filled p subshells in accordance with the octet rule, the vast majority of magnetic materials have historically contained transition metal atoms with partially filled d subshells. Nevertheless, some pure organic compounds with free radicals have been found to exhibit ferromagnetic intermolecular interactions, albeit at very low temperamres (several Kelvins). 

Lescop, C., Belorizky, E., Luneau, D. et al. (2002) Synthesis, stmctures, and magnetic properties of a series of lanthanum(III) and gadolinium(lll) complexes with chelating benzimidazole-substituted nitronyl nitroxide free radicals. Evidence for antiferromagnetic Gd - radical interactions. Inorganic Chemistry, 41, 3375-3384. 

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