The Exchange Interaction

Wliile the earliest TR-CIDNP work focused on radical pairs, biradicals soon became a focus of study. Biradicals are of interest because the exchange interaction between the unpaired electrons is present tliroiighoiit the biradical lifetime and, consequently, the spin physics and chemical reactivity of biradicals are markedly different from radical pairs. Work by Morozova et al [28] on polymethylene biradicals is a fiirther example of how this method can be used to separate net and multiplet effects based on time scale [28]. Figure Bl.16.11 shows how the cyclic precursor, 2,12-dihydroxy-2,12-dimethylcyclododecanone, cleaves upon 308 mn irradiation to fonn an acyl-ketyl biradical, which will be referred to as the primary biradical since it is fonned directly from the cyclic precursor. The acyl-ketyl primary biradical decarbonylates rapidly k Q > 5 x... 

The advantages of INDO over CNDO involve situations where the spin state and other aspects of electron spin are particularly important. For example, in the diatomic molecule NH, the last two electrons go into a degenerate p-orbital centered solely on the Nitrogen. Two well-defined spectroscopic states, S" and D, result. Since the p-orbital is strictly one-center, CNDO results in these two states having exactly the same energy. The INDO method correctly makes the triplet state lower in energy in association with the exchange interaction included in INDO. 

I have developed a simple theory of these potential barriers, described in the following paragraphs. According to this theory, the potential barriers are not a property of the axial bond itself, but result from the exchange interactions of electrons involved in the other bonds (adjacent bonds) formed by each of the two atoms, as determined by the overlap between the parts of the adjacent bond orbitals that extend from each of the two atoms toward the other. 

Fig. 12b). Since practically the same spectral shape is obtained at Q-band (35 GHz) (Fig. 12c), the commonly used criterion stating that the shape of an interaction spectrum is frequency-dependent fails to apply in this case. Actually, outer lines arising from the exchange interaction are visible on the spectrum calculated at Q-band (Fig. 12c), but these lines would be hardly detectable in an experimental spectrum, because of their weak intensity and to the small signal-to-noise ratio inherent in Q-band experiments. In these circumstances, spectra recorded at higher frequency would be needed to allow detection and study of the spin-spin interactions. 

When an organic radical is located near a high-spin metal ion, the g-tensor of the radical depends on the exchange interaction between the radical and the metal ion. 

If the charge distributions of the D and A overlap than a new class of interactions has to be considered, namely the exchange interaction between the electrons on D and on A. This type of energy transfer is called Dexter transfer [80, 96,98], Here we briefly outline the physical principles involved. 

Gayda, J.-P., Bertrand, P., and Theodule, F.-X. 1982. Three-iron clusters in iron-sulfur proteins An EPR study of the exchange interactions. Journal of Chemical Physics 77 3387-3391. 

Although extended ID systems comprising ferromagnetically coupled gadolinium and copper ions were obtained [46], the exchange interactions were too weak to provide significant effects and the increase of the x l product was observed only at very low temperatures. A much richer scenario was instead... 

Figure 4.8 (a) Representation of a magnetic chain in its ordered state, (b) The creation of a domain of reversed spins costs four times the exchange interaction, assuming unitary length for the spins, (c) If the reverse domain... 

It is thus evident from these selected examples that the exchange interaction between spin carriers constituted solely by anisotropic lanthanide ions has to be drastically enhanced to permit the observation of SCM behaviour at relatively high temperatures. 

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