Biradicals exchange coupling

Forbes M D E, Closs G L, Calle P and Gautam P 1993 The temperature dependence of the exchange coupling in polymethylene biradicals. Conclusions regarding the mechanism of the coupling J. Phys. Chem. 97 3384-9... 

Forbes M D E 1993 The effect of localized unsaturation on the scalar exchange coupling in flexible biradicals J. Phys. Chem. 97 3390-5... 

The spin Hamiltonian for a biradical consists of terms representing the electron Zeeman interaction, the exchange coupling of the two electron spins, and hyperfine interaction of each electron with the nuclear spins. We assume that there are two equivalent nuclei, each strongly coupled to one electron and essentially uncoupled to the other. The spin Hamiltonian is ... 

Figure 6.1 Stick spectra for a dinitroxide biradical with a= 13 G for various values of the exchange coupling constant J. (Several very small, widely spaced resonances have been omitted for J=2A and J=5A.)...

Figure 4 Comparison of the EPR spectra of the biradical Q Q a in R. sphaeroides at 325 GHz (solid line) with that at 94 GHz (dashed line), both measured at 80 K. Arrows indicate lines that are observed at 326 GHz but are absent in the 94 GHz spectrum. They correspond to transitions with small transition moments in the 4-level scheme of the RP (cf. references 162,163). These lines enable a direct determination of the exchange coupling J in the biradical at 326 GHz.

Figure 16.8 Pake pattern of a biradical with dipolar and exchange coupling. The edges and peaks do not behave like 1 2 (taken from Margraf ft al., 2009).

The exchange coupling can in certain cases be determined by an analysis of the hyperfine pattern due to nitrogen observed for instance in nitroxide biradicals in liquid solution. A procedure to extract the value of J was developed many years ago [43]. This procedure does not depend on the microwave frequency and measurements were usually made at X-band. Three cases may appear, (1) / [Pg.195]

Exchange coupling constants in organic biradicals, transition metal dimers or complexes with organic radicals bound to transition metal, depend, oftoi heavily, on the geometry of the systems. In turn, the geometry will depend up on the nature of the ligands and/or on the environment. A classical example of environment... 

The wanted information is usually the distribution of distances P(f). In other words, the distribution of orientations P(0) has to be factored out of the data. " - This is possible if exchange coupling J is neghgible and P(0) is known. Usually, a uniform distribution of spin-spin vector orientation with respect to magnetic field, P(0) = sin(0), is a fairly good approximation at X band, as we have found even for rigid biradicals. Some deviations can be seen in frequency domain, but with proper data processing, as explained below, they do not cause artifacts in the distance distribution P f). 

Kokorin, A. I. (2004). Regularities of the spin-exchange coupling through a bridge in nitroxide biradicals. Appl. Magn. Reson., Vol. 26, No. 1-2, 253-274, ISSN 0937-9347... 

Magnetic susceptibilities of 10a and 10b were measured on a SQUID suscep-tometer in microcrystalline form. %T-T plots are shown in Fig. 9.5. The data were analyzed in terms of a modified singlet-triplet two-spin model (the Blea-ney-Bowers-type), in which two spins (S = V2) couple antiferromagnetically within a biradical molecule by exchange interaction J. The best-fit parameters obtained by means of a least-squares method were 2J/kB = -2.2 + 0.04 K for 10a and -11.6 + 0.4 K for 10b. Although the interaction (2J/kB = -2.2 K) between the two spins in the open-ring isomer 10a was weak, the spins of 10b showed a remarkable antiferromagnetic interaction (2J/kB = -11.6 K). 

Although the switching of exchange interaction was detected by a susceptibility measurement of biradical 10, both open- and closed-ring isomers 10a and 10b had nine-line ESR spectra because the exchange interaction between the two radicals was much stronger than the hyperfme coupling constant in both isomers. For one... 

As described earlier, when two nitronyl nitroxides are magnetically coupled via an exchange interaction, the biradical gives a nine-line ESR spectrum. If the exchange interaction is weaker than the hyperfine coupling, the two nitroxide radicals are magnetically independent and give a five-line spectrum. In intermediate situations, the spectrum becomes complex. 

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