Electron spin exchange

Related compounds with other transition metals have been studied only sparsely, e.g., with nickel(II) [198], cobalt(III) [174], and rhodium(lll) [199, 200]. A series of dimeric copper(ll) complexes [[Cu(L BF2)S][X] is also known and exhibits interesting magnetic effects associated with electron spin exchange between the copper(ll) ions [201]. 

R. E. Turner, and R. F. Snider, and D. G. Fleming, Phys. Rev., A41, 1505 (1990). Spin Relaxation of Hydrogen-Atom Isotopes via Electron Spin Exchange with Paramagnetic Gases. 

A few papers have dealt with the effects of Heisenberg electron spin exchange on low-field DNP behavior. Bates and Drozdoski s initial description of spin exchange effects on DNP was tested against data collected at 7 mT,31 and further work by Bates showed the changes in the DNP-detected ESR spectra at 5-9.5 mT as radical concentration was increased 92 Tregubenkov et al. presented the calculated effects of spin... 

Relaxation of a nucleus (x) in a paramagnetic compound is generally dominated by contributions from nuclear spin-electron spin magnetic dipole-dipole interaction and isotropic (scalar) nuclear-electron spin exchange. The relaxation effects of the two mechanisms are additive... 

The primary salt effect on rates of electron spin exchange in solutions of Fremy s salt is consistent with bimolecular reaction species with a charge product (za b) of + 4, which supports a mechanism involving collision between two [(S03)2N0] anions (za = Zr = — 2). ... 

Electron spin exchange is a bimolecular reaction in which the unpaired electrons of two free radicals exchange their spin states. At low concentration (ca. I0 mol/L) the usual isotropic spectrum is observed. As the radical concentration is increased the lines gradually broaden until at high concentration (> I0 mol/L) the lines coalesce to a single line. This single line sharpens further as the concentration is increased. 

The spectrum is said to be exchange narrowed since the electron spins are exchanging so fast that the time average of the hyperfine field is close to zero. Electron spin exchange must be avoided if narrow lines are desired in solution and this is readily achieved if the concentration is maintained below ca. lO" mol/L. 

Effect of dielectric constant on the rate of electron-spin exchange in solution. ... 

The presence of a paramagnetic centre in a molecule may affect the nuclear relaxation times or the chemical shifts or both. If one of the conditions [19], A or > A, is met, where is the electronic spin relaxation, Te is an electronic spin exchange time, and A the hyperfine coupling constant, then it should be possible to observe contact-shifted resonances. The contact shift in some paramagnetic molecules is related to the hyperfine coupling constant by the following equation ... 

Once a molecule (Fig. 11) has been added at the sample Node, the user can add atoms and subsequently bonds (see 2.14) through the Molecule menu or by a right mouse click on the Molecule Node. In this context, the term bond is loosely defined to include not only bonds but also interactions between electron spins and either nuelear spins (superhyperfine interaction) or other electron spins (exchange interaetion). Adding an atom displays a Periodic Table (Fig. 12) from which the user ean select an atom by clicking (left mouse button) on the appropriate element. The atom is then added to the Molecule in the Explorer. 

The saturation factor s is a measure of the degree to which the electron transitions are saturated. When the electron transitions are completely saturated, s takes a value of 1. It depends on the microwave power and the relaxation of the electrons. The saturation factor becomes significantly more complicated in radicals with several hyperfine lines (two for three for " N), such as nitroxide radicals, where various relaxation mechanisms need to be considered if only one line is saturated. Bates and Drozdoski proposed a theoretical model taking into account electron spin exchange transitions [50], which has recently been extended by Armstrong [51] to account for the relaxation effect arising from (and N) nuclei. Recently Tiirke and Bennati... 

It should be noted that long electron spin-lattice relaxation times (Ti 10" to 10 s) impede recording of the NMR spectra of radicals. Therefore lines in the NMR spectra of radicals become observable only at high rates of electron spin exchange. Spin exchange correlation time Tex is directly proportional to the viscosity rj and inversely proportional to the temperature and radical concentration [R] rj/T[R ]. Therefore,... 

Dynamic aspects of the intramolecular electron spin exchange... 

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