Electron spin-lattice relaxation time

Electron longitudinal (spin-lattice) relaxation time Proton longitudinal (spin-lattice) relaxation time Electron transverse (spin-spin) relaxation time Characteristic temperature Glass transition temperature (K)... 

However, there is no indication that the presence of the observed signals correlates with the polymerization efficiency of the catalyst. In fact, systems which exhibit these signals are less effective catalysts and in some cases do not even polymerize ethylene under the chosen conditions. In contrast, systems without EPR signals correlated to Ti species are foimd to be catalytically active. It has to be emphasized at this point that the lack of an ESR signal associated to Ti + ions, in cases where no additional argon or electron bombardment has been applied, cannot be interpreted as an indication of the absence of Ti + centers at the surface. It has been discussed in the literature that small spin-lattice-relaxation times, dipole coupling, and super exchange may leave a very small fraction of Ti " that is detectable in an EPR experiment [115,116]. From a combination of XPS and EPR results it unhkely that Ti " centers play an important role in the catalytic activity of the catalysts. 

The saturation behavior of a spectrum - the variation of integrated intensity with microwave power - is related to the spin-lattice relaxation time, a measure of the rate of energy transfer between the electron spin and its surroundings. Saturation often depends on the same structural and dynamic properties as line widths. 

Gayda, J.-P., Bertrand, P., Deville, A., More, C., Roger, G., Gibson, J.F., and Cammack, R. 1979. Temperature dependence of the electronic spin-lattic relaxation time in a 2-iron-2-sulfur protein. Biochimica et Biophysica Acta 581 15-26. 

Furthermore, the method of orientation selection can only be applied to systems with an electron spin-spin cross relaxation time Tx much larger than the electron spin-lattice relaxation time Tle77. In this case, energy exchange between the spin packets of the polycrystalline EPR spectrum by spin-spin interaction cannot take place. If on the other hand Tx < Tle, the spin packets are coupled by cross relaxation, and a powder-like ENDOR signal will be observed77. Since T 1 is normally the dominant relaxation rate in transition metal complexes, the orientation selection technique could widely be applied in polycrystalline and frozen solution samples of such systems (Sect. 6). 

Fig. 13. Predicted magnetic field dependence of the electron spin lattice relaxation time. Solid line pseudorotation model dashed line spin dynamics calculation. Reproduced with permission from Odelius, M. Ribbing, C. Kowalewski, J. J. Chem. Phys. 1996,104, 3181-3188. Copyright 1996 American Institute of Physics.

Three parameters are readily obtainable from FiMR spectra which may be useful in studying binding interactions the chemical shift [jS], the linewidth (Av) or the apparent or effective spin-spin relaxation time (T2 ), and the spin-lattice relaxation time (Ti). C chemical shifts can reflect steric strain and change in the electronic environment within a molecule when it hinds to another species. Spin-lattice and spin-spin relaxation times can yield information on the lifetimes, sizes and conformations of molecular complexes. 

Electron spin resonance (ESR) measures the absorption spectra associated with the energy states produced from the ground state by interaction with the magnetic field. This review deals with the theory of these states, their description by a spin Hamiltonian and the transitions between these states induced by electromagnetic radiation. The dynamics of these transitions (spin-lattice relaxation times, etc.) are not considered. Also omitted are discussions of other methods of measuring spin Hamiltonian parameters such as nuclear magnetic resonance (NMR) and electron nuclear double resonance (ENDOR), although results obtained by these methods are included in Sec. VI. 

In these equations, the symbols have their customary meanings (see Toth et al. in this volume for an excellent review of the topic), and the correlation times given in Eq. (3) have the following typical values at 50 MHz in water Tle (electron spin-lattice relaxation time) =10 ns, T2e (electron spin-spin relaxation time) = 1 ns, rm (inner sphere water exchange correlation time) = 130 ns [3], and rR = 60 ps. These values, in the context of Eq. (1 - 3), show why rotational dynamics control relaxivity for such chelates. 

Fig. 21 Temperature dependence of the electron spin-lattice relaxation time Tle in powdered...

The temperature dependence of the electronic spin memory time was determined in powdered TDAE-C60 [106]. The memory time Tm is nearly temperature independent above Tc and is of the order of 30 ns. Below Tc it increases monotonically on cooling and reaches 92 ns at 4.2 K. No critical anomaly has been observed at Tc. The spin-lattice relaxation time (Fig. 21) was found to coincide with Tm-... 

S nuclear quadrupole coupling constants have been determined from line width values in some 3- and 4-substituted sodium benzenesulphonates33 63 and in 2-substituted sodium ethanesulphonates.35 Reasonably, in sulphonates R — SO3, (i) t] is near zero due to the tetrahedral symmetry of the electronic distribution at the 33S nucleus, and (ii) qzz is the component of the electric field gradient along the C-S axis. In the benzenesulphonate anion, the correlation time has been obtained from 13C spin-lattice relaxation time and NOE measurements. In substituted benzenesulphonates, it has been obtained by the Debye-Stokes-Einstein relationship, corrected by an empirically determined microviscosity factor. In 2-substituted ethanesulphonates, the molecular correlation time of the sphere having a volume equal to the molecular volume has been considered. 

Figure 4a. Electronic part Tle, dipole-dipole part Tld and the total spin-lattice relaxation times T. assuming disordered configuration of H0-atoms at all temperatures.

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