Spinning separation processes

A major limitation of CW double resonance methods is the sensitivity of the intensities of the transitions to the relative rates of spin relaxation processes. For that reason the peak intensities often convey little quantitative information about the numbers of spins involved and, in extreme cases, may be undetectable. This limitation can be especially severe for liquid samples where several relaxation pathways may have about the same rates. The situation is somewhat better in solids, especially at low temperatures, where some pathways are effectively frozen out. Fortunately, fewer limitations occur when pulsed radio and microwave fields are employed. In that case one can better adapt the excitation and detection timing to the rates of relaxation that are intrinsic to the sample.50 There are now several versions of pulsed ENDOR and other double resonance methods. Some of these methods also make it possible to separate in the time domain overlapping transitions that have different relaxation behavior, thereby improving the resolution of the spectrum. 

Spin-spin relaxation dynamics were also used in the study of the kinetics of the vulcanization of polysulphide rubbers. The T2 values decrease with the course of the reaction and the time dependence of log (T2/T ), where corresponds to the time equal to zero, exhibits an inflection. The inflection point is attributed to gel formation, and the reaction rate constants for the two separate processes are determined from the T2 data. It was also observed that the addition of carbon black reduces T2 by a factor of 2 or 3, because vulcanization occurs both through the thiol groups and by the chemical reaction between the polymer and carbon black 37>. 

Madic, C., Bourges, J., Dozol, J.F. 1994. Brief overview of the long-lived radionuclide separation processes developed in France in connection with the SPIN program. International Conference on Accelerator-Driven Transmutation Technologies and Applications, July, Las Vegas, NV. 

As mentioned above, for finite forcing amplitudes one already has periodic A solutions in the range e < 0. But, when the system is quenched into the two-phase region with e > 0, where one may choose for reasons of simplicity e = 1, the spin-odal decomposition sets in and the late stage of the phase separation process depends on the forcing amplitude a. It is an interesting question, for which parameter combinations the systems ends up in a A solution that is locked to the periodicity of the external forcing, independent of the initial conditions before the quench. 

The indirect spin—spin coupling can provide a relaxation mechanism by two separate processes. As we saw in Chapter 7, J is a tensor, and its aniosotropy can cause relaxation as molecules tumble, just as with CSA. However, for most nuclei, values of J (and the resultant anisotropy) are generally much smaller than chemical... 

According to Equation (5.35), energy transfer by the exchange mechanism is a short-range phenomenon since the exchange term decreases exponentially with the donor-acceptor separation / ao- Since it requires the intervention of an encounter complex (D A) it is also called the overlap or collision mechanism. The Wigner-Witmer spin-selection rules (Section 5.4.1) apply, and the spin-allowed processes... 

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