Polarization transfer, spin exchange

Ishikawa et detected the free-induction signals of xenon atoms polarized by spin-exchange optical pumping. The temperature dependence of dissolution and spin-polarization transfer of xenon atoms in ethanol is measured by simultaneous detection of both xenon and ethanol protons. 

Properties of the selective pulses are used therefore twofold in such experiments. Firstly, a selective pulse selectively perturbs the selected spin and the perturbation is distributed in the course of the experiment among the coupled spins, depending on the type of coupling (scalar, dipolar) and depending on the type of exchange mechanism (polarization transfer, cross polarization or cross relaxation). Secondly, the phase (selective 90° pulse) or the frequency (selective 180° pulse) of the selective pulse serve to label the response of both the selected and the residual coupled spins as positive or negative. 

The temperature dependence of the spectral spin diffusion and crossrelaxation was examined by Mueller et a/.287,288 with spin- and spin-1 systems. They showed that the diffusion rate can be strongly temperature dependent if it is motionally driven. It is therefore, unreliable to discriminate spin diffusion and chemical exchange by variable-temperature measurement of 2D exchange spectra. Mueller et al. suggested that the dependence of the polarization transfer rate on the spectral difference of the relevant resonances should be measured in a single crystal to safely distinguish the two different polarization transfer processes (see also ref. 289). They also explained satisfactorily why the relaxation of the quadrupolar order is much faster than the Zeeman order. This... 

The diamagnetic molecular combination product r-N is invisible by ESR, but we can state that the radical r was in proximity of the stable nitroxyl N, because the TR ESR spectrum of was observed. Polarization transfer from one radical to another occurs by spin exchange." It is believed that spin exchange does not require physical contact of the reagents, and it can occur at the distance of one or several molecular diameters between radicals. 

In the case of a reaction of r with (N-O-N) (Scheme 12.12), two polarization transfers occur by spin exchange with an appearance of (N-O-N) and by addition of r to (N-O-N) with the formation of polarized monoradical see Scheme 12.14. 

When the primary acceptor is prereduced, electron spin polarization can transfer by exchange interaction from BPh to Q, leading to an inversion of the EPR line of in RCs where was magnetically uncoupled from Fe [112] (Fig. 8). From a phenomenological treatment [112-114] it was concluded that the exchange interaction /(BPh QA) was 3 - 5 G, whereas /(P BPh ) was between 1 and 5 G. A more sophisticated treatment of the three-spin system P BPh QX [115] led to 7(P BPh ) between 0 and +8 G. (Note that for / = 0 polarization may develop if D = 0.) A positive value of / for a biradical state is unusual it might be explained by some form of superexchange via an intermediate (possibly one of the accessory bacteriochlorophylls). 

Unlike correlation spectroscopy based on spin diffusion, the adiabatic version enables, in principle, almost full exchange of magnetization between the two spins. As a result, the entire signal intensity will reside in the cross-peaks. Violation of the adiabaticity is characterized by the appearance of a diagonal peak and can be expected to occur if the rotation sweep is too fast compared to the interaction between spins. While numerical simulations indicate possible linear dependencies of the polarization transfer coefficient on spin coupling and the rate of the sweep over a range of practical values, the validity of this assumption remains to be tested. Here we present a semi-quantitative example of a relayed polarization transfer process. 

S-spin zero-quantum line shape is sufficiently broadened to allow S-spin polarization transfer. The broadening of the zero-quantum line can also be caused by chemical exchange ( motionally-driven spin diffusion ) [15, 18]. In a different approach one decouples the protons and removes the chemical-... 

Coherence transfer by polarization transfer results in a population exchange for one of the I transitions, e.g. the transition between the ajps and Pips states, with a signal enhancement for spin S of 1 yj/ys 1. 

It occurs only between nearest neighbors and is not a spatial process. In contrast, multi-polar resonemce processes do involve transfer over relatively large distances through the lattice. We can compare qq-coupling with a spin-exchange process by using ruby, i.e.- AI2O3 Cr3+, as an... 

Before proceeding further with this section, we will emphasize some important points closely associated with the dynamic processes encountered in solid-state NMR. In the most general sense, relaxation, spin diffusion/exchange and polarization transfer can be regarded as different aspects of coherence transfer. They all involve mutual transformation among... 

A list of second-order contributions and corrections in the ab initio calculation of J is available. The spin polarization mechanism of exchange is different from the charge transfer mechanisms discussed so far and is perhaps best illustrated by the exchange between metals ions bridged by the azide ligand (Scheme 3). 

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