Mutual spin-flips

Exchange of magnetization due to cross-relaxation (NOE, nuclear Overhauser effect) does lead to intensity changes of individual resonances which provide valuable information about spatial and motional characteristics of the spins involved [4, 5]. It is currently mostly measured in two-dimensional NMR, where the NOE is measured as cross-peak intensity. Cross-relaxation is caused by mutual spin flips in dipolar coupled spin pairs. 

Exactly the same results were obtained with spin diffusion experiments performed at ZSM-39. Frequencies can be affected by spin diffusion between sites having different NMR parameters, when, for example, magnetization is transported through a solid by means of mutual spin "flip-flops" that can occur even in the absence of atomic or molecular motion. By monitoring the correlation among frequencies in the different dimensions of a multidimensional NMR experiment, it is possible to learn about the mechanisms and rates of reorientation and diffusion processes in solids (32). 

Npp — Np ) will decrease (negative NOE). The manner in which the NOE varies with motion is a result of variation of the transition probabilities Wq and W2 with motion. W2 describes the probability for changes in the z components of magnetization of A and B, simultaneously, a Ti process. Such transitions become increasingly probable when the spins are moving at frequencies close to the Larmor frequency, as described above. Wq concerns the probability of mutual spin flips, a Tj process. As described above, this process becomes increasingly probable as molecular motions become slower and... 

These components will oscillate as a function of t]. The resultant Z-magnetizations will be in a nonequilibrium state and spin-lattice relaxation processes will work toward equilibrium. For the case of protons relaxation is primarily dipolar via other protons, this non-equilibrium magnetization will then be redistributed by mutual spin flip to other protons. The final 90° pulse monitors the extent of magnetization transfer. The 2D experiment samples all degrees of magnetization transfer as it increments t]. ... 

The dipolar relaxation mechanism is unique because the dipolar coupling interaction contains two spin terms involving mutual spin flips - zero quantum t J. J. t and double quantum J J < j j transitions. All other relaxation processes are limited to single spin interchange. While the decoupler is on, the spin population differences between the levels irradiated are equalized as the rate of energy input from the decoupler greatly exceeds the outflow by relaxation. In dipolar coupled systems, the availability of double and zero quantum relaxation pathways (cross-relaxation) produces non-Boltzmann spin populations in the energy levels of the observed nucleus. These perturbed populations are measured as the nuclear Overhauser effect (nOe) effect rj). The relationship is... 

Figure 5.5(c)), resulting in mutual spin flips. CP can be thought of as a flow of polarization from the abundant spins to the rare spins. 

In a conventional optical pumping experiment in solids, an intense pumping beam perturbs the net populations of various atomic or molecular energy levels in a sample, which are then monitored to provide information about the relaxation processes that cause the system to return to its equilibrium state. While this technique is sensitive to the net level populations of the atoms, it is relatively insensitive to processes which preserve net populations while redistributing them spatially through the sample volume, for example by mutual spin flip interactions or reabsorption... 

The diffusion constant due to mutual spin flip interactions of the ions is of order... 

RF channels, respectively. This means that the RF fields have to be applied simultaneously in such a way that the nutation frequencies of both nuclei around the respective RF fields are the same. As a result, energy exchange between dipolar conpled H and nuclei occurs through a mutual spin flip mechanism [2],... 

Thus, even when the interacting nuclei have very similar gyromagnetic ratios, the homonuclear second moment is larger by a factor of than the heteronu-clear moment. This is because dipolar coupling between unlike spins cannot lead to an energy conserving mutual spin flip. The second moment is thus very sensitive to the kind of neighbour. 

Nuclear cross relaxation in liquids is caused by mutual spin flips in pairs of dipolar-coupled spins. 

When a strong decoupling field is applied in the solid state, there are rapid transitions or mutual spin flips that occur at a rate approaching that of the inverse of the line width of the proton resonance. The time constant for this process is Ti (the spin-spin relaxation time), which ranges from 10 to 100 p,s for rigid solids [12]. 

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