Mutual spin flip interactions

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... 

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... 

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. 

The fact that mutually different cross sectional patterns were observed in the SC-2D NMR spectrum where 7 was chosen to be 0 ms indicates that both inter- and intramolecular cross relaxation rates and spin flip-flop rates between interacting pairs of protons are relatively slow. This can be understood if one considers that dipolar interactions are partially averaged out by fast translational and rotational molecular motions in the liquid crystalline phase in contrast to the solid phase. 

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