Spin flip-flop

A spin-Nuclear spin Fluctuation of A spin quantum state by molecular motion and nuclear spin flip-flop No... 

Switching on the 13C RF transmitter is represented by opening the valve between the reservoirs H and 13C. The relative powers of the proton and 13C RF transmitters are adjusted to maximize interactions between the two types of precessing nuclei. Polarization can then be transferred between neighboring nuclei through spin flip-flop processes. Optimization is achieved when the Hartmann-Hahn condition is met, i.e., the H and 13C RF field strengths are in a ratio set close to 1 4 (Pines et al. 1973). Magnetization is then transferred with a time constant TCH-... 

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). 

Spin exchange among the protons in a sample can occur by one of two mechanisms (1) spin-spin flip-flops... 

Problem (2) is that the spin-lattice relaxation rate of C NMR, 77c. should obey (6.14) as H NMR 77,j does, if the neutral soliton diffuses whole the sample. To investigate a role of spin flip/flop diffusion through H and C Scott and Clarke have measured 77] and 77c ll samples enriched by various ratios of C to D ( ) 98 0 (2) 90 98 and (3) 20 98 [152]. They observed ... 

The Korringa relationship [21] indicates thatl/Ti oc T, where T is the spin—lattice relaxation time and T is the absolute temperature of the sample. This unique temperature dependence of l/Ti is the NMR fingerprint of a metallic state. It results from the fact that only conduction electrons around the Fermi level can satisfy energy conservation for the electron-nuclear spin flip-flop relaxation process, and the fraction of these electrons is proportional to bT. When all relaxation mechanisms other than the first term in Eq. (1) can be neglected, and there are only x-like electrons at the Fermi level (such as in the alkali metals), the Korringa relationship takes its simplest form ... 

Echo detection of selectively-burned holes in photosynthetic systems led to estimates of distances between 25 and 50 A. If the contributions to spectral diffusion from motion, nuclear spin flip-flops, and instantaneous diffusion are smaller than the contribution from dipolar interaction between unpaired electrons, the spectral diffusion can be used to determine the interspin distance. (97-100). 

A new method has been recently proposed for spin dynamic studies in conducting polymers [33]. It basically relies on the following. Collisions between moving spins (species A) and unlike spins (species B), instead of giving rise to a motional narrowing, can result in a broadening of the line. Such an effect can be observed in two cases. First, in the slow exchange case, i.e., when the spin flip-flop frequency Larmor frequencies of the two species < Soab = weA Wcb. Second, in the fast... 

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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