Flip-flop spin interaction

For the dilute aH spins surrounded by a large number of the 2H spins, the heteronuclear aH-H dipolar interaction is dominant, lifting the spectral overlap between the 1H packets. H spin diffusion is driven by the flip-flop term of the 1H-1H dipolar interaction, which becomes secular in the presence of spectral overlap. Thus, spin diffusion would be accelerated if it had not been for the 1H-2H dipolar interaction. In order to confirm this prediction, they used another RF channel of the OPENCORE... 

Unpaired electronic density can be delocalized onto the various nuclei of the complex via through-bond scalar hyperfine interactions involving occupied orbitals containing s-character (direct interaction or polarization according to the Fermi mechanism, Wertz and Bolton (1986)). Random electron relaxation thus produces a flip-flop mechanism which affects the nuclear spin and increases nuclear relaxation processes (Bertini and Luchinat, 1996). Since these interactions are isotropic, they do not depend on molecular tumbling and re is the only relevant correlation time for non-exchanging semi-rigid complexes. Moreover, only electronic spin can be delocalized via hyperfine interactions (no orbital contribution) and the contact re-... 

Electron spin-electron spin interaction. The transition betwen a and P spin states takes place by the interaction between the A spins and the surrounding off-resonant spins (called B spins). The most important process in this type of the relaxation is cross relaxation. In the cross relaxation, the excess energy of the A spin system is resonantly transferred to the surrounding B spins through a flip-flop process. The relaxation rate depends on either the distance betwen the A and B spins or the number of the B spins surrounding an A spin. It is this relaxation mechanism which provides us with a means for studying the local spatial distribution of radical species. 

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-... 

Serious complications arise, however, if the spins are subject to strong nuclear electric quadrupolar interactions, which tend to modify the echo amplitudes measured. In such cases, it is still possible to extract dipole-dipole coupling information from spin echo decay spectroscopy, if the 71-pulses are applied entirely selectively to the central l/2>o -l/2> transition [6]. If the resonance frequencies between the coupled nuclei are sufficiently similar to allow for spin-exchange via the flip-flop mechanism, Eq. (5) turns into... 

The decoupling efficiency depends on two factors [Engl, Mehl] (1) The amplitude (Oil = - y fii/ of the decoupling field in comparison with the strength of the heteronuclear dipole-dipole interaction. (2) The modulation of the heteronuclear dipole-dipole coupling by flip-flop transitions in the system of the abundant / spins, which communicate by the homonuclear dipole-dipole interaction. In addition to this, the influence of thermal motion has to be considered [Mehl]. 

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). 

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