Relaxation mechanism interaction, types

Explicit expressions for Tivr were derived from models for the interactions of a proton with the local magnetic fields of its environment. Two types of environments which are important in hemoproteins will be discussed. First, if the water molecule is in a diamagnetic environment, dipole-dipole coupling between the two protons modulated by the anisotropic rotational tumbling of the molecule will be the dominant relaxation mechanism. The resulting relaxation rate is (Solomon (102) Abragam (/))... 

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. 

Type of Relaxation and Interaction Mechanism Concentration Dependence... 

Regardless of the types of molecular motion, relaxation occurs only if there is some specific interaction between the nucleus and its environment that can result in energy exchange. We now describe the six interactions that have been mentioned, beginning with the one that is always present—the interaction between nuclear magnetic dipoles. In the following treatment of each of these relaxation mechanisms, we shall focus on the relaxation rate R.x = 1/Tb as the overall relaxation rate is the sum of the rates produced by each mechanism. 

Double-resonance Experiments. - TROSY-type experiments have been traditionally based on the cross-correlation between dipolar and chemical shift anisotropy relaxation mechanisms. Tugarinov et al. extended the application of the relaxation compensation principles to cancellation of the intra-methyl H- H and dipole-dipole interactions. The analysis of the relaxation of the... 

Since quadrupole interaction constitutes such an effective relaxation mechanism it is only rarely that other types of interactions have to be considered for relaxation of covalent chlorine, bromine or iodine. However, it can be estimated that in paramagnetic molecules the interactions between the halogen nuclear spin and the unpaired electron briefly mentioned in sub-Section 1.3.2 may give significant contributions to the relaxation rate. For VCl modulation of the electron-... 

Cox [11] has discussed the relaxation mechanisms in this type of system in detail with experimental results and simulations to clearly demonstrate the various aspects. The most likely candidate that would account for the relaxation due to molecular dynamics is shown to be the fluctuation of the hyperfine interaction or the so called Fermi contact term. The hyperfine constant is in fact a thermal average over the different vibrational modes of the molecule. Therefore the molecular vibrations or librations will modulate the hyperfine constant. Hyperfine interaction is in general anisotropic, except in the gas or liquid state when the fast molecular tumbling averages out the anisotropy leaving the isotropic part. One can think of two separate mechanisms of relaxation depending on the modulation of either the isotropic part or the anisotropic part. 

The behavior of materials under dynamic load is of considerable importance and interest in most mechanical analyses of design problems where these loads exist. The complex workings of the dynamic behavior problem can best be appreciated by summarizing the range of interactions of dynamic loads that exist for all the different types of materials. Dynamic loads involve the interactions of creep and relaxation loads, vibratory and transient fatigue loads, low-velocity impacts measurable sometimes in milliseconds, high-velocity impacts measurable in microseconds, and hypervelocity impacts as summarized in Fig. 2-4. 

---