Spin-lattice relaxation mechanisms

The minimum in the spin-lattice relaxation time is more difficult to account for. It cannot be attributed to the onset of the diffusional motion, because the jump frequency does not match the Larmor frequency at the temperature where diffusion becomes important. For this reason it is necessary to postulate an additional kind of motion in the lithium-vanadium bronze—a side-to-side jumping from one side of the channel to the other. In the structure there are sites on both sides of the channel roughly 2 A. apart which are equivalent but only one of which is occupied to fulfill stoichiometry. This kind of motion should start at a lower temperature than the above diffusion and lead to a correlation frequency that matches the Larmor frequency at the spin-lattice time minimum. Because of modulation of quadrupolar interaction, side-to-side motion could provide an effective spin-lattice relaxation mechanism. 

More recently Tarasov et al. provided unambiguous evidence for the superposition of scalar and quadrupolar spin-lattice relaxation mechanisms in the case of Al in the corresponding mixed tetrahalogenoaluminates AlCl Br4 . (244) From the temperature dependence of Ti( Al) in AICl4 and AlBr the scalar contributions to the Al relaxation rates are determined, providing the following estimates for the respective scalar coupling constants J( Al- Cl) 650 Hz and J( Al- Br) = 750 Hz. 

The four spin-lattice relaxation mechanisms that are usually considered for nuclei of spin are (i) dipole-dipole interactions (DD) (ii) spin-rotation interactions (SR) (iii) scalar coupling (SC) and (iv) chemical screening anisotropy (CSA). (5) In Si NMR SC is not generally considered, except in the rare cases when Si is spin-coupled to I or other nuclei with resonance frequencies close to that of Si. Table XXX compares the contribution of the three remaining mechanisms with Si relaxation times. Definitions of these relaxation mechanisms can be found in ref. 229. Since a theoretical discussion of Si relaxation and NOE is detailed elsewhere [(5) and references therein] only general observations are made in this section. 

The scalar-coupling mechanism contributes to the spin-spin relaxation mechanism as the temperature is increased on the other hand, it contributes to the spin-lattice relaxation mechanism in the lower temperature region. The scalar-coupling mechanism is also absent. 

The dominant spin-lattice relaxation mechanism for nitroxides remains unknown. Experimental saturation recovery measurements of the small nitroxides tanone and tanol in an organic solvent in the liquid phase have been published by Percival and Hyde [41]. The values fall in the 10 -10 second range and exhibit an approximately half power dependence on t /T. These authors observed that neither substitution, thereby removing the nuclear quadrupole interaction, nor deuterium substitution affected electron Tj values. 

However, one advantage does result. It is usually possible by inspection of a 13C spectrum to recognize the nuclei that do not bear protons by their low intensity (peaks 1 and 2 in Fig. 5.1 h). The common spin-lattice relaxation mechanism for 13C results from dipole-dipole interaction with directly attached protons. Thus, non-protonated carbon atoms have longer Tx relaxation times, which together with little or no NOE, results in small peaks. It is therefore often possible to recognize carbonyl groups (except formyl), nitriles, nonproton-ated alkene and alkyne carbon atoms, and other qua-ternaryt carbon atoms readily. 

At least two T minima and two loss maxima are apparent in Figure 2, indicating the existence of at least three different mechanisms. Nonexpotential magnetization decay curves, decomposable into two parts each associated with a separate T value, were observed for P4MP1 crystals from 77°-170°K. The occurrence of two spin-lattice relaxation mechanisms at... 

One of the cross-polarization pulse sequences used to measure is shown in Figure 6.9. During the evolution time, At, of this pulse sequence, the magnetization is spin-locked along Any reorientation of the internuclear vectors induces fluctuations of the dipolar local fields, and the COcomponent of these fluctuating fields participates in the relaxation of the spins. This is a spin-lattice relaxation mechanism which is related... 

Director fluctuations are however not the only spin-lattice relaxation mechanism in nematics. Translational self-diffusion of nematic molecules modulates the inter-molecular nuclear dipole-dipole interactions and induces - as first emphasized by Vilfan, Blinc and Doane [36], another contribution... 

After the pulse, the tilted magnetization will (i) recover exponentially back to its equilibrium value along the z axis via a spin-lattice relaxation mechanism (time constant T ) and (ii) disappear in the xy plane due to a spin-spin relaxation mechanism (time constant T2) with T2 <... 

The studies to be reviewed below mainly refer to proton or deuteron magnetic resonance in melts or concentrated solutions of polymers of molecular masses close to or above the critical value M. Under such circumstances the predominant spin-lattice relaxation mechanism of protons (spin 1/2) is based on fluctuating dipole-dipole couplings of like spins, that is, we are dealing with the homonuclear case. Deuterons (spin 1), in contrast, possess a finite electric quadrupole moment which is subject to quadrupole coupling to local molecular electric field gradients which is much more efficient than dipolar interactions. 

---