Spin-lattice relaxation definition

Indeed, 13C spin-lattice relaxation times can also reflect conformational changes of a protein, i.e. helix to random coil transitions. This was demonstrated with models of polyamino acids [178-180], in which definite conformations can be generated, e.g. by addition of chemicals or by changes in temperature. Thus effective molecular correlation times tc determined from spin-lattice relaxation times and the NOE factors were 24-32 ns/rad for the a carbons of poly-(/f-benzyl L-glutamate) in the more rigid helical form and about 0.8 ms/rad for the more flexible random coil form [180],... 

The longitudinal relaxation time is associated with spin-lattice relaxation, and the transverse relaxation time with spin-spin relaxation. The definitions are... 

If the symmetry increases or decreases drastically, the spin-lattice relaxation process is fast, or the optical pumping is not successful in producing large spin alignment, the above rules would not be as definite. 

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. 

It remains to be seen whether there is a definite relationship between "shape" and "taste" for sucrochemicals and whether the spin-lattice relaxation method can provide useful information on this point. 

Relaxation measurements provide another way to study dynamical processes over a large dynamic range in both thermotropic and lyotropic liquid crystals (see Sec. 2.6 of Chap. Ill of Vol. 2A). The two basic relaxation times of a spin system are the spin-lattice or longitudinal relaxation time 7] and the spin-spin or transverse relaxation time T2. A detailed description, however, requires a more precise definition of the relaxation times. For spin 7=1, for instance, two types of spin-lattice relaxation must be distinguished, related to the relaxation of Zeeman and quadrupolar order with rates 7j"2 and Jfg. The relaxation rates depend on spectral density functions which describe the spectrum of fluctuating fields due to molecular motions. A detailed discussion of spin relaxation is beyond the scope of this... 

The definition of the lattice is somewhat arbitrary, and depends on which model is employed, but in general it includes each possible source of nuclear spin relaxation. The lattice does not include explicit reference to any nuclear spin... 

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