Relaxation in nmr

Spin relaxation in NMR is known to provide information about the dynamics of molecular entities and possibly about molecular geometry or electron distribution. Generally, dynamical information is obtained if the tensor of the relevant relaxation mechanism is known from independent determinations. Conversely, if parameters describing the dynamics of the considered molecule have been deduced beforehand, geometrical parameters may be derived. Only in particular situations, one can hope to access both types of parameters (dynamical and geometrical). For... 

H. W. Spiess, Rotation of Molecules and Nuclear Spin-Relaxation in NMR Basic Principles and Progress , Springer, New York, 1978, Vol. 15, p. 55. 

Comparing FTMS with Fourier transform nuclear magnetic resonance (FTNMR), we first notice how the frequency range to be covered here is very large. Second, relaxation in NMR is invariably linked with the interaction among liquid-phase or solid-phase molecules. In the gas phase, relaxation depends on the vacuum and on the stability of the ions being observed. If the vacuum is not sufficient, collisions slow the ions and their movement becomes incoherent. The observation of an ion is also limited to its lifetime. 

Decoherence in quantum systems is somewhat akin to transverse relaxation in NMR (T2 processes), in which the nuclear spins lose their phase coherence. The proposal that consciousness is somehow connected with collapse of the wavefunc-tion, although intriguing, does not appear to be relevant. 

Chapter 8 considers the important topic of relaxation in NMR. We start out by considering the effects of relaxation, concentrating in particular on the very important nuclear Overhauser effect. We then go on to consider the sources of relaxation and how it is related to molecular properties. 

H. W. Spiess, "Rotation of molecules and nuclear spin relaxation," in NMR, edited by P. Diehl, E. Fluck, and R. Kosfeld, (Springer Verlag, Berlin, 1978) pp. 55-214. 

The dominant mechanism for nuclear relaxation in NMR is via quadrupolar relaxation thus, for rapid molecular tumbling, the condition Ti=T2 holds. See (a) Butler, L. G. in 0 NMR Spectroscopy in Organic Chemistry, Boykin, D. W., Ed., CRC Press Boca Raton, 1991, Ch.l (b) Abragam, A. Principles 0/Nttckar Magnetism, 2nd ed. Clarendon Press Oxford, 1983, p. 314. 

Nuclear Relaxation in NMR of Paramagnetic Systems Ivano Bertini, Claudio Luchinat, and Luigi Messori... 

A major application of solid state NMR is the study of polymer morphology. Information potentially available includes the amount and orientation of crystalline phases in semi-crystalline polymers and the domain sizes in phase-separated polymeric systems. For the determination of crystallinity, a common method is to measure Ti relaxation in NMR (or NMR for deuterated polymers). The relaxation data can often be resolved into two (or more) components, which may correspond to magnetization arising from crystalline and amorphous phases (11-15,130-134). The development of the maximum entropy regularization method has permitted more facile and less subjective analysis of the data (143). In optimal cases, multiple components can be identified. 

Many books, review articles, and papers devoted to relaxation in NMR have been published since the "historical paper" of Bloembergen, Purcell, and Pound (1). It is pertinent to question the necessity of writing another review article on this subject, especially if we take into account the fact that many contribu-... 

This review article will be more oriented towards the physical meaning and the role of relaxation in NMR than on the extensive derivations of formulae which are given with details in basic treatises or review articles like those cited in the reference list (1-10). Due to the general scope of this volume, some emphasis will be put on the relaxation phenomena for nuclei characterized by I > 1/2. 

What we have described here is a relaxation process which corresponds to an energy relaxation. In NMR this energy relaxation is described as longitudinal (because the z direction, identical to the Bq direction, is known as the longitudinal direction). The term spin-lattice relaxation is also frequently used. 

---