Study of Internal Motion in Materials by Means of Direct Observation of Relaxation Resonances in NMR (NMRRR)
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Publisned 19 1 by Elsevier 'crth Holland, Inc. Kaufr:ann and Shenci. editorn 0
Nuclear and Electron
esconance S-ectroscooies Applied to Materials Science "
~521
STUDY OF INTERNAL MOTION IN MATERIALS BY MEANS OF DIRECT OBSERVATION OF RELAXATION RESONANCES IN NMR (NMRRR)
L. VAN GERVEN, P. COPPENS and P. VAN HECKE Laboratorium voor Vaste Stof -Fysika en Magnetisme, Leuven, 3030 Leuven (Belgium)
Katholieke Universiteit
ABSTRACT A method, different from the classical ones, for studying internal motions - in particular reorientational and tunneling modulation of rotations - via NMR is described. Instead of vs. Tlooking for matching or resonance maxima in NMR T 1 v1 with p: the correlation rate and (a): curves [Iwhere o 0opru the Larmor frequency] we look for such maxima in NMR T 95. w0 - curves [where w =,A'with w: the (tunneling)frequency].
These method, Nuclear Magnetic Resonance Relaxation Resonance (NMRRR), has marked advantages, which are discussed. A field cycling technique is presented. Typical results, for solid silane, are given.
INTRODUCTION Nuclear magnetic resonance is well known as a powerful tool for the detailed study of materials, in particular of organic materials, like e.g. polymers, and biological materials. In most cases this research, in particular in industrial laboratories, has been limited to the study of internal structures, i.e. of the static properties of materials, for which NMR, owing to the very high resolution of its spectra, is most adequate. Another major advantage, however, of radiospectroscopy over optical spectroscopy has not been widely used up to now in the study of materials. In radiospectroscopy, in contrast to higher frequencies spectroscopies, spontaneous (emission) relaxation transitions do not occur, and so other relaxation transitions, thermal transitions, induced by different internal relaxation mechanisms prevail. This opens the road to the study of dynamic processes in materials via NMR, in particular NMRR, nuclear magnetic resonance relaxation, a domain which has not yet been fully exploited. PHENOMENA.
SOME THEORY
The modulation of the space coordinates of nuclei r(t), 6(t), 0(t) in the nonsecular terms (q >0) of the dipolar Hamiltonian of nuclear spin pairs F (q) (r,0,f)A (q) HD=D q
(1)
[F(q) = coordinate functions; A(q) = spin functions]
± Research fellow of the Belgian Interuniversitairinstituut voor Kernwetenschappen Now with N.A.T.O.,
1110 Brussels (Belgium)
522 gives rise to modulations of the "internal" magnetic fields rt, stimulatinq spinlattice relaxation. In this way internal motions, often calleR molecular motions, motions of molecules or molecular groups, lie in many cases at the basis of spinlattice relaxation, are a relaxation mechanism par excellence. In fact the for NMR interesting modulations of 1 very often are brought about by modulations of internal motions. The direct cause is not the "fundamental" motions, translations or rotations - their frequencies are generally too high to induce the relatively low frequency Zeeman transitions -
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