Librational motion, spin-lattice relaxation times

The chain dynamics of the MGIMx copolymers has been studied in deuter-ated chloroform solutions through spin-lattice relaxation time, T y or more precisely through the Ti/T min ratio, in order to avoid the contribution of libration motions (Sect. 8.2.3.2). The temperature range considered extends from - 50 to 50 °C. 

Dynamic processes such as fluctuational motions of water molecules can be studied from the spin-lattice relaxation times of proton nuclear resonance . In the presence of highly mobile H atoms, the separation of the proton resonance disappears and the remaining signal sharpens as for isotropic liquids. Furthermore, the frequencies of H2O librations (see Sect. 4.4) can be computed . ... 

Levy and coworkers97 have measured 13C spin-lattice relaxation times, 7), for 3- and 4-aminobiphenyls in a number of solvent systems, and of the corresponding ammonium ions in acidic and nonacidic media. The observed 7) values indicated that the molecular tumbling is anisotropic for these species. In addition, the known biphenyl geometry allowed indentification and semiquantitative evaluation of internal rotation-libration motion. The protonated amine function is motionally more restricted by solvent-solute and ion-pair interactions than the corresponding neutral amine. Thus, in the 3-biphenylammonium ion, the principal axis for molecular reorientation is aligned close to the C3—NHj-bond, whereas in the amine the principal axis lies closer to the biphenyl C2-symmetry axis. In both 3- and 4-aminobiphenyls, the unsubstituted phenyl rings are less restricted due to rapid phenyl rotation or libration. Table 14 presents 13C Tj-data for 4-aminobiphenyl 37 (NH2 on C4) and 4-biphenylammonium acetate 38 and trifluoroacetate 39. 

In general, no large energy barriers are to be crossed for the motional phenomenon. The activation energy was evaluated by the relaxation values to be as small as 4.5 kj mol in PHTP, lower than a gauche/trans interconversion barrier [39]. Large librations of methylenes account for the short carbon spin-lattice relaxation times at room temperature. 

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