The Spin Lattice Relaxation of the Nuclear Dipolar Energy in Some Organic Crystals with Slow Molecular Motions

Abstract

The relaxation of nuclear dipolar energy to the lattice has been measured in three different organic solids (benzene, cyclohexane and hexamethylbenzene) as a function of temperature. In the cases of C6H8 and C6(CH3)6 very slow motions associated with rather high activation energy were detected near the melting point. They are thought to be thermally activated rotations of the molecules about axes other than the hexad axis. In the case of cyclohexane the activation energy for the process of molecular diffusion was determined directly from the experimental results without the need of a model for vacancy diffusion. A maximum in dipolar relaxation rate was always observed for correlation times of the order of the rigid lattice line width i. e. in the temperature region where the lines narrow.

Molecular rearrangements in organic crystals. I. Potential energy calculations for some cases of reorientational disorder

“…The rotation of hexamethylbenzene in the crystal was reported early (Andrew, 1950) and confirmed by further studies, dealing also with methyl rotation (Rush & Taylor, 1966;Rush, 1967;Allen & Cowking, 1967). Rotation about an axis other than the hexad axis has been reported at high temperatures for benzene and hexamethylbenzene (Van Steenwinkel, 1969). For various other derivatives, dielectric measurements have yielded approximate activation energies for the in-plane rotations (Brot & Darmon, 1970).…”

“…(c) Brot & Darmon (1970). (d) Allen & Cowking (1967);Van Steenwinkel (1969). (e) Boden, Davis, Stam & Wesselink (1973b).…”

NMR Relaxation in Thermotropic Liquid Crystals

“…Since many mechanisms may contribute to spin relaxation in liquid crystals, the relaxation rates are additive if the motions can be uncoupled on the basis of sufficiently different correlation times: i.e., interlintra separation in PAA was demonstrated? cornbined with TIP (the spin-lattice relaxation time in the rotating frame) data, in a study of order parameter T-I 1 --2 TLI = T; : + T;:, (48) fl~ctuations'~ near the clearing temperature Tc in the nematic phase of PAA. Lewis et al= have recently where Ti,' and Ti.'…”

NMR‐Relaxationsverhalten von adsorbiertem Benzol an einer methylierten Silika‐Oberfläche. I. Transversale Relaxationszeiten und Signalintensitäten. Gefriererscheinungen in Kapillarflüssigkeiten

“…In den vorausgegangenen Abschnitten wurde gezeigt, daR die Abweichungen der Signalintensitaten vom Curieschen &en -8OUnd -120°C o b e r Gefriererscheinungen in Adsorbaten, insbesondere auch des Benzols [l5, [28][29][30][31][32][33][34][35] Bei Zugrundelegung der Annahmen (i) und (ii) lassen sich zunachst aus den bei 23 "C gemessenen Adsorptionsisothermen [4] mit Hilfe der Kelvingleichung *) Im Benzolkristall (Raumgruppe Pbca, a, = 7,46 A, b, = 9,67 A, c, = 7,03 A ( -5°C) [26]) werden Reorientierungen der Molekiile um zweizablige Symmetrieachsen mit nur sehr kleinen Frequenzen dicht unterhalb des Schmelzpunktes gemessen [27].…”