NMR Spin-lattice Relaxation Times Research Articles

NMR Spin—Lattice Relaxation in Solid Solutions of C6H6 and C6D6: Evidence for Concerted Motions by Adjacent Molecules

The NMR spin—lattice relaxation time T1 has been investigated as a function of isotopic substitution in cyrstalline benzene. 1,3,5-C6H3D3 and six solid solutions of C6H6 and C6D6 were studied between 140°K and the melting point. The concentration dependence of T1 was used to separate the effects of intra- and intermolecular relaxation. The experimental intramolecular T1 contribution agreed quite closely with that calculated from known molecular dimensions. Comparison of the intra- and intermolecular correlation times are shown to give information regarding the relative motion of neighboring molecules in the solid. Independent molecular ``jumps'' are found to be relatively unimportant in crystalline benzene, and a free-volume model, leading to the simultaneous motion of adjacent molecules, is discussed.

Nuclear Magnetic Relaxation in Camphor

Solid solutions containing d- and l-camphor have been investigated by nuclear magnetic resonance and x-ray powder diffraction between 77° and 473°K. Striking differences are observed among the various solutions in the vicinity of the second-order phase transition. The transition temperature is found to vary with composition, passing through a minimum at the 3:1 d:l solution. The width of the λ transition, reflected in the NMR second moment, increases going from the pure isomer to the racemic mixture. Methyl group reorientation persists at 100°K, although molecular tumbling has been arrested at this temperature in all samples. The x-ray powder patterns of the pure isomer, the 1:1, and 3:1 mole ratio solids differ at 77°K, but are identical at 298°K. Plots of the NMR spin—lattice relaxation time versus reciprocal absolute temperature yield an activation energy of 2.35 kcal/mole for methyl-group reorientation in d-camphor below the λ point. Above the rotational transition, the x-ray and NMR data are independent of concentration. Activation energies of 2.8 and 14.6 kcal/mole are calculated for the processes of molecular tumbling and self-diffusion, respectively. The T1 data are quantitatively examined in terms of the BPP theory.

Nuclear Magnetic Resonance Studies of Molecular Motion in Natural Rubber

Abstract Nuclear magnetic resonance measurements have been made on natural rubber to examine how frequency, temperature, and crystallinity affect the nuclear relaxation. Moecular motions were studied by observing NMR linewidths and spin-lattice relaxation times at temperatures between −100° and 100° C, and at radio frequencies between 2 and 60 Mc. The effect of crystallinity was seen in measurements on stark rubber. The relation between frequency and temperature in the spin-lattice relaxation process is examined in terms of the Arrhenius equation and the WLF expression. The importance of using frequency as a variable in NMR studies of molecular motion is stressed.

Nuclear Magnetic Resonance Studies of Molecular Motion in Natural Rubber

Nuclear magnetic resonance measurements have been made on natural rubber to examine how frequency, temperature, and crystallinity influence the nuclear relaxation. Molecular motions were studied by observing NMR linewidths and spin-lattice relaxation times at temperatures between −100° and 100°C, and at radio frequencies between 2 and 60 Mc. The influence of crystallinity was seen in measurements on stark rubber. The relation between frequency and temperature in the spin-lattice relaxation process is examined in terms of the Arrhenius equation and the WLF expression. The importance of using frequency as a variable in NMR studies of molecular motion is stressed.