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Nuclear magnetic resonance study of molecular motion in some solids Ripmeester, John Adrian

Abstract

A number of solid substances ware examined by nuclear magnetic resonance methods with a view to investigating possible molecular motion. The possibility of using the adiabatic rapid passage technique as a method for investigating the molecular motion in the solid state was studied. Two systems, namely benzene and furan were studied. It was found that spin-lattice relaxation times and in some cases, second moments could be obtained using the adiabatic rapid passage technique; the results obtained were in good agreement with values obtained using standard techniques. Also it was found that the presence of a certain type of molecular motion severely affects the shape and amplitude of the adiabatic rapid passage signal. A number of charge-transfer complexes were investigated using standard broadline (1)H and (19)F nuclear magnetic resonance techniques. Some strong charge-transfer complexes studied in this manner include: a number of amine complexes of BF(3) and some halogen complexes of trimethylamine. Second moment and linewidth changes with temperature were interpreted in terms of reorientations of molecular groups within the complexes. Also studied were a number of weak complexes of benzene. Linewidth, second moment, and also spin-lattice relaxation time measurements showed that the benzene rings were reorientating about their hexad axes at frequencies greater than about 10(5) sec.(-1) at temperatures above 120°K. The activation energies for this motion depended strongly on the environment of the benzene ring in the complex. A study of some arene-chromium-tricarbonyl compounds indicated that the motional properties of the arene rings in the complexes resembled very much the motional properties of the free ring compounds; this suggests that specific bonding effects are relatively inimportant. Finally, linewidth, second moment and spin-lattice relaxation time measurements are reported for some soap systems, the alkali metal stearates and oleates. Transition temperatures observed could in some cases be correlated with values obtained by different methods. Some motional models were suggested in order to explain decreases in linewidths and second moments with increasing temperature. It was shown that methyl group rotation provides a relaxation mechanism in the low temperature phase for all the soaps studied. Activation energies found for this process ranged from 1.8-2.5 Kcal/mole. The effect of thermal history on phase transitions in the alkali metal stearates was also investigated. Thermograms were obtained using samples with different thermal histories using a differential scanning calorimeter. It was found that the thermal history of a sample may affect transition temperatures, and also the absence of presence of some transitions.

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