Molecular motion in succinonitrile and malononitrile by nuclear magnetic resonance

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Abstract
Measurements are reported of the proton spin-lattice relaxation time, T 1, at 21·5 and 14·3 Mhz and the relaxation time in the rotating frame, T 1ρ , at 14·3 Mhz for H 1=7·1 g, in liquid and solid polycrystalline succinonitrile, (CH2CN)2. Correlation times for molecular reorientation and diffusion have been obtained. In the liquid these agree with dielectric correlation times. In the upper temperature solid phase I, the N.M.R. reorientational correlational time has an Arrhenius behaviour with an activation energy greater than in the liquid and only agrees with the dielectric correlation time at lower temperatures. Reasons for the discrepancy at higher temperatures are discussed and an anomaly pointed out by Clemett and Davies [10] is resolved. At higher temperatures in phase I, T 1ρ is controlled by translational diffusion modulated interactions, as also, possibly, is T 1 just below the triple point. Values for the translational correlation time are obtained. It seems possible that a ‘defect diffusion’ mechanism [24] is active in this substance rather than the lattice diffusion usually accepted in plastic crystals with simpler globular molecules. It is confirmed that no large scale molecular motion occurs in the lower temperature phase II. It has been shown that malononitrile, CH2(CN)2, does not have a plastic-crystal phase.