Nuclear Magnetic Relaxation in 1,2-Dichloroethane

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Abstract
Analysis of thermal relaxation times in solid and liquid 1,2-dichloroethane suggests the following: (1) The mechanism causing thermal relaxation in the solid is a partial rotation of the molecule about the molecular axis (which is approximately the C–C axis) against a potential barrier of about 1.5 kcal per mole. (2) It is likely that the narrowing of the resonance line is a result of the completion of this partial motion. (3) At the melting point, a drop in the thermal relaxation time of about an order of magnitude occurs. (4) In the liquid, a new motion has set in, characterized by a lower activation energy (roughly 0.6 kcal/mole). This motion may be end-over-end tumbling. The spin-lattice relaxation time (``thermal'' relaxation time) in the solid ranges between 1 and 20 sec, and its length permits its measurement by a method which is slightly different from the conventional technique. This technique utilizes the fact that, immediately upon application of the radio-frequency field, the magnetization of the sample decays with a time which can be related to the thermal relaxation time and to the strength of the radio-frequency field. The measurement is a direct measurement of a characteristic time of the sample, and is therefore more reliable than the conventional indirect measurement. The technique also enables one to calibrate the radio-frequency field more easily.

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