Spin-Echo NMR Studies of Internal Rotation in 1,1-Difluoro-1,2-Dibromodichloroethane

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Abstract
A spin‐echo study of internal rotation in liquid 1,1‐difluoro‐1,2‐dibromodichloroethane is reported. Carr‐Pucrell spin‐echo (CPSE) trains were observed from −115° to +100°C with rf pulse spacings of 0.4 to 32 msec. Modulation of the echo trains was observed at temperatures below −60°C, and the modulation frequencies were found to agree quantitatively with predictions of a general theory for the combined effects of scalar coupling and chemical exchange in spin‐echo experiments. At −100°C, effects of the internal rotation are negligible and dipole‐dipole interactions give different relaxation rates for fluorines in different rotamers. Above −60°C the echo trains decay exponentially. The dependence of the observed decay rates on pulse spacing was used to obtain values of the free energies of activation for exchange between rotamers and values of the fluorine chemical shifts in the different rotamers. The free energies of activation are in agreement with values obtained by steady‐state NMR, and by analysis of CPSE trains at lower temperatures, but the apparent chemical shift values are about 25% too large.