Spinhyphen;Echo NMR Studies of Internal Rotation in 1,1hyphen;Difluorohyphen;1,2hyphen;Dibromodichloroethane

摘要

A spinhyphen;echo study of internal rotation in liquid 1,1hyphen;difluorohyphen;1,2hyphen;dibromodichloroethane is reported. Carrhyphen;Pucrell spinhyphen;echo (CPSE) trains were observed from minus;115deg; to plus;100deg;C with rf pulse spacings of 0.4 to 32 msec. Modulation of the echo trains was observed at temperatures below minus;60deg;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 spinhyphen;echo experiments. At minus;100deg;C, effects of the internal rotation are negligible and dipolehyphen;dipole interactions give different relaxation rates for fluorines in different rotamers. Above minus;60deg;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 steadyhyphen;state NMR, and by analysis of CPSE trains at lower temperatures, but the apparent chemical shift values are about 25percnt; too large.