An N.M.R. investigation of rotational isomerism in some halogenated alkanes

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Abstract
The variation with temperature and solvent of the N.M.R. vicinal coupling constants in 1,1,2-tribromo and trichloroethane, 1,1,2,2-tetrabromo and tetrachlorethane, and in 1,1-dichloro-2-methyl propane have been measured. The energy differences between rotamers have been estimated from the temperature dependence of the coupling constants. For 1,1-dichloro-2-methylpropane ΔE = 480 ± 150 cal/mole and is virtually independent of the medium. The results for the remaining four compounds are strongly dependent on the nature of the solvent and can only be interpreted satisfactorily by means of an electrostatic theory of the medium effect. Values of ΔE vap are estimated for these compounds. The results are compared with those obtained from infra-red and Raman spectroscopy. Agreement is satisfactory only when effects of the medium are taken into account.