Studies of rotational isomerism

Because the measured thermodynamic properties of ethane did not agree with those calculated from statistical mechanics, Pitzer proposed the existence of energy barriers hindering internal rotation about single bonds. A consequence of hindered rotation is that molecules may exist in more than one stable conformation: these conformations are referred to as "rotational isomers". Depending on the magnitude of the energy barrier relative to thermal energy, the conformers (or rotamers) may be individually isolable (as in ortho-substituted biphenyls) or in dynamic equilibrium (as in halogeno-ethanes). There is no simple explanation for the magnitudes of energy barriers: the various theories which have been proposed are reviewed by Dale and Lowe. Irrespective of the barrier height, the infra red spectrum of a mixture of two rotamers often shows doubling of characteristic group bands. Thus chloroacetone (I), in carbon tetrachloride solution, has two carbonyl bands which may be assigned to rotamers (la) and (Ib). [Please see pdf. for full abstract.