MEDIUM EFFECTS IN ROTATIONAL ISOMERISM .6. INCLUSION OF DIPOLE-DIPOLE INTERACTIONS IN POLAR SOLVENTS

An electrostatic theory of the effect of the medium on the energy differences between rotational isomers is extended by the inclusion of solvent-solute dipole-dipole interactions in polar solvents. This extension permits the calculation of the energy differences of rotational isomers in the vapor (ΔEv) and liquid (ΔE1) from measurements of the solvent dependence of the coupling constants in a molecule. In 1,1,2-trichloroethane the theory is in agreement with the observed energy differences in the vapor and liquid. For meso-2,3-dibromobutane these are obtained as ΔEv = 2.0 (±0.1) kcal/mol and ΔE1 = 0.90 (±0.1) kcal/mol. The trans and gauche vicinal HH couplings in the latter are 10.3 (±0.1) and 2.9 (±0.4) cps, respectively, and the trans and gauche oriented Me-C-CH couplings are 0.5 (±0.2) and -0.3 (±0.1) cps, respectively. The long-range couplings are similar to those found in related molecules and agree with present theories of these couplings.