An NMR and quantum mechanical investigation of solvent effects on conformational equilibria of butanedinitrile

Vicinal proton-proton NMR couplings and ab initio quantum mechanics have been used to investigate solvent effects on conformational equilibria of butanedinitrile. The trans and gauche conformations are about equally favored at room temperature in solvents of low dielectric constant while the equilibrium is essentially the statistical proportions of one-third trans and two-thirds gauche in water with a high dielectric constant. The coupling assignments were confirmed with the aid of stereospecific deuterium-labeled (R,R or S,S)-1,2-dideuteriobutanedinitrile. The calculations support the observed trends. Similar results were observed for 1,2-dibromo- and dichloroethanes.