Conformational analysis of meso- and (+/-)-2,3-dicyano-2,3-dicyclopropylbutane and 1,2-dicyanotetracyclopropylethane

The dipole moments of meso-2,3-dicyano-2,3-dicyclopropylbutane meso-1, (+/- )-2,3-dicyano-2,3-dicyclopropylbutane (+/-)-l and 1,2-dicyanotetracycloprop ylethane 2 in carbon tetrachloride and benzene have been measured over a ra nge of temperatures. Analyses of the relative permittivity data show that a t 25 degreesC, meso-1 and (+/-)-1 favour the trans form. However,, replacem ent of the methyl groups in I with cyclopropyl moieties completely reverses the trans reversible arrow gauche equilibrium, such that 2 exists in 74% g auche conformation. The experimentally derived values of the energy differe nce between the gauche and trans conformers and the gauche/trans population quotients were compared with values predicted by molecular orbital calcula tions. Theory predicts that the conformational preference of the substitute d dicyanoethanes is strongly influenced by solvent polarity. The crystal an d molecular structures of meso-1 and 2 were determined by single-crystal X- ray diffraction methods. Both compounds exist in the trans conformation in the solid state.