Conformational Properties of 1,3-Difluoropropane

The conformational energy order of 1,3-difluoropropane is identified as GG < AG < AA < GG‘ at various ab initio calculation levels. This result is analyzed on the basis of the molecular structures, partial charge distributions, and a molecular mechanics calculation. It is demonstrated that a strong dipole−dipole interaction between the highly polarized C−F bonds is the decisive factor determining the conformational energy preference between two gauche-gauche conformers (GG and GG‘). This observation suggests that, in addition to the gauche effect, the intramolecular electrostatic interaction should be considered for studying conformational behaviors of molecules with highly polarized bonds in general. The conformational energies obtained in this work propose a challenge to earlier interpretations of experimental data for 1,3-difluoropropane.