Conformational energies of perfluoroalkanes. Part 1.—Semi-empirical calculations

Abstract

Conformational energies have been calculated as functions of the internal rotation angles about the C—C bonds for n-perfluorobutane in the neighbourhood of its trans and gauche conformations, and for polytetrafluoroethylene (PTFE) in the vicinity of its all-trans form. The energy equation consists of a semi-empirical “6-exp” potential for the interactions between non-bonded atoms, a dipole-dipole interaction term, and a three-fold “intrinsic” torsion potential having a barrier height E₀ for each bond undergoing rotation. The various parameters were adjusted to reproduce the experimental barrier height of C₂F₆. When E₀≅1 kcal mole^–1, the predicted most stable conformation of an isolated PTFE chain is a helix in which each C—C bond is rotated about 15° from the planar trans position, and the energy difference between the trans and gauche states of n-C₄F₁₀ is about 2 kcal mole^–1 . The dependence of the conformational energies on the magnitude of E₀ is investigated and discussed.