Complete conformational analyses of perfluoro-n-pentane, perfluoro-n-hexane, and perfluoro-n-heptane

Complete conformational analyses of perfluoro-n-pentane, perfluoro-n-hexane, and perfluoro-n-heptane were carried out at both the semiempirical PM3 and the ab initio Hartree-Fock RHF/3-21G(*) levels of theory, and confirmed at the RHF/6-31G* level of theory, using the PC Spartan Pro computational package. These analyses indicated that the lowest-energy conformation of each of the perfluoroalkanes is helical. The conformational analyses also demonstrated that while helical or partially helical conformations dominate the lower-energy portions of the total conformational manifolds of these molecules, the fully staggered, zig-zag conformer analogous to the corresponding hydrocarbon molecules was also present in the lower-energy portions of the conformational manifolds of these molecules. The dihedral angle (∠F-C-C-F) leading to helical structures in these perfluorocarbon molecules ranges from approximately 46° at the ends of the molecules to approximately 49° near the centers of the molecular chains, compared to the 60° dihedral angle (∠H-C-C-H) expected for the fully staggered, zig-zag structure observed for hydrocarbons and also observed as higher-energy conformations in the perfluorocarbons.