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 (angle F-C-C-F) leading to helical structures in these perfluorocarbon molecules ranges from approximately 46 degrees at the ends of the molecules to approximately 49 degrees near the centers of the molecular chains, compared to the 60 degrees dihedral angle (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. (c) 2008 Elsevier B.V. All rights reserved.