Microwave spectroscopy and characterization of the helical conformer of perfluorohexane

The lowest energy conformer of perfluorohexane (C₆F ₁₄) is helical and its microwave rotational spectrum has been observed and assigned. The helicity along the carbon chain results in overall C₂ symmetry with the C₂ axis corresponding to the c-principal axis. The assignment of a c-type spectrum confirms the structure of C₆F₁₄ to be helical. The rotational constants are A 824.9001(9) MHz, B 202.2195(8) MHz, and C 198.3355(10) MHz. The molecular parameters were characterized by scaling computed models by the square root of the ratio of the observed second moments to those computed. The scaled structures exactly reproduce the observed second moments. From this scaling approach, the exterior C₁C₂C₃C₄ dihedral angle is 16.7o and the interior C₂C₃C ₄C₅ dihedral angle is 18.0o away from trans. Computed models at the PBE0/VTZ level of theory are in excellent agreement with the experimental results.