The structure and helicity of perfluorooctanonitrile, CF ₃-(CF₂)₆-CN

New molecular structural data is presented for a cyanide terminated oligomer of polytetrafluoroethene. The target molecule, CF₃-(CF ₂)₆-CN, has been seeded within a pulsed supersonic expansion of argon. The result of this action is to cool the species to rotational temperatures below 4 K. Within this state, the pure rotational spectrum of the oligomer has been recorded using two types of Fourier transform microwave spectroscopy. A total of 111 transitions have been identified involving rotational J levels between 6 and 40. Only a- and b-type transitions were observed. The spectrum has been analyzed using a Hamiltonian containing all three rotational constants and one centrifugal distortion constant, D _J. The experimental spectroscopic constants have been used to develop an effective molecular structure by scaling the quantum chemical calculated structure. The data shows that the seven carbon perfluorinated chain for the isolated oligomer twists ≈104°. This compares well to the C ₇F₁₃-twist of ≈97° anticipated from the X-ray structure of phase II polytetrafluoroethene.