Conformations of the semifluorinated n-alkane H–(CF2)8–H investigated using Fourier transform microwave spectroscopy and quantum chemical calculations

• 209 pure rotational transitions of 1H,8H-perfluorooctane recorded between 7.8 GHz and 16.2 GHz.
• Precise rotational constants have been obtained for the first time.
• Quantum chemical calculations support the experimental measurements.

The three lowest energy conformations of the title compound have been investigated using quantum chemical calculations and the lowest energy conformer has been observed using pure rotational spectroscopy. The lowest energy conformer possesses C2C2 symmetry, a helical CF2 backbone, with the hydrogens nearly eclipsing one another when looking down the long axis of the molecule. The technique of Fourier transform microwave spectroscopy in conjunction with quantum chemical calculations is demonstrated as a complimentary method to X-ray diffraction for structural determinations of small oligomers for which the location of hydrogen atoms may be important.

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