Molecular structure and vibrational spectra of 1,1,1,3,3,3-hexafluoroacetone (HFA) have been investigated by means of ab initio and density functional theory (DFT) calculations. The harmonic and anharmonic vibrational frequencies of HFA were calculated at the B3LYP and B2PLYP levels of theory. The calculated frequencies and the Raman and IR intensities were compared with the experimental results. In addition, a normal coordinate analysis was also done by using the normal mode eigenvectors obtained at the B2PLYP/6-31 + G(d) level. To study the internal rotation of CF3 groups, single CF3 rotation and synchronous rotations of both CF3 groups (clockwise–clockwise and clockwise–counterclockwise) were analysed. The internal rotation of CF3 groups around C-C bonds in HFA allow four hypothetical conformers (C2, Cs, and two C2V point groups). Aided by ab initio and density functional theory (DFT) calculations, the C2 conformer is the only stable form and the FCCO dihedral angle is in the 17.6–21.0 degrees range. Complete vibrational assignments have been reported for the stable C2 conformer, which is supported by normal coordinate analysis for all fundamentals.

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