Knowledge Bank

The vibrational spectrum of gaseous hexafluoroacetone has been reexamined. The use of current Raman techniques allows the accurate observation of intensity and polarization properties of bands in the spectrum in order to identify them with molecular symmetry species. As found $previously,1$ the characteristics of the spectrum are consistent with a $C2v$ molecular symmetry, in contradiction with the $C2$ structure found by electron $diffraction.2$ To investigate this discrepancy, a 3-21G SCF ab initio calculation was carried out to estimate the structure and the vibrational force field. The ab initio structures was found to be $C2$, in agreement with the electron diffraction result. The ab initio force field, weighted in the accepted manner, predicts vibrational frequencies which are readily identifiable with the observed spectrum. Further, it is found that, to a great extent, the symmetry coordinates which combine to make up a given normal mode all correlate with the same symmetry species under $C2v$. Using this fact we can understand how the observed spectrum may be interpreted on the basis of $C2v$ symmetry when the actual molecular symmetry is $C2$.$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$C.V, Berney, Spectrochim. Acta 21, 1809 (1965); F. A. Miller and F. E. Kiviat, Spectrochim. Acta 25A 1577 (1969). $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$R. L. Hilderbrandt, A. L. Andreassen and S. H. Bauer, J. Phys. Chem. 74 1586(1970).