Knowledge Bank

In order to derive information pertaining to the crystal symmetries of the chlorofluoromethanes, for which diffraction measurements have not been reported, we have obtained far infrared Raman spectra of solid $CF3C\ell ,CF2C\ell 2$ and $CFC\ell 3$. Spectra have been observed between $13-130\circ K$; no evidence of phase changes for any of these halomethanes over this temperature range is noted. The lattice spectra are consistent with crystal structures which vary smoothly between the low-temperature forms of $CF4(P21/c,Z=4)1$ and $CC\ell 4(C2/c,Z=32).2$ Natural exclusion between infrared and Raman activity is found, as expected for centric unit cells. The symmetry-forbidden $a2$ fundamental $(\nu 5)$ of $CF2C\ell 2$ is observed in the infrared spectrum of the crystalline solid, which indicates a site symmetry lower than the $C2v$ molecular symmetry. Three infrared peaks are observed for the $\nu 4(a1)$ mode; they are interpreted in terms of the $C\ell$ isotopes in natural abundance. Implications of the spectroscopic observations on the crystal symmetries of solid $CF3C\ell ,CF2C\ell 2$ and $CFC\ell 3$ will be discussed. Molecular packing $considerations3$ have also been emptoyed to these crystals. The resultant molecular geometries have been emleyed in lattice dynamics calculations for the chlorofluormenthanes, which will also be described. $\mathrm{<mrow\; data-mjx-texclass="ORD">\ast </mrow>}$Research supported in part by the U.S. National Science Foundation. $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$R. Powers and R. Rudean, J. Chem. Phy. 72, 1679(1989). $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$S. Cohen, R. Powers and R. Rudman, Anta Cryst. 835, 1670 (1979). $\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$D. E. Williams, PCK5/PCK6, QCPE 12, 373 (1975).