Normal Modes of CF4 (tetrahedral - Td symmetry)

    CF4 (carbon tetrafluoride or tetrafluoromethane) is a tetrahedral molecule with a number of industrial sources. There are also natural sources of CF4 to the Earth's atmosphere, but they are presently overwhelmed by human-induced emissions. C-F bonds are highly inert, giving CF4 a lifetime of thousands of years in the atmosphere. This allows emissions to build up in the atmosphere, to a present abundance of ~0.080 ppb, roughly double the pre-industrial conentration (1998, IPCC). Increased CF4 is estimated to add  0.003 Watts/m2  of greenhouse forcing (IPCC). The characteristic motions of the normal modes for CF4 depicted below were calculated at the Hartree-Fock level using the 6-31+G* basis set. Only ν3 and (to a lesser extent) ν4 interact strongly with infrared light.  Harmonic vibrational frequencies are from Wang et al. (2000, J. Chem. Phys. p.1353-1366), based an ab initio model adjusted to reproduce measured fundamental frequencies. The wavelength of light interacting with the fundamentals for ν3 and ν4 is also listed (in μm)

ν1 (A1) - Symmetric Stretch
921.6 cm-1

 ν3 (F2) - Asymmetric Stretch
1303.0 cm-1 absorbs at 7.8μm

ν2 (E) - Symmetric Bend
439.9 cm-1

 ν4 (F2) - Asymmetric Bend
637.9 cm-1 absorbs at 15.8μm

Go back to Molecular Vibrations