Characteristic Vibrations of CH4 (Td symmetry)

CH4 (methane) is a tetrahedral molecule, the simplest hydrocarbon. It is the main ingredient in natural gas, and is also produced directly by biological activity. Methane is a substantial contributor to the Earth's greenhouse effect, after CO2 and H2O. According to the IPCC, an increase of 1060 ppb (150%) in its atmospheric abundance between 1750 AD (pre-industrial) and 2000 AD amounts to a radiative forcing ≈ 0.48 Watts/m2.
The highly symmetric shape of the methane means that only two of the vibrational modes depicted below interact directly with infrared light (o3 and o4 -- the ones where the carbon and hydrogen both move), and these modes are the most likely to absorb or scatter infrared heat radiation from the Earth before it can escape to space.  The normal modes depicted below were modeled using hybrid density functional theory (B3LYP) and the cc-pVTZ basis set. Harmonic frequencies calculated by Gray and Robiette (1979, Mol. Phys. 37:1901-1920) are also shown. These are based on a force-field model fitted to spectroscopic measurements.

o1 = 3025.5 cm-1
A1 Symmetry

o2 = 1582.7 cm-1
E Symmetry
nu-1 vibration of methane
nu-2 vibration of methane

o3 = 3156.8 cm-1
F2 Symmetry

o4 = 1367.4 cm-1
F2 Symmetry
nu-3 vibration of methane nu-4 vibration of methane

 

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