Characteristic Vibrations of CH4 (Td
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
depicted below were modeled using hybrid density functional theory
(B3LYP) and the cc-pVTZ basis set. Harmonic frequencies
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.
= 3025.5 cm-1
= 1582.7 cm-1
= 3156.8 cm-1
= 1367.4 cm-1
to Molecular Vibrations