Alkanes
(Methane as example)

Molecular Vibrations
(The wavenumber range is that typically found for the relevant structural feature)

In methane, the three asymmetric stretching modes are degenerate, and the symmetric
stretching mode is not seen in the IR as the dipole moment does not change on vibration.
Also, in methane, due to the high symmetry, the two bending modes are degenerate, and
are not seen in the IR.  In larger alkanes these symmetry restrictions do not hold and many
peaks are seen in the CH stretching and HCH bending regions.
Here is an expanded CH stretching region for propane:


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