Every line in an IR spectrum arises from activating a molecular vibration. In the "functional group region" these are generally simple: only one bond, or a collection of two in concert. In the "fingerprint region" these tend to be more complex combinations of bond vibrations.

Select an IR band in the spectrum of methylphenylketone (marked in red along the top) to activate the vibration in the structure to the right. grid on grid off show peak list Vibration off

Look at several different molecules with different functional groups:
1-Hexene Distinguish between sp² and sp³ C-H stretches. The weak C=C stretch is also visible, as are out-of-plane vinyl C-H vibrations in the fingerprint region.
Benzaldehyde The C=O is most significant; the aromatic C-H OOP vibrations can be seen in the fingerprint region.
Benzoic Acid The O-H absorption extends across 1000 cm^-1, and overlaps the C-H stretches.
1-Methylcyclopentanol The O-H stretch is most significant.
Phenylethyne The alkyne shows both the terminal C-H stretch and the C-C stretch.
Hexanenitrile This spectrum is in μm; the C-N stretch is still quite evident.
Ethyl Acetate We can see three distinct C-O stretches: two different single bonds, and one double.
n-Butylamine The N-H bands are clearly evident.
Acetamide Here we see both the N-H and C=O.

Last updated: 9/22/2018