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. You may want to click-and-drag over a region to expand the spectrum.
(ref: Mohrig, Secs. 21.1-21.2, pp. 311-316)
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Look at several different molecules with different functional groups; click the compound name, then the links in each description to highlight relevant peaks. Click on the peak in the loaded spectrum to activate the vibration in the 3-D molecule to the right.
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. The C=O band is, of course, present.
1-Methylcyclopentanol The O-H stretch is most significant. In this case, both monomeric (short, sharp) and H-bonded (strong, broad) forms are evident.
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 (and much stronger than the weak C-C stretch in the alkyne).
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.

Normal modes calculated at the B3LYP/6-31G** level and assigned based on the calculated spectrum using Gaussian09. Gaussian 09, Revision D.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski, and D. J. Fox, Gaussian, Inc., Wallingford CT, 2009.
Page last updated: 12/16/2014