Spectral Interpretation


  Once you collect a spectrum, the real work begins. IR spectra of organic compounds have two general areas:
 
4000-1500 cm-1 1500-400 cm-1
The Functional Group Region 

Peaks in this region are characteristic of specific kinds of bonds, and therefore can be used to identify whether a specific functional group is present.

 The Fingerprint Region   

Peaks in this region arise from complex deformations of the molecule.  They may be characteristic of molecular symmetry, or combination bands arising from multiple bonds deforming simultaneously. 

A word about units. Most spectra using electromagnetic radiation are presented with wavelength as the X-axis.  Originally, IR spectra were presented in units of micrometers.  Unfortunately, a linear axis in micrometers compresses the region of the spectrum (10-15 μm) that usually has the largest number of peaks.  One could rectify this by presenting the speectrum on a linear scale vs. frequency (Hz), but the magnitude is unwieldy (10 μm = 3 x 1013 Hz).  A different measure, the wavenumber, is given the unit cm-1.  The relationship can be derived by the relationship
    
ν (cm -1)= 10,000/λ (μm)

The spectra may appear to be "backward" (large wavenumber values on the left, running to low values on the right); this is a consequence of the μm to cm-1 conversion; old spectra in microns did read from low wavelength on the left to large wavelength on the right.


  The two regions of the spectrum overlap to a degree.  (In fact, one always finds overlap between different regions of any spectrum; these designations are "guideposts" to help you orient yourself.)  For example, carbon-chlorine bonds appear at around 800 cm-1, and C-O single bonds appear at around 1200-1300 cm-1.  Also, benzene rings show "overtones" in the 1500-1700 cm-1 region, even though these arise from complex ring deformations. 
     The normal way to approach interpretation of an IR spectrum is to examine the functional group region to determine which groups might be present, then to note any unusually strong bands or particularly prominent patterns in the fingerprint region.  Finally, if you think you have identified the compound (usually you need additional information) you can compare the spectrum with a reference.  Matching the fingerprint region is a very rigorous test.

Look at the page about molecular vibrations and see this relationship for yourself.

Some important IR-active functional groups, and examples of spectra.


Group Region Examples of spectra. (Try to find the characteristic peaks.)
C-H 3000-3100 cm-1 (sp2) 

2800-3000 cm-1 (sp3)
Zoom in (click the spectrum to activate it first).
Reset zoom		 1-Hexene:
The alkene C-H band is at 3090 cm-1, and the alkyl C-H bands are lower in frequency.
C=O
 1600-1800 cm-1  

Acids: 1650-1700 
Esters: 1740-1750 
Aldehydes: 1720-1750 
Ketones: 1720-1750 
Amides:1650-1715		 Benzaldehyde:
O-H 
(alcohol)		 3300-3600 cm-1 

Monomeric forms: sharp. 
H-bonding leads to broadening.
Zoom in
Zoom out		 1-Methylcyclopentanol:
O-H 
(acids)		 2400-3000 cm-1 

Very broad, medium intensity 
Highlight O-H
(Note overlap with C-H bands which are visible here at 3000-3100 cm-1)
 Benzoic Acid:
C≡C 
C≡N		 2200-2100 cm-1 

Usually weak; maybe not visible 
in internal alkynes. 
Nitriles are quite strong.
Highlight C-C stretch
Note also the very strong C-H band at 3342 cm-1

Highlight C-N stretch		 Phenylethyne:

Hexanenitrile:
C-O 1200-1300 cm-1 

Often difficult to assign, 
depending on fingerprint region.		 Ethyl Acetate (both C=O and 2 C-O bands visible):
N-H 3400 cm-1 

Usually sharper than O-H. Two peaks indicate NH2; a single peak is NH (secondary amine/amide). Tertiary amines/amides have no N-H bond and so show no peak.
Highlight (click each spectum first to activate)		 n-Butylamine:

Acetamide:

A final word about symmetry.

Molecular vibrations give rise to IR bands only if they cause a change in the dipole moment of the molecule.  (This comes out of the quantum mechanics of molecular absorption of energy, so we aren't concerned too much with why, yet.)  If a stretch does not change the dipole moment, there won't be any IR band.  This is why O2 and N2 in the atmosphere don't show any IR bands.  CO2, however, has a stretch where one O moves in and the other moves out:

 Comparison of N2 (one symmetric stretch) with CO2 (oxygens can vibrate symmetrically (in/out at the same time) or asymmetrically (one in, one out)

Thus we see this band at 2400 cm-1.