Infra-Red (IR) Spectroscopy

Chapter 13: Spectroscopy

Infra red (IR) spectroscopy deals with the interaction between a molecule and radiation from the IR region of the EM spectrum (IR region = 4000 - 400 cm-1). The cm-1 unit is the wave number scale and is given by 1 / (wavelength in cm).

IR radiation causes the excitation of the vibrations of covalent bonds within that molecule. These vibrations include the stretching and bending modes.

An IR spectrum show the energy absorptions as one 'scans' the IR region of the EM spectrum.  As an example, the IR spectrum of butanal is shown below.

In general terms it is convenient to split an IR spectrum into two approximate regions:

functional group / fingerprint regions of a typical infra red spectrum


When analysing an IR spectrum avoid the temptation to try to assign every peak.
The fingerprint region, however, can be useful for helping to confirm a structure by direct comparison with a known spectra.

Infra-Red (IR) Spectroscopy

Hookes' Law
To help understand IR, it is useful to compare a vibrating bond to the physical model of a vibrating spring system.  The spring system can be described by Hooke's Law, as shown in the equation given on the left. 

Consider a bond and the connected atoms to be a spring with two masses attached.

Using the force constant k (which reflects the stiffness of the spring) and the two masses m1 and m2, then the equation indicates how the frequency, u, of the absorption should change as the properties of the system change.

Consider the following trends:

1. for a stronger bond (larger k value), u increases.
As examples of this, in order of increasing bond strength compare:
CC bonds: C-C (1000 cm-1), C=C (1600 cm-1) and CºC (2200 cm-1),
CH bonds: C-C-H (2900 cm-1), C=C-H (3100 cm-1) and CºC-H (3300 cm-1),
(n.b. make sure that you understand the bond strengths order)
2.  for heavier atoms attached (larger m value), u decreases.
As examples of this, in order of increasing reduced mass compare:
C-H  (3000 cm-1)
C-C  (1000 cm-1)
C-Cl (800 cm-1)

C-Br (550 cm-1)

C-I   (about 500 cm-1)
The following diagram reflects some of the trends that can be accounted for using Hookes' Law.  It also gives an approximate outline of where specific types of bond stretches may be found.

approximate regions for common types of bonds

Infra-Red (IR) Spectroscopy

Important absorptions:

The more important absorptions that you should probably learn to recognize, in order of importance are:

Base Value
Strength / Shape
s, "finger"
Exact position depends on type of carbonyl
s, brd
Broad due to H bonding
Can tell primary from secondary
Also check for OH and C=O
w alkene 
m-s aromatic 
Alkene w due to low polarity 
Aromatic usually in pairs
w, sharp
Most obvious in terminal alkynes
As hybridisation of C changes sp3-sp2-sp, the frequency increases
m, sharp
Characteristic since little else around it

If you know these, then you can identify most of the functional groups of interest. Note that it is rarely useful to look for C-C since the large majority organic molecules will have them.

You should also be aware that the exact substitution pattern of a particular bond causes shifts in the position of the absorption and, therefore, ranges of values are typically given in most tables.

It is possible to rationalize the shifts of absorbances based on electronic effects due to proximal groups, conjugation and / or ring strain.

In general, when you are trying to work out what a molecule is, you will not just have the IR spectrum, but you will have other information as well, such as the formula or most likely the NMR. Always cross check between these sets of information For example, if the molecular formula indicates only one O, then you can not have an ester (CO2R) or a carboxylic acid (CO2H) !

Sample IR Spectra :
    By looking at IR spectra that contain known functional groups and comparing and contrasting them with other IR spectra, one can develop the skills required to be able to "interpret" an "unknown" IR spectra.  Remember that for an organic chemist, the primary role of IR is to identify the functional groups that are present. A few examples reflecting some of the more important functional groups are provided below.
Compare them to try to appreciate the subtle differences, comparing frequency, intensity and shape.

In the first example, of the aromatic hydrocarbon, toluene, we can see both the aromatic and aliphatic CH stretches, and two absorptions for the aromatic C=C stretches.

IR spectrum of methylbenzene (toluene)

Acetone (2-propanone) is the "classic" carbonyl containing compound with the obvious C=O stretch in the middle of the spectra. Note that the peak is a very strong absorption. Compare it with the C=C in the previous case which are weaker and sharper.

IR spectrum of 2-propanone


The characteristic absorption of the alcohol, 2-propanol, is the broad band due to the hydrogen bonded -OH group.

IR spectra of isopropanol


Carboxylic acids contain both C=O and OH groups. Note the broadness of both absorptions due to the hydrogen bonding and that the C=O is typically at slightly lower frequency than that of a ketone.

IR spectrum of butanoic acid


An ester has the follwoing key absorptions, the C=O and typically two bands for the C-O (not always easy to identify) since there are sp3 C-O and sp2 C-O bonds.

IR spectrum of methyl ethanoate