Infrared Spectroscopy and Organic Functional Groups

IR spectroscopy is a powerful tool for determining the presence (or absence) or particular bond types in molecules. In thinking about IR it is useful to think of chemical bonds as springs, and the absorption of IR light results in exciting the motions of those springs. In Chemistry 105 you learned that a particular vibrational motion in a molecule will only absorb IR light if the dipole moment of the molecule changes during the course of the vibration. Two factors influence the frequency (usually expressed as wavenumber) of IR light absorbed by a particular bond type - the strength of the bond (as reflected in the bond force constant - if you have encountered Hooke's Law in physics this is similar to the spring constant in that expression) and the masses of the atoms in the bond of interest (the dependence is on the reduced mass - if the masses of the two atoms are m1 and m2, the reduced mass is (m1m2)/(m1+m2)). Keeping these two factors in mind can greatly aid in understanding why particular vibrations occur where they do in the IR spectrum. As an example, consider a C-H stretch and a C=C stretch. The CH single bond is clearly weaker (has a smaller force constant) than the CC double bond, but the CC double bond has a smaller reduced mass

Here we examine the IR absorption characteristic of various bonds in organic molecules. For each type of functional group the vibrations characteristic to that functional group will be shown for a particular test molecule. Along with the vibrations will be the wavenumber range within which that vibration is commonly found. One thing you will notice is that in many of the vibrations, more than two atoms are moving. This is due to the fact that in a vibration, the center of mass of the molecule must remain fixed (otherwise the molecule would move through space, and that is not a vibration). However, the dominant feature is associated with the particular functional group being examined. 

The Functional Groups - Click on the Desired Group Name

phenols
aliphatic alcohols
carboxylic acids
esters

Additional Information About IR Spectroscopy

Two factors influence the frequency (usually expressed as wavenumber) of IR light absorbed by a particular bond type - the strength of the bond (as reflected in the bond force constant - if you have encountered Hooke's Law in physics this is similar to the spring constant in that expression) and the masses of the atoms in the bond of interest (the dependence is on the reduced mass - if the masses of the two atoms are m1 and m2, the reduced mass is (m1m2)/(m1+m2)). Keeping these two factors in mind can greatly aid in understanding why particular vibrations occur where they do in the IR spectrum. As an example, consider a C-H stretch and a C-C stretch. These two vibration occur in quite different regions of the IR spectrum. The wavenumber of a vibration (directly proportional to the energy absorbed by the vibration) is proportional to the square root of the ratio of the force constant to the reduced mass.

The reduced mass of a CH bond is 12/13 = 0.923, while that of a CC bond is 144/24 = 6. Even if the bonds had the same force constant, you can show (try this) that the ratio of the CH stretch wavenumber to that for the CC stretch would be on the order of 2.5.

How Your Spectra Differ From The Spectra Included Here

The spectra shown here are computed by high level quantum mechanical calculations and refer to a single molecule in the gas phase. Your spectra will be those of many molecules in a condensed phase. You will probably notice that your spectra differ from the ones here in several ways:

  • the bands are broader
  • there are more peaks
  • the relative peak intensities differ

There are a number of reasons for this. Condensed phase spectra invariably have broader absorption bands as the same bond finds itself in a variety of slightly different environments. This is the predominant reason for the broader bands. An extreme example of band broadening is found for OH stretching vibrations. In a condensed phase there are many hydrogen bonding interactions possible leading to quite broad bands for the vibrations.

The calculated spectra include only the fundamental vibrations. These correspond to exciting one quantum of each vibration. Other peaks can arise in spectra due to overtone vibrations - exciting more than one quantum of a particular vibration - that occur at about twice the frequency of the fundamental vibration for the first overtone (if you are a musician the concept of overtones is like octaves). These overtones usually have small intensities but can often still be scene.

It is also possible to have combination bands, in which absorption is due to the simultaneous excitation of one quantum of several different vibrations, with the resultant peak occurring at the sum of the wavenumber of the individual vibrations. In general the combination bands also have low intensity.

However, if one of these low intensity bands is close in energy to a high intensity band, a process known as intensity stealing can occur. The most common example of this is a process known as Fermi resonance and requires that the two peaks arise from processes of the same symmetry. The discussion of the role of symmetry in IR spectroscopy is beyond what we are doing here, but is discussed in detail in Chemistry 341. Fermi resonance leads to peaks that you would expect to have low intensity becoming more intense, and altering peak intensities from those that are calculated.