Infrared Spectroscopy,Characterisation of Organic Compounds

 This discussion is on infrared (IR) spectroscopy which tells about the functional groups in the molecule.

William de Wiveleslie Abney

Almost any compound having covalent bonds absorbs various frequencies of electromagnetic radiation in the infrared region of the electromagnetic spectrum. This region lies at wavelength longer than those associated with visible light, which range from approximately 400 to 800 nm, but lies at wavelengths shorter than those associated with microwaves, which are longer than 1 mm. For chemical purposes, we are interested in the vibrational portion of the infrared region. It includes radiations with wavelengths between 2.5 μm and 25 μm.


In the early use of IR spectroscopy in 1882, William Abney managed to identify 52 benzene derivatives from the IR spectroscopy. IR spectroscopy is used to see the the functional group in the molecule as the bonds vibrates at certain wavenumber and it only vibrate at only certain allowable frequencies. 

The covalent bonds in a molecule can be described in similar way with a 2 balls (atoms) that connected with a spring (the bond). The bond distance continually changes, but an equilibrium or average bond distance can be defined. Whenever the spring is stretched or compressed beyond this equilibrium distance, the potential energy of system increases. As the spring moves in a harmonic oscillation, the energy is proportional to the frequency of vibration which is determined by the force constant (K) of the spring and the masses of two bonded atoms. The natural frequency of vibration of a bond is given by the equation

which is derived from Hooke's Law for vibrating springs. The reduced mass, μ, of the system is given by

From those equations, two things should be noticeable immediately. One is that stronger bonds have a larger force constant K and vibrate at higher frequencies than weaker bonds. Therefore, triple bonds will vibrate at higher frequencies (higher wavenumber) than double bonds or single bonds.

The second is that bonds between atoms of higher masses vibrate at lower frequencies than bonds between lighter atoms as shown below.

Besides that, when a bond vibrates, not all modes of vibration are allowed. When vibration modes do not provides no dipole moment change, so it is not allowed; Only vibration modes give dipole moment change that is allowed. The result of unallowed vibration modes is there is no peak or signal in the spectra.

1-octyne (left) and 4-octyne (right)

Alkenes

Alkenes show many more peaks than alkanes. The principal peaks of diagnostic value are the C-H stretching peaks for the sp² carbon at value greater than 3000 cm^-1, along with C-H peaks for the sp³ carbon atoms. Also the C=C stretching peak near 1650 cm^-1, with higher intensity of cis- than trans- configuration.

cis-pentene (above) and trans-pentene (below)

O-H and N-H stretching

The signals for O-H and N-H stretching occur around 3300 cm^-1, but they look different. O-H stretching broad bands centering between 3400 and 3300 cm^-1. In solution, it will also be possible to observe a free O-H stretching band at about 3600 cm^-1 (sharp and weaker) to the left of the hydrogen bonded O-H peak.

1-butanol

Meanwhile, primary amines, R-NH₂, show two N-H stretching bands in the range 3500-3300 cm^-1, whereas secondary amines, R₂N-H, show only one band in that region. Tertiary amines will not show an N-H stretch. Because of these features, it is easy to differentiate among primary, secondary, and tertiary amines by inspection of the N-H stretch region.

butylamine (above), dibutylamine (middle), and tributylamine (below)

Carbonyl stretching

The carbonyl group is present in aldehydes, ketones, acids, esters, amides, acid chlorides, and anhydrides. This group absorbs strongly in the range from 1850 to 1650 cm^-1 because of its large change in dipole moment. In figure below provides the normal bas value for the C=O stretching vibrations of the various functional groups. The C=O frequency of a ketone, which is approximately in the middle of the range, is usually considered the reference point for comparison of these values. In this section we will focus on aldehydes, ketones, acids, and esters.

Normal base value for the C=O stretching vibrations for carbonyl groups

Aldehydes show a very strong band for the carbony group that appear in the range of 1740-1725 cm^-1. A very important doublet can be observed in the C-H stretch regio for the aldehyde C-H near 2850 and 2750 cm^-1. The presence of this doublet allows aldehydes to be distinguished from other carbonyl-containing compounds. In the other sides, ketones show a very strong band for C=O group that appears in the range of 1720-1708 cm^-1.

Nonanal (left) and 2-nonanone (right)

Carboxylic acids show a very strong band for the C=O group that appears in the range of 1730-1700 cm^-1 and the O-H stretch appears in the spectrum as a very broad band extending from 3400 - 2400 cm^-1. This broad band centers on about 3000 cm^-1 and partially obsecures the C-H stretching bands. If the very broad O-H stretch band is seen, along with a C=O peak, it almost certainly indicates the compound is a carboxylic acid.

Nonanoic acid

Besides, there are variations of carbonyl compounds that can shift the C=O stretch frequency. The first one, there is conjugation of C=O with C=C, it lowers the stretching frequency to around 1680 cm^-1.

4-methyl pent-3-en-2-one (mesityl oxide)

The carbonyl amide absorbs at an even lower frequency, 1640-1680 cm^-1, but the carbonyl ester absorbs at higher frequency, 1730 - 1740 cm^-1.

Butyl propanoate (left) and N-butyl propanamide (right)

Besides that, carbonyl groups in small rings (5 carbons or less) absorb at an even higher frequency.

The C=O stretching vibrations for cyclic ketones

Carbon-Nitrogen Stretching

The C-N stretching absorbs around 1200 cm^-1, and as the bond stronger the C=N stretch absorbs around 1660 cm^-1 and is much stronger than the C=C absorption in the same region. The nitriles group absorb strongly just above 2200 cm^-1. The alkyne C=Csignal is much weaker and is just below 2200 cm^-1.

Octanenitrile

To summarise this section, we will see the approximation range of those signals in IR spectroscopy.

IR spectroscopy correlation chart