Coupon Accepted Successfully!


Vibrational Modes

Chemical bonds have characteristic natural frequencies with which they vibrate (bending and stretching) in certain specific energy levels. The special range of frequencies used in IR spectrometry is within the range of the natural vibrational frequencies of the bonds in organic compounds. By looking at the peaks, we can tell whether a particular functional group is present or absent in the compound that we are analyzing. The bonds present in the molecules of the sample vibrate when they absorb characteristic frequencies.
Molecules are always in a state of vibration. The bonds present in molecules have their own unique stretching and bending modes. Let’s consider a hypothetical molecule A which has n atoms per molecule. Then, theoretically molecule A will have 3n β€“ 6 modes of vibration. But this does not mean that the vibrational modes strictly follow this rule. Some of the vibrational modes have frequencies that do not coincide with the frequencies of the IR that we use. We will consider the vibrational modes of the methylene group.


The stretching vibrational modes of methylene group. The two drawings show stretching vibrations – symmetric (on the left) and asymmetric on the right.


The bending vibrational modes of methylene group The top two diagrams from the left are scissoring and rocking modes. The bottom two diagrams show wagging and twisting modes.
Besides the fundamental vibrational frequencies, overtones and combination tones can appear in the IR spectrum. For example, the overtone of a carbonyl group can appear close to 3400cm-1 (the carbonyl peak is close to 1700 cm-1). Combination frequencies are peaks that appear when two frequencies intermingle, resulting in two weak frequencies.



Analyze the IR diagram shown below.




The given IR spectrum is that of phenol. Look at some of the important peaks that are present in this IR spectrum of phenol.


1) The peak 1 around 3400 cm-1 represents the peak caused by the -OH group.
2) The peak 2 at 3050 cm-1 represents the aromatic -CH.
3) The peak 3 denotes the benzene ring stretch.






In the IR spectrum shown above, identify the most likely peak which represents the carbonyl group.



The carbonyl peak is roughly around 1700 cm-1. So Peak 2 indicated in the IR diagram is the correct answer.

Infrared (IR) spectroscopy

Infrared spectroscopy is a widely used tool to identify and analyze compounds. As you know, the infrared radiation is part of the electromagnetic spectrum. The IR has wavelengths between those of the visible light and the microwaves. From the analytical point of view, only a portion of the entire infrared range is actually useful for the spectroscopic analysis.

In order to analyze organic compounds, certain properties at the molecular level are put to use in the infrared spectroscopy. In this technique, the infrared light is passed through organic compound samples. By doing this, some of the frequencies are absorbed by the sample and the other frequencies simply pass through. The percentage absorbance or transmittance is plotted against the frequency or, to be more precise, the wave number. The wave number (cm-1) is the reciprocal of wavelength expressed in centimeters. The plot can be used to analyze the structure of a compound by identifying the peaks that are characteristic to the groups present in the compound.


Example : The carbonyl group (>C=O) shows absorbance close to 1700 cm-1 in the IR spectrum. The peaks around this value indicate the presence of carbonyl group in the compound that is analyzed. By knowing the IR values, we can predict the structure of a compound. Our discussion from the MCAT point of view will be limited to some of the common and important IR peaks.


Other very complex IR diagrams can only be interpreted by skilled and experienced professionals.

Test Your Skills Now!
Take a Quiz now
Reviewer Name