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IR Spectroscopy

Page history last edited by Ben Levin 13 years, 4 months ago

Infrared Spectroscopy

Group B: Ben Levin, Patrick Kurecka, William Rasmussen, Andy Lin

               Infrared spectroscopy has become an indispensible tool in analytical chemistry. It is a nondestructive technique used to detect functional groups in substances. Examples of its uses include identification of organic and inorganic compounds, determination of functional groups, quantitative determination of compounds in mixtures, and classification of isomers (both structural isomers and stereoisomers). In addition to providing a great deal of data, it is a very simple and quick technique to perform: It takes less than ten minutes for the machine to run, and preparation of the compound is as simple as grinding up a solid or dissolving it into a solvent, while liquids and gases are simply put into the machine. The utility of this technique is based on quantum mechanical theory, specifically, on the quantization of vibrational energies in chemical bonds.


From a classical perspective, a bond can be thought of as two masses on the end of a spring. This is due to the Coulombic attractive and repulsive forces between the nuclei and electrons that bond them. If the nuclei move too far from their rest position, the attractive forces between the positive nuclei and the negative electrons of the bond will serve to pull the nuclei closer together. However, if the atoms move too close together, the repulsive interactions between the two positively charged nuclei will force them apart. In this manner, any displacement from the equilibrium bond length will cause a restorative force to move the nuclei back toward their initial positions.

Bonds can absorb energy from electromagnetic radiation. This will result in the bond becoming excited, which is analogous to the masses on the spring vibrating harder and faster. The energy from the electromagnetic radiation essentially goes into increasing the vibrational frequency. The energy levels are described by the equation below, where ω=kμomega is equal to the square root of the spring constant divided by the reduced mass. The spring constant can be thought of as the stiffness of the spring. A spring that is more stiff is harder to compress and will respond less to excitation from electromagnetic radiation. The stiffness is determined by the strength of the bond, or in other words how strongly the two atoms are held together. Atoms that have a higher bond order, evidenced by a double or triple bond, are more strongly bound together and thus have a higher spring constant. The reduced mass is discussed later during the section on the isotope effect. This equation has important effects on the nature of vibrational energy states. First, the vibrational energy must be quantized, because there are specific energy differences between the levels. This result arises from quantum mechanics. It is not possible to absorb half an energy level’s worth of energy: a bond must absorb a wave that has energy exactly equal to the spacing between its energy levels. This means that only photons of specific energies can be absorbed


IR spectroscopy exploits the differences in vibrational frequencies between two molecules to create a distinctive spectrum. As every different bond will have either different atoms or a different bond strength, the frequency of light that will excite the bond will vary from bond to bond, allowing us to determine what bonds are present in a molecule through IR analysis.

First, the material to be analyzed is put into an IR spectrometer (solids are often either dissolved in a solvent or ground with KBr for better resolution). Then the spectrometer emits electromagnetic radiation of varying frequencies, which pass through the sample of compound. The compound will absorb only those frequencies that correspond to the vibrational energy levels of the compound. After the molecule absorbs energy, it will quickly release that energy. The released energy will be of the same frequency, and so the frequency of the bond oscillations can be recorded. This is the principle behind all IR spectrometers. There are two types of spectrometers: Dispersive spectrometers are older and go through each frequency separately, but the more common Fourier transform spectrometers release all of the different frequencies simultaneously. The name “Fourier transform” is derived from the mathematical operation that is used to separate all of the frequencies and to obtain the spectra.

The Isotope Effect:

If you model the bond between two atoms as a spring, you can calculate its vibrational frequency with this equation.


The spring constant (k), which is different for each bond, is a measure of how hard it is to stretch said bond. The reduced mass (μ) is a mathematical concept used in two-body problems which represents the mass of the system if the two masses were to be considered as one object, greatly simplifying the math needed to solve the problem. The reduced mass is calculated by multiplying the two masses together and dividing it by the sum of the two masses:


Each element can have several different isotopes, based on the number of neutrons it has. For example, oxygen has three stable isotopes with masses of 16 amu (8 neutrons), 17 amu (9 neutrons), and 18 amu (10 neutrons). The mass of each isotope will affect the frequency at which the bond will stretch. Generally the heavier the isotope the harder it is to stretch the bond. This is the isotope affect. As a result of differing isotopes, depending on which isotope is in the sample, the reduced mass will be affected. This in turn affects the frequency at which this bond will vibrate and ultimately changes how the spectra look. One specific example is that the vibrational frequency of two O-16 atoms is 832 and for two O-18 atoms it is 788.


               The isotope effect in infrared spectroscopy has numerous analytical uses, and it can tell us about much more than the mere structural makeup of a given compound. Isotopes are easy to detect, since their presence is indicated by a clear shift in the absorption peaks of the bonds within which they are present, providing us with a useful analytical tool. In particular isotope effects can be used to help us understand how chemical reactions progress, by “tagging” certain atoms or bonds with isotopes and tracing their fates with IR analysis. In reactions involving large organic molecules, for example, it can be difficult to trace the fates of every single atom and bond. IR spectroscopy can only give us clues as to the structure of the reactants and products, since one cannot very well place a running reaction into an IR spectrometer. Isotope effects can give us clues as to what bonds break, which are formed, and where each atom goes by controlling the isotopic composition of the reagents.

Take for example the combustion of glucose, in which glucose and oxygen are converted into carbon dioxide and water:

C6H12O6+ 6O26CO2+ 6H2O

Imagine a scientist wanted to determine the fate of the oxygen consumed during the reaction. To do so, the scientist could combust 16O-glucose in an atmosphere containing only 18O2gas. The scientist knows already that all the oxygen gas cannot end up in water, since 12 gaseous oxygen atoms are consumed and only six end up in water. This leaves two possibilities: either all the gaseous 18O is incorporated into CO2, or some ends up in both CO2and H2O. Therefore, all the scientist has to do is take the IR absorption spectrum of the water produced – if the 18O2is incorporated exclusively into CO2, the single absorption peak for the water should be consistent with the 16O-H bond. If the oxygen in the H2O is derived exclusively from the 18O2, there should be a single 18O-H peak. If the oxygen in the H2O is derived from both the 16O-glucose and the 18O2, there will be a mixed peak.

To sum everything up, IR spectroscopy is based on quantized vibrational energy levels. Bonds are modeled as a spring. When IR light is put onto bonds, the bonds increase in vibrational frequency. This can be recorded and features, such as bond strength, can be determined. As this is being modeled as a spring, different isotopes of the atoms can be used to provide more information, as they will have different masses. In summary, IR spectroscopy is a very powerful method of chemical analysis that only works because of the quantum nature of matter and energy.




Oxtoby, David W. Principles of Modern Chemistry. 6thEdition. Brooks/Cole: 2007.


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