Infrared Spectroscopy, Chemistry tutorial


Infrared spectroscopy is a spectroscopic process of analysis employed qualitatively to recognize and study chemicals and quantitavely to measure the concentration. Whenever a molecule interacts or absorbs radiation at the infrared region, it causes the molecule to experience vibrational transitions. Let us pause here to obtain a clearer picture or understanding of what happens whenever a molecule absorbs the radiation.

There are three fundamental methods via which a molecule can absorb the radiation: all involve increasing the molecule to a higher internal energy level, the increase in energy being equivalent to the energy of the absorbed radiation (hv). The three kinds of internal energy are quantized; that is, they exist at discrete levels. Primarily, the molecules rotates about different axes, the energy of rotation being at definite energy levels, therefore the molecule might absorb radiation and be increased to a higher rotational energy level. In a rotational transition, this kind of transition (that is, rotational transition) takes place whenever molecules absorb radiation at the far infrared region and microwave region of the electromagnetic spectrum. Secondly, the atoms or groups of atoms in a molecule vibrate relative to one other, and the energy of this vibration takes place at definite quantized levels. The molecule might then absorb a discrete amount of energy and be increased to a higher vibrational energy level, in a vibrational transition. This kind of transition takes place having absorption of close to infrared radiation. Third, the electrons of a molecule might be increased to higher electron energy, corresponding to an electronic transition. This kind of transition takes place at the ultraviolet and visible region. These transitions takes place only at definite wavelengths corresponding to energy equivalent to the difference of the discrete energy levels comprised in the transition.

The infrared part of the electromagnetic spectrum is generally categorized into three regions; the near-, mid- and far- infrared, The higher-energy near-IR, around 14000-4000 cm-1 (0.8 to 2.5 μm  wavelength) can excite overtone or harmonic vibrations. The mid-infrared, around 4000 - 400 cm-1 (2.5 - 25 μm) might be employed to study the basic vibrations and related rotational-vibrational structure. The far-infrared, around 400-10 cm-1 (25-1000 μm), lying adjacent to the microwave region, consists of low energy and might be employed for rotational spectroscopy. Many analytical applications are confined to the middle IR region as absorption of organic molecules is high in this region.

The procedure or technique of infrared spectroscopy employs an instrument termed as an infrared spectrometer (or spectrophotometer) to generate an infrared spectrum. The fundamental IR spectrum is necessarily a graph of infrared light absorbance (or transmittance) on the vertical axis versus frequency or wavelength on the horizontal axis. Common units of frequency employed in IR spectra are reciprocal centimeters (at times termed as wave numbers), abbreviated as cm-1. The units of IR wavelength are generally given in microns, abbreviated as μm that are related to the wave numbers in a reciprocal manner.

Principle of Infrared Spectroscopy:

Whenever molecules absorb radiation at the infrared region, the energy of wavelength absorbed causes a vibrational transition whenever the energy absorbed is equivalent to the quantized jump in the internal energy that is, the energy difference between the vibrational energy levels of the atoms or group of atoms in the molecule. The vibrational energy of the atoms or groups of atoms in the molecule is increased to a higher vibrational energy. The energy absorbed appears as absorption peaks at the wavelength it corresponds to the IR spectrum.

Not all the molecules can absorb in the infrared region. For absorption to take place there should be change in the dipole moment (or polarity) of the molecule. The diatomic molecule must encompass a permanent dipole (that is, polar covalent bond in which a shared pair of electrons is shared unequally) in order to absorb.

Various functional groups absorb features frequencies of IR radiation. Therefore provides the characteristic peak value. Thus IR spectrum of a chemical substance is a finger print of the molecule for its recognition.

Types of Vibrations:

In an organic molecule there are two main kinds of fundamental vibrations. Such are stretching and bending vibrations. The stretching vibration could either be symmetrical or asymmetrical. Bending vibration is of four various kinds namely scissoring, rocking, wagging and twisting. The energy needed to bend a bond is not great and falls in the range of 400 to 1300 cm-1. This region is known as the finger print region since absorption in this region is extremely dependent on the molecular environment. Therefore, this region is employed to set up the identity of the chemical compounds. The energy needed to stretch a bond is a little bit higher. This falls in the region of 1300 to 4000 cm-1. Absorption in this region is mainly caused due to functional groups and is independent of other parts of the molecule and is employed to detect the functional groups in the molecules.

Group frequencies:

Group frequencies are the absorption bands or signals which take place at certain frequencies because of stretching or bending vibration in a molecule. For illustration, the bands at 3300cm-1 and 1050cm-1 are features of the OH group in alcohols.

Correlation of structure and frequency:

Most of the thousands of infrared spectra have been recorded and from these, it has been possible to empirically tabulate the correlations between absorption frequencies and kinds of bonds or chemical groups. The table described below summaries some of the correlations for different kinds of vibrations.

Table: Group Frequencies

Vibration          Type of molecule        Group frequencies (cm-1)

C - H stretch       Alkanes, alcohols                2800 - 3000

C - H stretch           Aldehydes                       2700 - 2900

C - H stretch            Alkenes                          3010 - 3095

O - H stretch       Alcohols, phenols                3200 - 3600

O - H stretch             Acids                            2500 - 3000

O - H bend        Alcohol, phenol                   1260 - 1410

N - H stretch           Amines                           3300 - 3500

C = C stretch           Alkenes                           1620 - 1680

C = O stretch         Aldehydes                          1720 - 1740

C = C stretch          Alkynes                             2100 - 2140

C = N stretch          Nitriles                              2000 - 2500

C = O stretch          Ketones                            1705 - 1725

IR Spectroscopy Experimental Procedure:

The infrared spectrum of a sample is recorded via passing a beam of infrared light via the sample. Whenever the frequency of the IR is similar as the vibrational frequency of a bond, absorption takes place. The energy absorbed appears as absorption peaks at the wavelength it corresponds to on the IR spectrum. A fundamental IR spectrum is necessarily a graph of infrared light absorbance (or transmittance) on the vertical axis versus frequency or wavelength on the horizontal axis. Well resolved and sharp peak/peaks at wavelength/wavelengths it corresponds to, is/are matched by the wavelength range on the group frequency table to recognize the functional group/groups present.

The analysis of a sample via IR spectroscopy comprises:

Sample preparation:

Gaseous samples need a sample cell having a long path length to compensate for the diluteness. The path length of the sample cell based on the concentration of the compound of interest. A simple glass tube having length of 5 to 10 cm equipped by infrared-transparent windows at the both ends of the tube can be employed for concentrations down to some hundred ppm. Sample gas concentrations well below ppm can be evaluated by a white's cell in which the infrared light is guided by mirrors to travel via the gas. White's cells are available by optical path length beginning from 0.5 m up to hundred meters.

The liquid samples can be sandwiched between the two plates of a salt (generally sodium chloride, or common salt, however a number of other salts like potassium bromide or calcium fluoride are as well employed).The plates are transparent (don't absorb in the IR region) to the infrared light and don't introduce any lines to the spectra.

Solid samples can be made up in a variety of ways. One general process is to crush the sample by an oily mulling agent (generally Nujol) in a marble or agate mortar, having a pestle. A thin film of the mull is smeared to salt plates and measured. The second procedure is to grind a quantity of the sample by a specially purified salt (generally potassium bromide) finely (to take out scattering effects from the large crystals). This powder mixture is then pressed in a mechanical press to make a translucent pellet via which the beam of the spectrometer can pass. A third method is the 'cast film' method that is employed mostly for polymeric materials. The sample is primarily dissolved in an appropriate, non hygroscopic solvent. A drop of this solution is deposited on the surface of KBr or NaCl cell. The solution is then evaporated to dryness and the film made on the cell is analyzed directly.

Simple spectra are obtained from the samples having a few IR active bonds and high levels of purity. More complex molecular structures lead to more absorption bands and more complex spectra.

Comparing to a reference:

To take the infrared spectrum of a sample, it is essential to evaluate both the sample and a 'reference' (or control). This is due to each measurement is influenced by not just the light absorption properties of the sample, however as well the properties of the instrument (for illustration, what light source is employed, what infrared detector is employed and so on). The reference measurement makes it possible to remove the instrument influence. Mathematically, the sample transmission spectrum is categorized by the reference transmission spectrum.

The suitable 'reference' based on the measurement and its goal. The simplest reference measurement is to simply take out the sample (replacing it by air). Though, at times a different reference is more helpful. For illustration, if the sample is a dilute solute dissolved in water in a beaker, then a good reference measurement may be to measure pure water in the similar beaker. Then the reference measurement would cancel out not just all the instrumental properties (such as what light source is employed), however as well the light-absorbing and light-reflecting properties of the water and beaker, and the final outcome would just exhibit the properties of the solute (at least roughly).

A general way to compare to a reference is sequentially: first measure the reference, then substitute the reference by the sample and measure the sample.

Measuring IR absorption bands:

As by electronic (uv-visible) spectra, the utilization of infrared spectra for quantitative determinations based on measuring the intensity of either the transmission or absorption of the infrared radiation at a particular wavelength, generally the maximum of a strong, sharp, narrow, well-resolved absorption band. Most of the organic compounds will have some peaks in their spectra that satisfy such criteria and that can be employed so long as there is no substantial overlap by the absorption peaks from other substances in the simple matrix.

The background to any spectrum doesn't generally correspond to a 100 percent transmittance at all wavelengths; therefore measurements are best made by what is termed as the baseline process. This comprises choosing an absorption peak to which a tangential line can be drawn. This is then employed to establish a value for Io by measuring vertically from the tangent via the peak to the wave number scale. Likewise, a value for 'I' is obtained via measuring the corresponding distance from the absorption peak maximum. Therefore for any peak, the absorbance will not be the value corresponding to the height of the absorption, evaluated from the horizontal axis of the chart paper; rather than it will be the value of Acalc gets from the equation:

Acalc = log (1/T) = Io/I

Here, Io and I are values measured by employing the tangential baseline.

This method has the great benefit which some potential sources of error are removed. The measurements don't based on accurate wavelength positions as they are made with respect to the spectrum itself, and any cell errors are ignored by using the similar cell of fixed path length. Measuring the Acalc removes any variation in the source intensity, the instrument optics or the sensitivity.

Beer's law: Quantitative IR spectra

Infrared spectra are recorded by using either or both absorbance (A) and percentage transmittance (T) merely as they are in visible ultraviolet electronic spectra and Beer's law,

A = εcl = log (1/T) = log (Io/I) applies equivalently to infrared spectra as it does to electronic spectra.

Use of a calibration graph:

The calibration curve overcomes any problems made because of non-linear absorbance or concentration characteristics, and it signifies that any unknown concentration run beneath the same conditions as the sequence of standards can be found out from the graph. The method needs that all the standards and samples are computed in the similar cell of fixed path length, however the dimensions of the cells and the molar absorptivity for the selected absorption band are not required; they are constant for all the measurements.

Experiment: Identification of Functional groups of an unknown substance

Purpose: This experiment comprises the instrumental method to get the spectrum and also the analysis of the spectral data to find out the functional groups present in the unknown sample.


A critical portion of the infrared experiment is getting the infrared radiation to interact by the sample devoid of losing an important part of the infrared radiation from non-sample interactions (that is, mirrors which absorb light instead of reflect, scratches on any optical surface that reflect light in the wrong direction, sample surfaces that reflect light and so on). Classically, liquids are analyzed either neat [that is, suspended among the two sodium chloride plates (that is, sodium chloride doesn't absorb infrared radiation in the spectral range of concern)] or in solution in a solvent like carbon tetrachloride that doesn't absorb too much infrared radiation in the spectral range of concern. Solids, having problems related by crystal surfaces reflecting away most of the radiation, are evaluated either in solution (that is, at least the few solids which dissolved in appropriate IR solvents) or more generally as a mull, a KBr pellet, or a melt (that is, if the melting point was low adequate). Such sampling methods are essential to give adequate sample for the classical dispersive optical-null double-beam prism and (later) grating spectrometers. Though, the ready availability of low cost powerful laboratory calculating has allowed the routine utilization of more sensitive non-dispersive Fourier Transform Infrared Spectrometers (FT-IR). FT-IR lets for the collection of all the spectral data in seconds compared to 3 to 10 minutes for the classical grating spectrometer. Though, FT-IR needs powerful computing to mathematically examine the collected data. Computers as well let for the collection of numerous spectra in a short time and the averaging of the spectra to remove most arbitrary noise.

Experimental Procedure:

Generally you will not have to run a 'background'. (A background evaluates the quantity of energy which in reality gets to the detector devoid of any sample in the sampling device. The energy reaching the detector is not constant at all wavelengths because of: absorption via spectrometer mirrors and windows; scattering via flaws, scratches and dirt; absorption through condensed compounds and the atmospheres; variable output via the infrared source; and so on. The computer in spectrometer  subtracts  the  background  from  your  sample  data  to  produce  the  spectrum). Though, if you are the first person to utilize the spectrometer for the day, or if the spectrometer has been run for quite awhile, or if your spectrum is problematic, you must run a background.

Getting a spectrum of a liquid by using salt plate:

a) Get the salt plates, holder and O-rings from the desicator.

b) Clean the salt plates via wiping by a Kim-wipe moistened with absolute alcohol. Be very careful not to touch the surface of the salt plates by your fingers. (Your fingers are wet adequate to dissolve the salt).

c) Place an O-ring on the salt plate holder.

d) Put a clean salt plate on the O-ring. Again, don't touch the flat surface of the salt plate.

e) Put a drop or two of a dry liquid sample on the salt plate.

f) Put the other clean salt plate on top of the sample. Ensure that there are no air bubbles in the sample.

g) Put the other O-ring on the upper salt plate.

h) Put the other metal holder on the O-ring.

i) Lightly tighten the four knurled nuts. If you tighten very hard you might crack the salt plates or squeeze your entire sample out from between the salt plates.

j) Put the holder in the spectrometer. Get your spectrum. Print a hard copy and save your spectrum on the network.

k) Disassemble the sample holder and clean the salt plates again via wiping by a Kim-wipe moistened by absolute alcohol.

l) Return the clean salt plates in their container to the desicator. As well, return the salt plate holder and O-rings to the desicator.

From the IR spectrum, find out the functional group of your unknown.

Experiment: Determination of Concentration of Cyclohexane Using IR Spectroscopy

Purpose: To find out the concentration of cyclohexane by using IR spectroscopy

Experimental Procedure:

Run infrared spectra for the pure cyclohexane and pure nitro methane. From the spectra choose a cyclohexane absorption that is not influenced by, or overlapping by, those of the nitro methane. Make a sequence of solutions of known concentrations of cyclohexane in nitro methane covering the range from 0% to 20% (w/v). By using a cell of fixed path length 0.1 mm, evaluate the absorbance for the solutions at the selected peak absorption by employing the baseline process and plot the calibration graph. Make use of this graph to find out the unknown concentration of cyclohexane in the sample.

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