Introduction:

The Vibrational transitions in molecules cause absorption in the infrared region of the electromagnetic spectrum. The entire substances absorb infrared radiation. A simple description of this absorption method can be obtained via picturing the molecules making the substance as solid balls representing the atoms joined by springs representing bonds. This structure will be in a state of perpetual wobbly motion, this movement taking the form of the bonds stretching, contracting, bending and twisting. The energy of absorbed radiation is dissipated in the molecule via increasing the intensity of this molecular movement. This is possible to relate the wavelength of the absorption to a particular bond.

Principles of Infrared Spectroscopy:

Electromagnetic radiation ranging between 400 cm^-1 and 4000 cm^-1 (2500 and 20000 nm) is passed via a sample and is absorbed through the bonds of the molecules in the sample causing them to stretch and bend. The wavelength of radiation absorbed is characteristic of the bond absorbing it.

The infrared region is as illustrated in table shown below.

Table: Infrared Ranges

The middle infrared range is generally employed for structural confirmation, however near-infrared Spectrophotometry that has been employed for many years to control products like flour and animal feed, is finding increasing applications in quality control in the pharmaceutical industry.

The rules regulating transitions in the infrared region of the spectrum needs that, in order to absorb, the dipole moment of the molecule should change throughout vibration. These vibrations are stated to be IR active.

In order for the electrical component in electromagnetic radiation to interact by a bond, a bond should encompass a dipole. Therefore symmetrical bonds, like those in O₂ or N₂, don't absorb infrared radiation. The electrical field related by electromagnetic radiation will interact by the molecule to change its electrical properties. Several molecules (example: HCl) encompass a dipole moment due to charge separation and will interact by the field. Others might acquire a dipole whenever they vibrate.

For illustration, methane, CH₄ has no dipole, however when one of the CH bonds stretches, the molecule will build up a temporary dipole. Even if the molecule doesn't encompass a dipole, the electric field, E, might distort the electron distribution and polarize the molecule. The majority of organic molecules have plenty of asymmetry. Even in the small organic molecules the modes of vibration are complex. This is described by the vibrational modes which can take place in methylene group (figure shown below). 

Fig: Vibration modes of a Methylene Group

The huge numbers of bonds in polyatomic molecules signify that the data obtained through IR analysis is very complex and gives a unique 'fingerprint' identity for the molecule. Quite a lot of structural information can be obtained from the IR spectrum, however even with the modern instrumentation it is not possible to fully 'unscramble' the complex absorbance patterns present in the IR spectra.

Factors determining the Intensity and Energy Level of Absorption in IR-Spectroscopy

Intensity of Absorption:

1) The intensity by which a bond absorbs radiation based on its dipole moment. Therefore the order of intensity of absorption for the given C-X bond is:

C-O > C-Cl > C-C-OH > C-C-H

Likewise:

OH > NH > CH

2) The intensity based on the relative electronegativity of the atoms comprised in the bond.

3) The intensity of stretching of carbon-carbon double bonds is increased if they are conjugated in a polar double bond. The order of intensity is as shown below:

C=C-C=O > C=C-C=C > C=C-C-

Energy Level of Absorption:

The equation which finds out the energy level of vibration of a bond is illustrated below:

Evib α √k/µ

'k' is a constant associated to the strength of the bond, example: double bonds are stronger as compare to the single bonds and thus absorb at a higher energy than single bonds. 'µ' is associated to the ratio of the masses of the atoms linked by the bond.

µ = (m₁m₂)/(m₁ + m₂)

For illustration: 

For O-H bonds,

µ = (16 x 1)/17 = 0.94 for C-O bonds,

µ = (12 x 16)/28 = 6.86

Here, m₁ and m₂ are the masses of the atoms comprised in the bond. According to the µ term, the highest energy bonds are the X-H (OH, NH and CH). The order of energy absorption for some common bonds is as follows, that reflects 'µ' and the strength of the bonds:

O-H  >  N-H  >  C-H  > C= C  >  C=O  >  C=C  >  C-F  >  C-Cl

Instrumentation of Infrared Spectroscopy:

Two kinds of instrument are generally employed for obtaining IR spectra: dispersive instruments that make use of a Monochromator to choose each wave number in turn in order to monitor its intensity after the radiation has passed via the sample and Fourier transform instruments that make use of an interferometer. The latter produces a radiation source in which individual wave numbers can be monitored in a ca 1 sec. pulse of radiation without dispersion being needed. In recent years, Fourier transform instruments have become very common. A simple diagram of continuous wave instrument is illustrated in figure shown below. The actual arrangement of the optics is much more complex than this however the diagram exhibits the necessary component parts for a dispersive IR instrument. The filament employed is made up of metal oxides example: zirconium, yttrium and thorium oxides and is heated to incandescence in air.

Fig: Continuous Wave IR Instrument 

The sample is contained in different ways in discs or cells made up of alkali metal halides.  Once the light has passed via the sample, it is dispersed in such a way that an individual wave number or small number of wave numbers can be monitored in turn via the detector across the range of the spectrum.

Fig: Michelson Interferometer used in FT-IR Instruments

Therefore generating an interferogram that can be transformed by using an equation termed as the 'Fourier transform' in order to extract the spectrum from a series of overlapping frequencies.  The benefit of this method is that a complete spectral scan can be acquired in around 1 second, compared by the 2-3 minutes needed for a dispersive instrument to obtain a spectrum. As well, due to the instrument is joined to a computer; some of the spectral scans can be taken and averaged in order to enhance the signal-noise ratio for the spectrum.

Sample Preparation:

=> Gases:

Gases are generally present in the lower concentrations as compared to pure solids and liquids (example: 0.04 M for nitrogen in air, 17.4 M for liquid ethanol), longer path lengths are needed. The gas-phase spectrum of HCl at 0.2 atm might be studied in a 10 cm glass cell having NaCl windows.

Low concentrations of exhaust gases might require a 10 m cell that reflects the IR beam to accomplish the long path length.

=> Liquids:

These are more concentrated and might be studied directly as a thin film between the NaCl plates.  For more quantitative work, precisely prepared solutions in solvents which don't absorb in the region of analytical interest like CCl₄ or CS₂ in the NaCl cells, having a known path length given by a space, might be used. Most of these are as well applicable to NIR, and short path length silica cells might as well be employed.

=> Solids:

Whenever a solid organic powdered sample is positioned in an IR beam, the particles scatter the light, and light is transmitted. Thus, for routine analysis, the sample is generally ground to a fine powder and mixed by paraffin oil (Nujol) to form a paste or mull. This decreases the scattering at the powder surface and provides a food spectrum, with the drawback that the bands due to the oil (at around 2900, 1450, 1380, 750 cm^-1) are superimposed on the spectrum. Alternatively, the fine powder KBr and the mix pressed in the hydraulic press between smooth stainless steel dies to provide a clear KBr disk. Solutions of solids might as well be employed and tetrachloromethane, CCl₄, is often employed as solvent, as it consists of few IR-active bands, mostly at the low wave number end of the spectrum. These should be ignored whenever the spectrum is interpreted. Thin films of solids like polymers might be supported directly in the IR beam. Polystyrene is a helpful calibration sample to check the performance of an IR spectrometer.

A more recent growth in the sample preparation is the use of reflectance spectra.

Reflectance Spectra:

Reflectance spectra can be assessed in three ways. A powder put in the incident beam and allowed to interact through diffuse reflectance. The reflections are collected through a mirror. If the beam is reflected off a flat sample surface, specular reflectance results and this might provide a good spectrum.   

If the sample is positioned in good contact with the surface of an optical device of high refractive index (like a prism of KRS-5) and illuminated via the prism by IR, the beam passes into the layers in contact and is attenuated before being completely internally reflected by the system. This is termed as attenuated total reflectance or ATR. If the beam interacts several times, then we encompass multiple internal reflectances (MIR) and if the surface is horizontal, which is a benefit in setting up the sample and then it is horizontal attenuated total reflectance (HATR).

It must be noted that the detail of spectra obtained via reflectance methods might be different from that obtained in solution or with KBr disk methods. Modern instruments possess software to transform reflectance spectra to resemble the more usual transmission spectra.

Analysts frequently deal by samples of extremely small size or analyze a small area of a large sample. One method is to reduce the size of the IR beam by using a beam-condensing accessory. A more versatile modern development is the IR microscope.

The Infrared Spectrum:

The infrared spectrum of a compound is generally represented as a plot of transmittance against wave number, the reciprocal of wavelength. Absorptions are recorded as the downward peaks.  (The spectrum is generally taken by employing a dilute solution of the compound in an appropriate non-aqueous solvent or a solid solution in potassium bromide or a nujol mull. Spectra might as well be taken in the form of liquid films and the vapor state. These different sampling methods can influence the appearance of the spectrum of a specific compound. Spectra taken in non-polar solvents like tetrachloromethane and alcohol free trichloromethane are preferred as fewer intermolecular forces are found in these solutions. As an outcome, the resolution of the spectrum will be better, that is, it will encompass sharper and better defined peaks.  Intermolecular forces like hydrogen bonding, tends to widen the absorption peaks and in several cases the result is a broad absorption band instead of a narrow peak in the spectrum.

Note the differences between the spectra of each and every compound and as well the 'dead pan peaks' (A) where the interaction of the absorptions of the solvent and compound yields in no signals reaching the recorder's pen.

Note as well the peaks due to the nujol at 2850-2950 cm^-1 (strong), 1460 and 1370 cm^-1 (weak).

Interpretation of Infrared Spectra:

Most of the functional groups absorb at characteristic wavelengths in the infrared region of the electromagnetic spectrum. The positions of such absorptions exhibit little variation with change in the molecular environment and therefore can be employed in reverse to recognize the presence of a functional group in a molecule. The initial interpretation of a spectrum is made by using correlation tables from text-books. Such tables are of a general nature and are not likely to have been compiled under the similar conditions that an investigator would make use of to run a spectrum in the laboratory. A functional group might give mount to a peak in the spectrum that is at a significantly different wave number from that recorded for that structure in the correlation table. This should be borne in mind whenever interpreting spectra.

This is not feasible to interpret all the peaks in a spectrum form tables of this kind however with practice it is possible to pick out the key ones and relate them to functional groups in the molecule. Carbonyl groups for illustration, show strong absorptions in 1600 to 1780 cm^-1region. Further examination of the spectrum might let one to speculate further on the exact nature of this carbonyl group. Aldehydes, for illustration, encompass a C-H stretching absorption at around 2700 to 2900 cm^-1; ketones and esters don't absorb in this region even as acids and amides encompass wide O-H stretching absorption bands in the 2700 to 3600 cm^-1region. Most of the functional groups can be detected through this process however deductions of this nature must be backed up by other evidence like chemical tests and other forms of spectroscopy. A more detailed interpretation of the spectrum can be obtained via consulting specialized tables of absorptions for the specific kind of structure being studied.

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