Spectroscopy refers to the interaction between electromagnetic (EM) radiation and matter as a function of its frequency or wavelength. From this, a number of quantitative and qualitative measurements defining aspects like sample concentrations, structure, chemical compositions etc, can be obtained depending on the spectroscopic technique and spectrum of EM radiation used. When EM radiation interacts with matter, its frequency can undergo absorption, diffraction, inelastic scattering, emission and impedance by the atoms of the material substance. In addition, the interaction increases the internal energy transitions of the atoms and depending on the type of EM radiation, it causes the atoms to rotate in the case of microwave radiation, vibrate for infrared, have electron excitation in case of visible light or undergo ionization with UV and x-ray.
Infrared spectrophotometry, also referred to as Infrared spectroscopy (IR spectroscopy) is one of the most useful techniques for structural and functional group analyses and it has been used widely since its inception to identify unknown substances. This technique utilizes the ability of atoms to absorb infrared frequencies that match their internal vibrational frequency leading to the generation of an absorption spectra specific to the chemical bonds within the sample under analysis.
The technique has found wide applicability in various fields that require identification of organic substances. In research science, IR spectroscopy has been used in chemistry labs to identify and elucidate the structure of chemical substances. In industry, it has been used in food and drug administration to investigate the presence of (restricted) substances and for quality control to analyze the purity of food and drugs. The technique has been used in the manufacturing industry to evaluate on-going reactions by measuring the appearance or disappearance of particular reactants or detect the formation of polymers. Infrared spectrophotometry has also been used in the art world to verify authenticity of prized art by testing paint pigments while in forensic science, it has been used to identify substances of interest from crime scenes.
Theoretical basis of Infrared Spectrophotometry
At temperatures above 0o Kelvin, chemical bonds between atoms are not static, they undergo various types of vibrations, referred to as vibrational modes, which alter the length and angle of bonds. The type and number of vibrations can range from single stretching vibrational mode seen in diatomic molecules to more complex vibrations found in non-linear polyatomic molecules.
The total number of possible vibrational modes in a molecule can be calculated for both linear and nonlinear molecules using the formula 3n-5 and 3n-6 respectively, where n is the number of atoms in the molecule. For example, a diatomic molecule like N2 has only one possible vibrational mode while H20, a nonlinear polyatomic molecule, has 3 types of vibrations. Not all molecular bonds are active in infrared spectrum, meaning, not all vibrational modes can be observed.
In infrared spectrophotometry, diatomic molecules that are symmetrical (homonuclear molecules) like N2, Cl2, and certain bonds in polyatomic molecules, like CO2, do not absorb infrared frequency and therefore have no IR spectrum. This means that one might observe less number of absorption spectrum bands compared to the predicted (by calculation) vibrational mode in a molecule. This is because for a molecular bond to be ‘’Infrared active’’, the vibrational mode must involve change in the dipole moment.
Why do Molecules Only Absorb IR Frequency Resonant to Their Internal Vibrational Frequencies?
When atoms in a molecule absorb EM radiation, they do so in discrete quantities of energy or quanta. Different frequencies of the EM radiation have different levels of energy. The frequencies within the IR spectrum have just enough energy to cause molecules to vibrate. The lowest vibrational state of a molecule is the ground vibrational state while vibrational states higher than this are called the excited vibrational states. Molecules will only absorb energy of the frequency that is equivalent to the energy gap required to move them from one vibrational state to the next. The energy difference between the various vibrational states depends on the bond strength and mass of the elements in the bond. This is why the IR absorption spectra is specific for a particular functional group.
Therefore, what we observe in infrared spectrophotometry is the energy changes required to excite a molecule from the ground vibrational state to subsequent excited vibrational states.
Types of vibrations
The type of vibrations or vibrational modes refer to the changes in the position of the atoms making up the bond. Vibrational modes are divided into two main categories namely, stretching and bending, as follows;
This involves changes in the bond length between the atoms. The atoms can either move closer to each other, thereby shortening the bond, or further apart.
Stretching can be subdivided into;
Symmetric stretching: where two atoms simultaneously move toward or away from a central atom.
Asymmetric stretching: where the two atoms, joined to a central atom, move in different directions.
This type of vibration refers to the changes in the ‘angle’ between two bonds and can further be subdivided into;
Rocking: where two atoms move either clockwise or anticlockwise on the same plane.
Scissoring: where two atoms are moving towards or away from each other on the same plane.
Twisting: when the two atoms are moving out of the plane, where one moves forward and the other backward.
Wagging: an out of plane vibration where the atoms move simultaneously away and toward each other in a v-shape.
Classification of IR regions
The most common classification of the IR spectrum divides it into three IR regions named in relation to their distance from the visible light spectrum as follows;
The near IR region: This is the range bordering the visible light region, it has the highest energy and the shortest wavelength of the three regions. It traverses the 14000-4000 cm-1 (wavenumber,ῦ ).
The mid IR region: This has the range of 4000-400 cm -1 and is the region where most organic substances absorb IR radiation. It is further divided into two ranges – the fingerprint region at 400-1400 cm-1 and the functional group region at 1400-4000 cm-1.
The far IR region: This borders the microwave spectrum and is at the range of 400-10 cm-1. This region is useful for analysis of inorganic substances and gases because this is the frequency range where their fundamental vibrations occur.