Shining Light on Molecules: The Magic of Infrared Spectroscopy

Ever wondered how scientists can peek into the hidden world of molecules? Enter Infrared Spectroscopy, a cool technique that helps us understand the building blocks of stuff at a super tiny level.

Infrared spectroscopy is like using a special flashlight that we can’t see but shines on tiny things to make them reveal their secrets. Molecules, the tiniest bits of everything around us, respond to this invisible light by vibrating in specific ways. By watching these vibrations, scientists can figure out what’s in a substance, and how it’s put together, and even discover new things they didn’t know were there.

In this blog post, we’ll take a simple dive into the world of infrared spectroscopy. We’ll learn how it works, and how it helps us uncover hidden details in the molecules that make up our world. Join us as we shine a light on the invisible and make sense of the small stuff through the magic of infrared spectroscopy!

Principle of Infrared Spectroscopy

Infrared spectroscopy operates on the principle that molecules absorb infrared light at specific wavelengths, causing their atoms to vibrate. Molecules consist of atoms connected by bonds, and these bonds act like springs. When infrared light passes through a sample, it can make these bonds vibrate or stretch. The energy required for these vibrations corresponds to specific infrared wavelengths.

The instrument used in infrared spectroscopy measures which wavelengths are absorbed by the sample, creating a unique “fingerprint” for different compounds. By analyzing these absorption patterns, scientists can identify the types of bonds present and gain insights into the molecular structure of a substance.

The working principle of IR spectroscopy is based on Hooke’s law of simple harmonic motion which states that the strain of a substance is proportional to the applied stress within the elastic limit of that substance. It gives the frequency of motion as

𝜈 = 1/2πœ‹c * βˆšπœ…/πœ‡
πœ‡ = m₁mβ‚‚ / (m₁ + mβ‚‚)
where,Β 

𝜈 = frequency
c = speed of light
k = force constantΒ 
πœ‡ = reduced mass of individual atoms
m₁ and mβ‚‚ = mass of two atoms in a molecule

One essential factor in the vibrational transition of a molecule is the change in the dipole moment during the vibrations. That concludes that if there is no dipole moment then no vibration will occur in the IR region. The quantity produced by this is the vibrational energy which is given by:

E(vib) = [ v + ½] h𝜈
𝜟E(vib) = h𝜈
where,Β 

𝜟E(vib) = energy difference between two vibrational levels
v = number of vibrational levels
h = Planck’s constant
𝜈 = vibrational frequency of molecular bond

Instrumentation of Infrared Spectroscopy

An Infrared spectroscopy instrument is composed of similar components as in Ultraviolet-Visible spectroscopy but has few differences and modifications. A typical IR spectroscopy instrument is composed of the following components:

I. Radiation Source

The Infrared radiation sources are usually hot bodies similar to black bodies that continuously emit the radiation. IR spectroscopy uses the following sources:

  • Incandescent Lamps: In this lamp, a nichrome coil is used which can be raised to incandescence by resistive heating. A black oxide film formed on the coil gives acceptable emissivity. The maximum temperature can reach up to 1100 degrees Celsius. It produces less intense radiation but is a very reliable source.Β 
  • Nernst Glower: It is the most commonly used radiation source in IR spectroscopy. The oxides of zirconium, yttrium, and thorium are fused and molded in the form of hollow tubes or rods about 1-3 mm in diameter and 2-5 cm in length. The rod ends are cemented to short ceramic tubes for mounting and short platinum leads are provided for power connections.Β  It produces intense radiation and almost thrice that from nichrome wire and globar source.
  • Globar Source: A globar source is a rod of diameter 6-8 mm and length 50 mm, made up of sintered silicon carbide, enclosed in a water-cooled brass tube with a lot for emission of radiations. It is usually operated at 1300 degrees Celsius. One feature is that it is self-starting and is electrically heated.
  • Mercury Arc Lamp: A mercury arc is enclosed in a quartz jacket to reduce the loss of radiation. It produces radiation similar to the black body and is usually used for producing very far IR radiation.
  • Tungsten Filament: It produces radiation when a tungsten filament is heated. It is useful only for the near-infrared region.

II. Monochromators

The radiations emitted from the Infrared source are of varied frequencies, therefore a monochromator is installed so that only desired frequencies can pass through the sample. The two types of monochromators used in IR spectroscopy are:

  • Prism Monochromators: Prisms are very simple and have a greater range, therefore used as monochromators. Prisms made up of halogen salts are used in IR spectroscopy. Crystalline sodium chloride prisms are used for analytical work from 5-15 microns and are also adequate for 2.5 microns. Potassium bromide and cesium bromide are used for far IR regions. For near IR regions lithium fluoride is used.
  • Grating Monochromator: Grating is a series of parallel straight lines cut out into a plane surface (usually plastic or glass coated with aluminum). The gratings minimize the number of scattered radiations and concentrate them into a single order.

III. Sample Holder

Demountable sample cells made from the rock salt are used. Teflon spacers are used to adjust the path length. Since they are made up of alkali metals, they become foggy easily due to moisture. Therefore, regular polishing is required to render them useful again. 

IV. Detectors & Recorder

As the name suggests, detectors detect the radiation transmitted from the sample. The following detectors are used by Infrared spectroscopy:

  • Thermal Detectors: These detectors produce potential differences depending upon the amount of radiation. There are various kinds of thermal detectors which are as follows:
    • Thermocouple DetectorIt is constructed by joining the dissimilar metal stripes having different thermoelectric properties. These strips are welded with blackened gold foil. One welded joint is kept at a constant temperature, called the cold joint, and the other welded joint is exposed to radiation called the hot joint. The hot joint due to exposure causes a rise in temperature while there is no rise at the cold junction. Due to this difference in the temperatures at the two ends, there is a generation of potential difference that is used to produce signals.
    • Bolometers– It is a Wheatstone bridge based on the fact that the electrical resistance of a metal increases approximately 0.4% for every Celsius degree increase in temperature. When Infrared radiation falls on the metal, there is a change in the temperature which results in a change of resistance. This change in resistance is regarded as the measure of the amount of radiation that has fallen on the bolometer.
    • Thermistors– They are similar to bolometers but made up of metal oxides. Another difference is that in thermistors with the increase in temperature, the resistance is decreased.Β 
    • Golay Cell It is a small metallic cylinder closed by a blackened metal plate at one end and at the other end a flexible metalized diaphragm is used. Xenon gas is filled in the cylinder and the cylinder is sealed. When IR radiation falls on the blackened metal plate, the heat produced by it expands the gas-filled which results in the movement of the metallic diaphragm. The movement of the diaphragm results in the production of signals in the form of light reflected onto a photocell.
  • Photodetectors: These detectors detect the intensity of the light radiations from the sample. They are of the following types:
    • Photoconductivity Cell It is a detector that consists of a thin layer of lead sulfide or lead telluride supported on glass and enclosed into an evacuated glass envelope. When IR radiation falls on the lead telluride, its conductance increases and causes more current flow.Β 
    • Semiconductor Detectors Semiconductors such as doped germanium are used. When IR radiation falls on the detector, an electron is displaced in the detector, changing its conductivity greatly. This results in the production of signals which are easily recorded.

Sample Preparation For IR Spectroscopy

One of the most important processes in IR spectroscopy is the sample preparation of the analyte. There are various methods applied for the sampling of the analyte which depends on the state of the analyte. 

I. Solid Samples

The solid analytes are prepared by various methods such as:

  • Dissolving in a Solvent This is the simplest method in which the solid analyte is dissolved in appropriate solvents such as chloroform, carbon tetrachloride, alcohols, cyclohexane, acetone, and carbon sulfide.
  • Solid Film In this method the sample solution is placed on a potassium bromide or sodium chloride surface and allowed to evaporate. As a result, the sample forms a thin film on the surface.
  • Mull Technique The solid sample is mixed with heavy mineral oil called Nujol to form a paste. This paste is placed in between the two salt plates and then used for the spectral measurements.
  • Disk Method It is also known as the pressed pellet method, in which a small amount of finely ground solid sample is intimately mixed with 100 times its weight of powdered potassium bromide in a mortar pestle. This mixture is then pressed under a high pressure of 25000psi/g in an IR tablet press to form a transparent pellet.

Liquid Samples

Liquid samples are usually handled in their pure forms in a variety of absorption cells such as demountable cells, sandwich cells, or cavity cells. These cells are made up of sodium bromide, thallium bromide, or potassium bromide.Β 

Gas Samples

The dried gas sample is introduced into the gas cell made up of a glass or metal cylinder of about 10 cm long whose end walls are made up of sodium chloride.

Conclusion

Infrared Spectroscopy has proven itself as an indispensable tool for unraveling the secrets of molecular structures. From its foundational principles to its diverse applications, this technique has provided scientists with a unique perspective on the world of chemical compounds.

By deciphering the vibrational transitions within molecules, infrared spectroscopy continues to play a crucial role in diverse scientific fields. As technology advances, the future promises even more profound insights into the microscopic realm, solidifying the enduring significance of this analytical method in scientific exploration.

Suksham Gupta

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