Molecular Spectroscopy

Spectroscopy is the study of the spectra generated by matter when it emits or interacts with a source of electromagnetic radiation. Several types of spectroscopic techniques are used to study the different structures of atoms as well as molecules. The numerous wavelengths emitted by these particles helps in the investigation of their structures and electronic configurations. Molecular spectroscopy is a branch of spectroscopy that is utilised to access compositional information of samples. It also helps determine the structures of various compounds. This article unpacks the fundamentals of molecular spectroscopy along with its different types and applications.

What is meant by molecular spectroscopy?

Molecular spectroscopy involves the interaction between matter (molecules)and electromagnetic radiations to generate an absorption pattern or spectrum. This type of spectroscopy is utilised to gather information about the structure or composition of a compound.

The major difference between atomic and molecular spectroscopy is:

●  Atomic spectroscopy deals with electromagnetic radiations emitted or absorbed by atoms.

●  Molecular spectroscopy deals with electromagnetic radiations emitted or absorbed by molecules.

Molecular spectroscopy is a part of optical spectroscopy that deals with the emission and interaction of light by a particular substance.  

Fundamentals of molecular spectroscopy

●  Molecular spectroscopy deals with the interaction of molecules with electromagnetic radiations. Due to such interaction, a spectrum (absorption pattern) is generated. This spectrum helps you to access compositional as well as structural details of the material.

●  When molecules interact with various electromagnetic radiations, these particles move from one energy level to another. This type of motion produces the molecular spectrum.

●  Unlike atoms, molecules show interactions between different electrons and nuclei. Although the mechanisms are quite the same, the process is more complex in the case of molecules.

●  There are three types of interactions that take place in the case of molecular spectroscopy. They are rotational, vibrational or electronic. Various spectrometric techniques are used for the study of the compounds based on the types of interactions.

Types of molecular spectra

In molecular spectroscopy, specific material is exposed to electromagnetic radiation of varying wavelengths to obtain molecular spectra. Molecules of substances absorb different wavelengths to attain higher rotational, electronic, or vibrational energy levels. Each series of wavelengths that the molecules absorb produces a separate molecular spectrum. Hence, you will find three types of spectra in molecular spectroscopy.  

According to the Born-Oppenheimer approximation rule, the total energy of a molecule is equivalent to the total of all energies exhibited by the molecule and its constituent particles. In other words, the energies of a molecule is an additive property of each individual motion in the molecule. These individual energy levels are termed translational, vibration, rotational and electronic spectra of the molecule. Translational energy is often neglected due to its negligible value. The total energy is the molecular spectra, representing the transitions of energies occurring within the molecule.

The molecular spectra are represented in the form of an equation as:

E = Er  + Ev  + Ee

Where,

E is the total molecular energy in the spectrum.

Er is the total rotational energy

Ev is the total vibrational energy, and

Ee is the total electronic energy

The above spectral energies also follow a specific order of magnitude as: Er  <<  Ev  << Ee. 

The various types of molecular spectra based on their energy levels are as follows:

●  Rotational spectra – This type of spectrum is observed in the microwave region of the spectrum and is hence also known as microwave spectra. The spectra are measured using the values of angular momentum and electron spins within a molecule. Examples of molecules giving rotational spectra are polar molecules like water (H₂O), nitrous oxide, and nitrogen gas (N₂O).

●  Vibration spectra – This type of spectrum is observed in the infrared region of the spectrum and Raman spectra. However, the two differ in their physical origins. Molecular vibration occurs when a molecule absorbs a certain amount of energy equal to its vibrating frequency. This sets in a periodic motion of the molecules, and associated spectra are called the vibration spectra. Examples of molecules exhibiting vibration spectra are water (H₂O), hydrochloric acid (HCl), and carbon dioxide (CO₂).

●  Electronic Spectra – This type of spectrum is observed in the ultraviolet to near-infrared regions of the spectrum. Electronic spectra occur when kinetic energy increases, resulting in a shift of the electron from a lower to a higher orbital plane. However, this shift is limited for the electrons. An example of electronic spectra is observed in benzene (C₆H₆) molecules.

Steps involved in Molecular Spectroscopy

●  Different spectrometric devices are used in molecular spectroscopy, such as Fourier-transform or emission spectrometers.

●  These devices are designed with a sample, a radiation source and a system that detects and analyzes samples. The sample is appropriately placed in the spectrometer. The radiation source should be able to focus on the sample.

●  Then, two samples in a cuvette are placed adjacent to one another in the spectrophotometer. The concentration of one sample is known while the other isn’t.

●  The radiation source of the spectrophotometer passes light of a specific wavelength through the sample.

●  This wavelength of light is recorded by the detectors of the spectrophotometer and converted to digital values.

●  With these values, you can plot a graph of absorbance against the concentration of the sample. This plot is utilised to determine the unknown concentration of the sample used in the experiment.

Applications of molecular spectroscopy

Molecular spectroscopy is widely used in the fields of phytochemistry, medicine, analytical chemistry, and biological analysis.

This method is also incorporated into other analytical techniques such as elemental analysis, chromatography and other analytical methods.

The compositional and structural data of substances can be obtained from molecular spectroscopy. The intensity of light emitted or absorbed by the substance gives its concentration. Whereas you can get the nature of the compound from the wavelength of light, it emits or absorbs.

Conclusion

Molecular spectroscopy is the technique by which the interaction of molecules with electromagnetic radiations is measured. The fundamentals of molecular spectroscopy involve the excitation of molecules from the ground state to the excited state. These charged up molecules either emit or absorb various wavelengths of radiation and move from one energy level to another. As a result of this movement, spectra are produced. Molecular spectroscopy utilises a spectrophotometer device to determine the structure and composition of different substances.