What is Molecular Spectroscopy?

Molecular spectroscopy is the study of atoms or molecules of chemicals. When atoms or molecules interact with electromagnetic radiation then it produces an absorption pattern. It helps to investigate the structure of the molecule, the bond length of the bond, rotation of bonds, the rigidity of bonds, electronic configuration of atom in the ground state, and excited states.

Applications of Molecular spectroscopy

• Spectroscopy is used to determine the structure of atoms and molecules.
• It is used to investigate the electronic configuration of atoms in the ground state and excited state.
• Analytical methods of spectroscopy are finding the composition of the material in unknown chemical composition. 
• The spectroscopic technique is used to identify the temperature and velocity of the objects in space.
• It is used to measure the masses of the molecule.
• It is used to measure the bond length and bond angle of the molecule.

Diffraction grating

When light sources pass through the diffraction grating then light splits into different wavelengths. The light source produces two types of patterns that are shown as destructive and non-destructive. The light source is in the form of quanta that is quantitation.

Electromagnetic radiation

Electromagnetic radiation is the form of photoelectron that propagates magnetic field radiation and electric field radiation. The electrical field radiation, the magnetic field radiation, and the propagation of radiation waves are perpendicular to each other. Both fields travel in the field of light.

There are seven types of electromagnetic radiation named as: 

1. Gamma rays
2. X-Rays
3. Ultraviolet rays
4. Visible light
5. Infrared radiation
6. Microwaves
7. Radio waves

As we move from left to right energy of the radiation increases and as we move from left to right wavelength decreases and frequency increases.

Interaction of these waves with matter

Some spectroscopic methods have been discussed as followed:

• When radio-wave interacts with the matter or chemical then it gives the information of changing the nuclear and electron spin under the NMR Spectroscopy, ESR spectroscopy, EPR spectroscopy.
• When the Microwave interacts with the matter or chemical then it gives the information of changing the molecular rotation under the Rotational spectroscopy.
• When Infrared radiation interacts with the matter then it gives the information of changing the molecular vibration under vibrational spectroscopy.
• When Visible radiation interacts with the matter or chemical then the transition of outer shell electron occurs.
• When Ultra-violet radiation interacts with the matter then it causes molecular dissociation.
• When X-Rays interact with the matter then it causes the transition of the inner shell electron.
• When Gamma-rays interact with the matter then it causes the nuclear transition.

Rotational spectroscopy

Rotational spectroscopy occurs in the range of microwave region and is concerned with the study of rotating the molecules. The rotation of the molecule depends on the moment of Inertia.

$I=\mu r^2$

Where the reduced mass of the molecule is $\mu$

and the radius of the atom is r from the center of gravity.

Rotational energy is in the form of quantized energy level, when the molecule has the quantized energy level then molecules rotate along their center of gravity. The quantized energy level is containing definite values depending on the shape and size of the molecule. The energy values so-called rotational energy levels. So, the energy of rigid diatomic molecules is:

$E=\frac{h^2}{8{\mathrm{\pi }}^2\mathrm{I}}J\left(J+1\right) in joules$

where h is Planck's constant, I is the moment of inertia, J is the energy level.

Vibrational spectroscopy

Vibrational spectroscopy occurs in the range of the Infrared region but does not occur in the near-Infrared region, so it is also known as infrared spectroscopy. The atom does not have any fixed relative position but vibrates about the mean position of the molecule. A diatomic undergoes vibration and rotation simultaneously. The vibration diatomic molecule is called the simple harmonic oscillator model.

In a simple oscillator model, the bond between diatomic molecules is elastic and contains vibrational frequency. Vibrational frequency mainly depends on the force constant and mass of the system. In a simple oscillator model, the bond between diatomic molecules is elastic in nature and contains vibrational frequency. Vibrational frequency mainly depends on the force constant and mass of the system.

$\nu =\frac{1}{2\pi }\sqrt{\frac{K}{\mu }}$

where K is the force constant,

$\mu$ is the reduced mass of the molecule.

Vibrational energy is in the form of quantized energy level when a molecule has the quantized energy level then molecules Vibrations along with its fixed position. The energy values are so-called Vibrational energy levels. So, the energy of a simple harmonic oscillator for diatomic molecules is:

$E=\left(n+\frac{1}{2}\right)hv$

where, n is the energy levels, h is the Planck's constant, v is the vibrational frequency of the molecule.

Electronic spectroscopy

Electronic spectroscopy is the transition of electrons from the ground state energy level to the higher energy level of the excited state within a molecular orbital. It is also known as absorption spectroscopy. It determines the shape of the atomic orbitals.

Four atomic quantum numbers are 

• Principle quantum number (n): It gives information about the energy of orbitals and the size of orbitals. The value of n lies from 1,2,3,...
• Orbital (or Azimuthal) quantum number (l): It gives information about the shape of the orbital and the electronic angular momentum. The value of l lies from 0,1,2,...,(n-2), (n-1).
• Magnetic quantum number (m): It gives information about the direction of an orbital and electronic behavior in the magnetic field. The value of m lies from -l, -(l-1), ..., -1, 0, 1, ... (l-1), l.
• Spin quantum number (s): It gives information about the axial angular momentum of the electron. The value of n is +1/2 and -1/2.

Electronic energy of hydrogen atom spectra

The energy of the hydrogen atom spectrum is:

$E=-\frac{m{e}^4}{8{\mathrm{\pi }}^2{{\mathrm{\epsilon }}_{\mathrm{o}}}^2{\mathrm{n}}^2}$

Where m is the mass of the atom, e is the charge on the electron, h is the Planck's constant, n is the number of orbitals,

${\epsilon }_o$is the vacuum permittivity.

NMR spectroscopy

NMR means nuclear magnetic resonance. This is a spectroscopic technique that observes the magnetic field around the nucleus. It occurs in the range of radio-wave frequency.

The principle of NMR spectroscopy is that every nucleus of the atom has spin, and every nucleus is electrically charged. When the external magnetic field is applied then the nuclei jump from ground state energy level to higher state energy level and again it gets deexcited and emission takes place then emission spectra are obtained. As the magnetic field strength increases then it gives high-resolution spectra.

The energy difference between the two energy levels is:

$∆E=\frac{h\gamma B_o}{2\mathrm{\pi }}$

Where, h is the Planck's constant, 

$\gamma$ is the gyromagnetic ratio,

$B_o$is the strength of the applied magnetic field.

Context and applications

Molecular spectroscopy is studied under courses like

• Masters of Science in Organic chemistry
• Masters of Science in Inorganic chemistry
• Bachelors of Science in Physical chemistry
• Bachelors of Science in Physics
• Masters of Science in Analytical chemistry

Practice problems

1. Which spectroscopic application lies in the range of radio wave region?

1. NMR spectroscopy
2. Vibrational spectroscopy
3. Fluorescence spectroscopy
4. Infrared spectroscopy

Answer- a

Explanation: NMR spectroscopic method lies in the range of radio wave regions.

2. Which quantum number gives the information about the behavior of the magnetic field?

1. Principle quantum number (n)
2. Orbital quantum number (l)
3. Magnetic quantum number (m)
4. Spin quantum number (s)

Answer- c

Explanation: Magnetic quantum number (m) gives information about the direction of an orbital and electronic behavior in the magnetic field.

3. Which spectroscopic technique is used to determine the shape of the orbitals?

1. NMR spectroscopy
2. vibrational spectroscopy
3. Electronic spectroscopy
4. Infrared spectroscopy

Answer- c

Explanation: Electronic spectroscopic technique is used to determine the shape of the orbitals

4. What is the full form of NMR?

1. Nuclear Matter Research
2. Nuclear Magnetic Resonance
3. Nucleus of Magnet Research
4. Nucleus Magnetic Resonance

Answer- b 

Explanation: The full form of NMR is Nuclear Magnetic Resonance.

5. How many types of electromagnetic radiation are there?

1. 2
2. 5
3. 7
4. 10

Answer- c

Explanation: There are seven types of electromagnetic radiation are gamma rays, X-rays, ultraviolet rays, visible light, infrared radiation, microwaves, radio waves.

Common Mistakes

It is important to concentrate on the units of the variables. Sometimes questions gave different units then convert into given units and then substitute into the formula. Firstly, convert it into S.I. units.