The key difference between rotational and vibrational spectroscopy is that rotational spectroscopy is used to measure the energy of the transitions that take place between quantized rotational states of molecules in the gas phase, whereas vibrational spectroscopy is used in measuring the interaction of IR radiation with matter through absorption, emission, or reflection.
Spectroscopy is a branch of science that concerns the investigation and measurement of spectra produced when matter interacts with or emits electromagnetic radiation. This interaction occurs due to electronic transitions. Electronic transitions in molecules take place when electrons in the molecule become excited from one energy level to another. The electrons tend to move from a low energy level to a high energy level. The change in the energy that is associated with this transition provides information about the structure of the molecule and helps in determining the molecular properties such as colour. The relationship between the energy and the frequency of radiation that is used in the transition process can be given by Planck’s relation.
CONTENTS
1. Overview and Key Difference
2. What is Rotational Spectroscopy
3. What is Vibrational Spectroscopy
4. Side by Side Comparison – Rotational vs Vibrational Spectroscopy in Tabular Form
5. Summary
What is Rotational Spectroscopy?
Rotational spectroscopy is the measurement of the energy of the transitions that take place between quantized rotational states of molecules in the gas phase. Sometimes, this method is known as pure rotational spectroscopy. This is because it helps to distinguish the rotational spectroscopy from rotational-vibrational spectroscopy. Rotation spectroscopy is governed by rotational transitions.
Figure 01: Rotational Spectrum
Rotational transitions of molecules refer to the abrupt change in the angular momentum of that molecule. This definition is given depending on the theories of quantum physics, which states that the angular momentum of a molecule is a quantized property, and it can only equal certain discrete values that correspond to different rotational energy states. The rotation transition refers to the loss or gain of angular momentum, which causes the molecule to move either to a higher or a lower rotational energy state.
What is Vibrational Spectroscopy?
Vibrational spectroscopy is the measurement of the interaction of IR radiation with matter through absorption, emission, or reflection. This spectroscopic technique is useful in studying and identifying chemical substances or functional groups in solid, gas or liquid compounds. Vibrational spectroscopy is governed by vibrational transitions.
Figure 02: A Vibrational Spectrum
The vibrational transition of a molecule refers to the movement of the molecule from one vibrational energy level to another. We can also name it a vibronic transition. This type of transition occurs in between different vibrational levels of the same electronic state. In order to evaluate the vibrational transition of a particular molecule, we should know the dependence of the molecule-fixed components of the electric dipole moment on the molecular deformations. Generally, Raman spectroscopy is based on vibrational transitions.
What is the Difference Between Rotational and Vibrational Spectroscopy?
Rotational spectroscopy and vibrational spectroscopy are governed by electron transitions. The key difference between rotational and vibrational spectroscopy is that rotational spectroscopy is useful to measure the energy of the transitions that take place between quantized rotational states of molecules in the gas phase, whereas vibrational spectroscopy is useful in measuring the interaction of IR radiation with matter through absorption, emission, or reflection.
Below is a list of differences between rotational and vibrational spectroscopy in tabular form.
Summary – Rotational vs Vibrational Spectroscopy
Rotational spectroscopy and vibrational spectroscopy are governed by electron transitions. The key difference between rotational and vibrational spectroscopy is that rotational spectroscopy is useful to measure the energy of the transitions that take place between quantized rotational states of molecules in the gas phase, whereas vibrational spectroscopy is useful in measuring the interaction of IR radiation with matter through absorption, emission, or reflection.
Reference:
1.“Vibrational Spectroscopy.” ScienceDirect Topics, Available here.
Image Courtesy:
1. “CF3I spectrum2” By Nnrw – Own work (CC BY-SA 3.0) via Commons Wikimedia
2. “Bromomethane IR spectroscopy” By Unknown author – NIST (Public Domain) via Commons Wikimedia