Some of the salient features of vibrational-rotational spectra are as follows :
- Vibrational-rotational spectra arise from the transition between vibrational energy states associated with the same electronic state of the molecules.
- These spectra are observed in the near-infrared region.
- These spectra are observed only for the molecule which has the permanent dipole moment, i.e. heteronuclear diatomic molecules. e.g. HCl, HBr, etc.
- These spectra are observed in absorption.
- The internuclear distance changes when the nuclei of these molecules vibrate related to each other. Thus these molecules possess an oscillating dipole moment.
Vibrational-Rotational Spectra for HCl Molecule :
In practice, vibrational-rotational spectra are observed in absorption. The vibrational-rotational spectra of HCl molecule can be explained in the following points clearly :
- Spectra of a molecule like HCl has an intense band called the fundamental band.
- The spectrum consists weak bands called overtones that appear at wave number approximately double, triple, quadruple,…. of fundamental bands.
|Fig. Fundamental bands in HCl|
- On close examination, the fundamental band is consist of two closed maxima which is an isotope effect.
|Fig. Isotope effect|
- Under the high resolution, it is found that each band consists of a series of lines that are not equidistant.
- A line is found missing in center of the band and is called null line or zero line.
- The line seems to converse towards higher wave number side and band is said to be degraded, toward the low wave number side. i.e. towards red.