Some of the Salient features of vibrational-rotational spectra are as follows :
- Vibrational-rotational spectra arises from transition between vibrational energy states associated with the same electronic state of the molecules.
- These spectra are observed in near infrared region.
- These spectra are observed only for molecule which have permanent dipole moment, i.e. hetronuclear diatomic molecules. e.g. HCl, HBr, etc.
- These spectra are observed in absorption.
- The internuclear distance changes when the nuclie of these molecule vibrate related to each other. Thus these molecule possess an oscillating dipole moment.
Vibrational-Rotational Spectra for HCl Molecule :
In practice, vibrational-rotational spectra is observed in absorption. The vibrational-rotational spectra of HCl molecule can be explained in following points clearly :
- Spectra of a molecule like HCl has intense band called fundamental band.
- Spectrum consists weak bands called overtones that appears 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 high resolution it is found that each band consists of a series of lines that are not equidistant.
- A line is found missing in centre of band and is called null line or zero line.
- 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.