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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.