Vibrational Spectroscopy of Nonlinear Molecules

Moving on to nonlinear molecules, there are few truly new concepts. The number of vibrational degrees of freedom is now 3N — 6, and the list of descriptions for the vibrations increases somewhat. Perhaps the biggest difference in considering nonlinear molecules is how the symmetry of the molecule affects the number of independent vibrations of the molecule. 

FIGURE 14.32 The normal modes of vibration for ammonia, NH3. All are IR-active. 

Unless otherwise noted, all art on this page is Cengage Learning 2014. 

Spectra and Molecular Structure, Vol. II, 1991, Krieger Publishing Company. Adapted with permission. 

There is a strict group-theoretical method for determining exactly the number of allowed vibrational transitions expected to be observed in a vibrational spectrum. The method is presented here as a sort of recipe to be followed, and 

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