A given molecule may vibrate in up to six ways (modes) each of which is called the vibrational mode. Say for instance that a molecule comprises N number of atoms; linear molecules have 3n – 5 degrees of vibrational modes. On the other hand, non-linear molecules have 3n to 6 degrees of vibrational modes.
Take water, for instance, as a non-linear molecule. From Bock’s experiment, “H20 would have 3 × 3 – 6 = 3 degrees of vibrational freedom, or modes.” (Bock, 1993 p 567) much in the same way, simple diatomic molecules consist of just one bond and as a result, they have one vibration band. Given that the molecule in question is symmetrical, say for instance Nitrogen – N2, the band is only observable in the Raman spectrum but not in the IR spectrum aided by the Fourier transform infrared (FTIR) spectroscopy. On the other hand, unsymmetrical diatomic molecules such as CO absorb in the infrared spectrum.
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