Homonuclear diatomic molecules bond stretch

Any homonuclear diatomic molecule has no permanent dipole moment and therefore no allowed rotational spectrum. Because the dipole moment remains zero as the bond stretches, homonuclear diatomics also have no allowed vibrational spectrum. Only electronic spectroscopy in homonuclear diatomics is allowed by electric dipole selection rules, and high precision measurements of the rotational and vibrational constants in molecules as simple as H2 and N2 can be quite difficult. 

This means that the dipole transition can only be nonzero if m = n 1 Thus if the oscillator is in state n > the transition will only occur to a state m >, which is one level above or one level below n >. That was a lot of complicated reasoning but it was necessary to get a very useful result It should be noted that the dipole mechanism depends on the fact that there is a permanent nonzero dipole moment in the molecule. In the case of a homonuclear diatomic molecule, such as O2, N2, or Li2, there is no dipole moment and stretching the bond will not produce a dipole so another technique (the Raman effect) must be used to detect transitions that depend on a change in... 

If we carry out the same analysis on a homonuclear diatomic, such as Hj, then additional operations are present because the molecule is now in Dooh and so is unaltered if the two atoms swap over. This introduces the inversion centre i at the middle of the bond and the improper rotation class 2So. However, the vibration remains totally symmetric, so that for H2 the stretching mode belongs to the irreducible representation S+. 

The stretching or compression of a bond changes the electronic wave function and therefore changes the polarizability. The vibration of a diatomic molecule is Raman active, whether the molecule is homonuclear or heteronuclear. A vibrational normal... 

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