Disruption of the centre of symmetry occurs for various reasons, such as the Jahn-Teller effect and asymmetric vibrations.
2.
However, forbidden transitions are allowed if the centre of symmetry is disrupted, and indeed, such apparently forbidden transitions are then observed in experiments.
3.
In the special case of a homonuclear diatomic molecule, there is an extra symmetry since in addition to the axis of symmetry provided by the internuclear axis, there is a centre of symmetry at the midpoint of the distance between the two nuclei ( the symmetry discussed in this paragraph only depends on the two nuclear charges being the same.
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