Discussion
The spectra shown below is due to :
Answer: A
The band is due to perpendicular vibrations in a linear polyatomic molecule.
A vibration-rotation band normally has a P branch (ΔJ = -1) and an R branch (ΔJ = +1), the two broad humps. A sharp, intense central Q branch (ΔJ = 0) appears only when the vibration carries angular momentum about the molecular axis, which is the case for a perpendicular vibration of a linear polyatomic molecule, such as the bending mode of CO2 or HCN.
A diatomic molecule's vibration-rotation band (and a parallel band of a linear molecule) has only P and R branches with a gap at the centre, not a spike. Nuclear spin statistics alter the intensities of alternate lines, and breakdown of the Born-Oppenheimer approximation does not produce a Q branch.
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