Question

Carbon dioxide (CO2) and hydrogen cyanide (HCN) are both triatomic linear molecules. How would you expect

their IR spectra to differ? Explain your reasoning structurally and physically

Answer 1

Whenever there is a dipole change, the molecule can interact with radiation and energy exchange can occur giving rise to an emission or absorption spectrum. Although CO₂ is linear triatomic molecule but because of its symmetric arrangement it possesses a net zero dipole moment and thus no interaction with radiation can occur. Hence for symmetric stretch in CO₂, it is IR inactive. Whereas in HCN there is net non zero dipole moment because of its asymmetric structure making it IR active.