Question

Which vibrational modes will be IR active? Briefly Explain.

a. Symmetric N-O stretching (sN-O) for the nitrate ion, NO3^-2?

b. Symmetric C-O stretching (sC-O) for the carbonate ion, CO3^-2?

c. Antisymmetric C=C stretching (asC=C) for allene:

Answer 1

Which vibrational modes will be IR active? Briefly Explain.

a. Symmetric N-O stretching (sN-O) for the nitrate ion, NO3^-2? (non active)

b. Symmetric C-O stretching (sC-O) for the carbonate ion, CO3^-2? (non active)

c. Antisymmetric C=C stretching (asC=C) for allene: (Correct)

For a molecule to be IR active, the vibrations modes should result in the change in polarity of bond. antisymmetric C=C stretching in allene will cause for change in polarity of double bond on the other hand, due to symmetric stretching there is no change in polarity and hence IR inactive.