Question

Indicate which vibrational modes will be IR-active and Raman-active. Briefly explain.

Acetylene, symmetric triple bond stretch

Benzene, symmetric ring CC stretch

Carbon dioxide, asymmetric C=O stretch

trans 1,2 dibromoethene, sym C=C

Answer 1

Whenever there is change in dipole moment of molecule due to stretching of bonds then the molecule said to be IR active.

Raman activity occurs due to scattering of light by the vibrating molecules which results of absorption of light by vibrating molecules thence vibration is Raman active if it causes a change in polarisability of the molecule.

Acetylene CC stretch is IR inactive and raman active due to there is no change dipole moment in stretching of bond.

Benzene symmetric stretch is also IR inactive and raman active

CO₂ asymmetric stretch results change in the dipole moment hence it is IR active. For the asymmetric stretch of CO₂ the movement of the two oxygens are in the same direction due to this the volume increase on the left side is not equal by a volume decrease on the right and the asymmetric stretch is said to be Raman inactive

Symmetric stretch between the trans-1,2-dibromo ethene donot results in change in dipole moment so it is IR inactive. whereas the change in volume on both left and right are equal soo it is Raman active.