Question

For the following compounds and the indicated vibration, indicate if the vibrational transition will

result in an observed (IR – active) absorption band. Explain.

cis – 1,2-dichloroethene, C=C stretch

trans – 1,2-dichloroethene, symmetric C-Cl stretch

formaldehyde, C=O stretch

1,1,1-trichloroethane, antisymmetric CH stretch

Answer 1

A particular vibration will be IR active if there is a change in dipole moment due to that vibration.

1. In cis-1,2-dichloroethene c=c stretching there will be change in dipole moment so this will be IR active

2.Symmetric stretching of C-Cl bond in trans-1,2-dichloroethene will not produce any net dipole moment change in the molecule so it is IR inactive.

3.C=O stretch in formaldehyde gives rise to change in dipole moment so it is IR active.

4. Anti symmetric C-H stretch results in change of dipole moment so It is IR active.