Question

For a molecule to absorb radiation in the IR region of the spectrum, it must undergo a net change in its dipole moment due to its vibration. The vibration is said to be active depending on whether there is a net change in dipole moment and the molecule absorbs IR radiation, or it is said to be inactive if there is no net change in dipole moment and the molecule does not absorb IR radiation .

A. State whether the C—C stretching vibrations in the two molecules below are active or inactive:

1. CH3—CH3

2. CH3—CCl3

B. Carbon dioxide is a linear molecule. It has a symmetric and an asymmetric stretching vibration mode.

1. Calculate the number of normal vibration modes of carbon dioxide.

2. Discuss whether the two stretching vibration modes identified are active and/or inactive and will allow carbon dioxide to absorb IR radiation?

Answer 1