Which structural feature must a molecule possess in order to absorb infrared (IR) radiation?

• A. A symmetrical structure with no polar bonds
• B. A delocalized pi electron system
• C. Only single bonds between identical atoms
• D. A bond with a changing dipole moment during vibration

Answer: (D)

Verified step by step guidance

1. Understand that infrared (IR) spectroscopy measures the absorption of IR radiation by molecules, which causes molecular vibrations.
2. Recognize that for a molecule to absorb IR radiation, its vibrations must lead to a change in the dipole moment of the molecule; this means the bond must be polar or become polar during vibration.
3. Note that symmetrical molecules with no polar bonds do not have a changing dipole moment during vibration, so they generally do not absorb IR radiation.
4. Realize that a delocalized pi electron system or only single bonds between identical atoms do not guarantee IR absorption unless the vibration changes the dipole moment.
5. Conclude that the key structural feature required is a bond whose vibration causes a change in dipole moment, enabling the molecule to interact with IR radiation.