IR selection rules: Why must a molecule’s vibration change its dipole moment for IR absorption to occur?

Introduction / Context:
IR selection rules state that a vibrational mode is IR-active only if it produces a time-dependent dipole moment. Understanding this principle explains why homonuclear diatomics (e.g., N2, O2) are IR-inactive and why polar bonds yield strong IR bands.

Given Data / Assumptions:

• IR light is an oscillating electric field.
• Interaction strength depends on transition dipole moment.
• We focus on vibrational (not electronic) transitions.

Concept / Approach:
During a vibration, if the molecular dipole changes with time, the oscillating dipole can couple to the oscillating electric field of IR radiation, allowing energy absorption. No change in dipole means negligible coupling and no observable IR band (though Raman may still be active).

Step-by-Step Solution:
Recall selection rule: Δμ ≠ 0 during the vibration.Recognize that coupling requires an interaction between EM field and time-varying dipole.Select the option that directly expresses this interaction requirement.

Verification / Alternative check:
CO has a strong C≡O stretch in IR (polar bond), whereas N2 lacks an IR band but is Raman-active—classic demonstration of the rule.

Why Other Options Are Wrong:

• Lowering energy for electronic transitions: Not relevant to vibrational IR absorption.
• Allow deformation: Deformation happens regardless; IR activity specifically depends on dipole change.
• All of the above: Combines incorrect statements.
• Ionic solids only: IR absorption is not limited to ionic materials.

Common Pitfalls:
Confusing IR and Raman activity; they have complementary selection rules.

Final Answer:
Because for absorption to occur, the radiation must interact with the electric field caused by changing dipole moment.