Question

The CORRECT statement regarding the molecules BF$_3$ and CH$_4$ is

• A. Both BF$_3$ and CH$_4$ are microwave active
• B. Both BF$_3$ and CH$_4$ are infrared active
• C. CH$_4$ is microwave active and infrared inactive
• D. BF$_3$ is microwave active and infrared active

Hint:

Microwave activity requires a permanent dipole moment, while infrared activity requires a change in dipole moment during vibration. Highly symmetrical molecules (like CH$_4$ and BF$_3$) are microwave inactive but can be infrared active.

Answer 1

The Correct Option is B

Step 1: Check molecular symmetry and dipole moment.
- BF$_3$: Trigonal planar ($D_{3h}$ symmetry), no permanent dipole moment → microwave inactive.
However, its asymmetric B–F stretching vibrations change dipole moment → infrared active.
- CH$_4$: Tetrahedral ($T_d$ symmetry), no permanent dipole moment → microwave inactive.
Some vibrational modes (asymmetric stretches) change dipole moment → infrared active.
Step 2: Conclusion.
Both BF$_3$ and CH$_4$ are microwave inactive but infrared active. Hence, the most appropriate choice is **(B)**.