Discussion
What is the selection rule for infrared absorption?
Answer: D
A vibration absorbs infrared radiation only if it changes the molecule's dipole moment, so that the oscillating field has something to couple to. Polarisability change is the Raman selection rule, which is why the two techniques are complementary. Double bonds and stretches are not required. That a symmetric molecule can vibrate vigorously and absorb nothing follows directly, and it is why nitrogen and oxygen are transparent in the infrared. Framing the question as which quantity the change acts upon makes the answer follow without calculation.
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