Practice question

Infrared absorption obeys fundamental selection rule requiring change in molecular dipole moment during vibration. When bond stretches or bends, charge distribution shifts, creating oscillating dipole that can absorb photon energy matching vibrational frequency. Nonpolar bonds undergoing symmetric motion produce no dipole change and remain infrared silent. Hydrogen bonding influences frequency and intensity through polarity modulation but is not mandatory. Double bonds, resonance or conjugation shift absorption positions yet do not alone confer activity. Core prerequisite remains dynamic dipole generation, explaining why polar carbonyls absorb strongly while symmetric alkanes absorb weakly in biological macromolecules.