Practice question

Infrared absorption follows the quantum selection rule that a vibration must cause a change in molecular dipole moment to interact with oscillating infrared electric field. The absorption intensity is proportional to the square of the dipole change during motion. Highly polar bonds such as carbonyl, hydroxyl and amide groups undergo large dipole variations, yielding intense IR bands, whereas weakly polar bonds absorb poorly. Concentration influences absorbance via Beer-Lambert law, but intrinsic molecular property determining strength is dipole modulation. Thus, vibrations with substantial charge redistribution dominate spectra and serve as sensitive reporters of protein secondary structure.