Practice question

Vibrational modes differ energetically because stretching requires overcoming strong covalent bond force constant to change interatomic distance, while bending only alters bond angle with lower restoring force. According to molecular mechanics, stretching force constants are roughly tenfold larger than bending constants. Consequently stretching absorptions occur at higher wavenumbers, 4000-1500 cm-1, whereas bending modes such as scissoring, rocking, wagging and twisting appear at lower frequencies, typically below 1500 cm-1 and often near 1400-700 cm-1. Lower energy explains why bending vibrations dominate fingerprint region and provide conformational sensitivity in lipids and proteins.