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The mid-IR window spans 4000–400 cm⁻¹. Its functional-group region, from 4000–1500 cm⁻¹, helps identify common groups, while the fingerprint region below 1500 cm⁻¹ contributes distinctive patterns. Bond strength and atomic mass influence vibrational frequency; dipole change determines whether a vibration is IR-active and affects band intensity. Characteristic absorptions include O–H, N–H, C–H, nitrile, alkyne, carbonyl, and alkene bands. Hydrogen bonding can broaden O–H and N–H features, while conjugation or resonance lowers carbonyl frequency and ring strain raises it. IR provides useful structural evidence, but overlapping peaks and other limitations mean it is best interpreted alongside complementary techniques such as NMR and mass spectrometry.