Topic overview

Start with the big picture

IR absorption occurs when photon energy matches a molecular vibration and the transition involves a change in dipole moment. The spectrum’s diagnostic region, 4000–1500 cm⁻¹, helps identify functional groups, while the fingerprint region, 1500–600 cm⁻¹, supports compound-specific confirmation. Bond strength and atomic mass influence vibrational frequency, and characteristic bands provide useful clues about groups such as O–H, N–H, C=O, and C–H. In pharmaceutical work, IR can support identification, polymorph discrimination, and other analyses. The deeper lesson develops these principles alongside quantitative analysis, sampling methods, FTIR instrumentation, data processing, and controls for common interferences. It also considers technique limitations and how IR complements Raman spectroscopy.