Infrared (IR) spectroscopy measures molecular vibrations in the 4000 to 400 cm⁻¹ region. The spectrum is divided into two analytical zones: the functional group region (4000-1300 cm⁻¹), where characteristic group frequencies appear, and the fingerprint region (1300-400 cm⁻¹), which contains complex patterns unique to each molecule. Successful interpretation requires systematic examination of both regions.
Each functional group absorbs within a characteristic frequency window due to the mass of the atoms and the force constant of the bond. Key reference frequencies include: O-H stretching (broad, 3200-3600 cm⁻¹), N-H stretching (3300-3500 cm⁻¹, typically sharper than O-H), C-H stretching (2850-3000 cm⁻¹ for aliphatic, 3010-3100 cm⁻¹ for aromatic), C=O stretching (1680-1750 cm⁻¹), C=C stretching (1620-1680 cm⁻¹), and C-O stretching (1050-1300 cm⁻¹). Hydrogen bonding significantly broadens O-H and N-H bands and shifts them to lower wavenumbers, providing information about intermolecular interactions in the sample.
Sample preparation affects spectral quality. The KBr pellet technique involves grinding 0.5-1% sample with dry KBr and pressing into a transparent disk. Attenuated total reflectance (ATR) eliminates sample preparation entirely by pressing the sample against a high-refractive-index crystal (diamond, ZnSe, or Ge) and measuring the evanescent wave. ATR has become the dominant sampling method due to its speed, reproducibility, and ability to measure solids, liquids, and pastes without modification.
Spectral subtraction is a powerful data-processing technique for difference spectroscopy. By digitally subtracting a reference spectrum (e.g., solvent or matrix) from the sample spectrum, the analyst can isolate the analyte’s absorption bands. This approach is particularly useful for identifying trace components, monitoring reaction intermediates, and analyzing formulated products where the active ingredient is present at low concentration.
Combined interpretation with other spectroscopic techniques dramatically improves structural elucidation. IR identifies functional groups, NMR provides carbon-hydrogen framework information, and mass spectrometry determines molecular weight and fragmentation patterns. In practice, IR interpretation often serves as the first step — a quick scan reveals which functional groups are present, guiding the subsequent NMR and MS analysis toward a complete structural assignment.
