Infrared spectroscopy

Infrared spectroscopy identifies molecules through vibrational absorption. When infrared radiation matches the energy difference between vibrational states, a molecule absorbs that frequency; the resulting spectrum records absorption or transmission against wavenumber. Stretching and bending motions are the principal vibrations, and their frequencies depend on bond strength, atomic masses, and the surrounding molecular structure.¹

A vibration is infrared-active when it produces a change in the molecule’s dipole moment. Polar bonds such as carbonyl groups therefore often give strong bands, whereas some highly symmetrical vibrations are weak or absent. The functional-group region, usually above about 1500 cm⁻¹, supports assignments such as O–H, N–H, C–H, and C=O absorptions; the fingerprint region below it contains complex, structure-specific patterns that are especially useful for comparison with reference spectra.

Fourier-transform infrared (FTIR) instruments obtain spectra by measuring an interferogram and applying a Fourier transform. An interferometer, commonly based on a Michelson design, allows broad spectral coverage and rapid acquisition, while a laser supplies an internal wavenumber reference.²

Samples may be examined as gases, liquids, solutions, thin films, powders, or solids. Transmission measurements pass radiation through the sample; attenuated total reflectance (ATR) uses an infrared-transparent crystal to probe only a shallow region at the surface; and diffuse-reflectance methods are useful for powders and rough materials. Reflectance, microscopy, and gas cells extend the technique to coatings, microscopic contaminants, atmospheric samples, and spatially resolved chemical maps.

Water and carbon dioxide can interfere with measurements, and strongly absorbing or optically thick samples may require dilution, thinner path lengths, or ATR. Background spectra, baseline correction, resolution, and appropriate reference materials are central to reliable interpretation.

Infrared spectra are widely used to identify functional groups, verify reaction products, characterize polymers, detect contamination, and monitor industrial processes. In pharmaceuticals, the method supports raw-material identification and quality control; in environmental analysis, it helps study gases, aerosols, soils, and organic pollutants. Coupled techniques such as gas chromatography–infrared spectroscopy separate mixtures before their infrared signatures are measured.

Interpretation combines band positions, intensities, shapes, and the whole spectral pattern rather than relying on one peak. Hydrogen bonding can broaden and shift absorptions, conjugation can alter carbonyl frequencies, and isotopic substitution changes vibrational frequencies. Quantitative analysis is possible because absorbance can depend on concentration and optical path length, although calibration, matrix effects, scattering, and overlapping bands must be controlled. Spectral libraries and chemometric methods can assist classification, but their conclusions remain dependent on sample preparation and the quality of the reference data.

Infrared spectroscopy also reveals dynamics, surfaces, and materials that are difficult to study by ordinary bulk analysis. Time-resolved measurements follow transient intermediates, while temperature- or pressure-dependent spectra expose phase changes, hydrogen bonding, and molecular rearrangements. Infrared microscopes and focal-plane detectors can map composition across paint layers, cells, pharmaceutical particles, or microplastics without requiring complete bulk homogenization.

The far-infrared and terahertz regions contain low-frequency lattice, torsional, and collective modes that are valuable for crystals, inorganic solids, and intermolecular interactions, although instrumentation and atmospheric absorption impose practical limits.³ Synchrotron infrared sources provide exceptional brightness for microscale studies, and infrared emission spectroscopy can examine hot gases or surfaces rather than relying only on an external lamp. These extensions make the technique relevant to planetary materials, cultural-heritage conservation, catalysis, and biological tissue analysis.

Infrared spectroscopy usually identifies chemical features rather than determining a complete molecular structure by itself. Different compounds can share similar bands, mixtures may produce overlapping absorptions, and weak or symmetry-forbidden vibrations may be missing. Quantitative results can also be distorted by particle size, scattering, path length, and variable contact in ATR measurements.

Complementary methods resolve these limitations. Nuclear magnetic resonance spectroscopy provides detailed information about molecular environments, mass spectrometry supplies accurate mass and fragmentation data, Raman spectroscopy often complements infrared selection rules, and X-ray diffraction determines long-range crystal structure. Used together, these methods distinguish isomers, establish molecular connectivity, and separate chemical identification from more demanding structural determination.⁴

Wavenumber

The reciprocal of wavelength, expressed in cm⁻¹; it is proportional to the energy of infrared radiation and is the usual horizontal scale for infrared spectra.

Functional-group region

The higher-wavenumber portion of a mid-infrared spectrum, commonly above about 1500 cm⁻¹, where characteristic bond-group absorptions are often assigned.

Fingerprint region

The lower-wavenumber region containing a complex pattern that can be highly distinctive for a particular compound.

ATR

Attenuated total reflectance, a sampling method in which an evanescent wave probes material contacting an internal-reflection crystal.

FTIR

Fourier-transform infrared spectroscopy, which derives a spectrum from an interferogram using a Fourier transform.