IRabsorption

IRAbsorption refers to the process by which molecules absorb infrared radiation as they transition between vibrational energy levels. Because vibrational transitions involve changes in the dipole moment, only IR-active modes are observed. Homonuclear diatomic molecules (for example O2 or N2) lack a changing dipole and are largely IR-inactive.

Infrared spectroscopy measures the wavelengths at which absorption occurs. The most widely used form is Fourier-transform infrared spectroscopy (FTIR), which records an interferogram and converts it to a spectrum. Spectra are typically plotted as absorbance or transmittance versus wavenumber (cm^-1).

Typical spectral range is 4000–400 cm^-1, with strong diagnostic bands: O-H and N-H stretches near 3200–3600 cm^-1; C-H stretches around 2850–2960 cm^-1; C=O stretches around 1650–1750 cm^-1; below 1500 cm^-1 lies the fingerprint region, where many compounds show characteristic patterns.

Fundamental vibrational bands are complemented by overtones and combination bands that occur at higher wavenumbers but are weaker. In gases, individual rotational lines modulate the bands, while in liquids and solids the bands broaden.

Quantitative use relies on the Beer-Lambert law: A = εlc, where A is absorbance, ε is molar absorptivity, l is path length, and c is concentration. In practice, FTIR is used for qualitative identification of functional groups and compounds, as well as for quantitative monitoring in chemistry and materials science. Interferences from water vapor and CO2 can affect spectra and are commonly corrected.