Infrared spectroscopy

Abstract

Infrared (IR) spectroscopy covers the region of the electromagnetic spectrum between the red end of the visible range and the microwave region. Standard spectrometers scan the mid-IR region between 4000 and 400 cm⁻¹ (2·5–25 μm). The near-IR extends from 12 500–4000 cm ⁻¹ (0·8–2·5 μm), and the region from 400–50 cm ⁻¹ (25–200 μm) is known as the far-IR. IR radiation causes transitions between vibrational energy levels and also between rotational energy levels. Transitions between rotational energy levels are manifested in the gas phase, but spectra in the liquid or solid state show broad bands in which the peaks arising from the individual rotational transitions are not resolved. The frequency of absorption due to transitions between vibrational energy levels allows the identification of functional groups within the molecule. The vibrations of molecules may also be usefully investigated by Raman scattering. (Details of this technique are given in ‘Further reading’.)