Instrumental Techniques for Analytical Chemistry

Introduction
Infrared (IR) spectroscopy is one of the most common spectroscopic techniques used by organic and
inorganic chemists. Simply, it is the absorption measurement of different IR frequencies by a sample
positioned in the path of an IR beam. The main goal of IR spectroscopic analysis is to determine the
chemical functional groups in the sample. Different functional groups absorb characteristic frequencies
of IR radiation. Using various sampling accessories, IR spectrometers can accept a wide range of sample
types such as gases, liquids, and solids. Thus, IR spectroscopy is an important and popular tool for
structural elucidation and compound identification.

IR Frequency Range and Spectrum Presentation
Infrared radiation spans a section of the electromagnetic spectrum having wavenumbers from roughly
13,000 to 10 cm–1, or wavelengths from 0.78 to 1000 μm. It is bound by the red end of the visible region
at high frequencies and the microwave region at low frequencies.
IR absorption positions are generally presented as either wavenumbers ( ) or wavelengths (l).
Wavenumber defines the number of waves per unit length. Thus, wavenumbers are directly proportional
to frequency, as well as the energy of the IR absorption. The wavenumber unit (cm–1, reciprocal centimeter)
is more commonly used in modern IR instruments that are linear in the cm–1 scale. In the
contrast, wavelengths are inversely proportional to frequencies and their associated energy. At present,
the recommended unit of wavelength is μm (micrometers), but μ (micron) is used in some older literature.
Wavenumbers and wavelengths can be interconverted using the following equation:
n in cm–1 ( ) 1
l (in μm)
------------------------ 104
(15.1)
IR absorption information is generally presented in the form of a spectrum with wavelength or
wavenumber as the x-axis and absorption intensity or percent transmittance as the y-axis (Fig. 15.1).