Page 183 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN007C-340 July 10, 2001 14:45

794 Infrared Spectroscopy

THE INFRARED (IR) spectrum results from the interac- vibration absorption in the IR. The low wave number limit
tion of radiation with molecular vibrations and, in gases, is more variable since it is more or less an instrumental
with molecular rotations. The spectrum itself is a plot of limitation. The region between the visible and the mid-IR
sample transmission of IR radiation as a function of wave- regions is called the near-IR region. This region of the IR
length or related units. Infrared spectroscopy is the physics has been used for many applications, especially for quan-
−1
that deals with the theory and interpretation of this spec- titative analysis. The region beyond ∼50 µm (200 cm )
trum and is one of the most popular techniques for identi- is called the far-IR region. This region is used for studying
fying molecules. The IR spectrum can be used as a type of low-frequency vibrations and some molecular rotations.
“fingerprint” unique to a molecule. In addition, the pres- Electronic transitions give rise to absorption in the ul-
ence or absence of many chemical functional groups such traviolet and visible regions of the spectrum, and pure
as phenyls and carbonyls usually can be established from rotations of gaseous molecules give rise to absorption in
the spectrum. Quantitative analyses of mixtures can be the far-IR and microwave regions of the spectrum. In-
obtained. Infrared spectra can be run for liquids, solids, tramolecular vibrations of molecules give rise to absorp-
or gases without special difficulties. Different types of tion throughout most of the IR region.
spectrometers can be used, and a wide variety of sam-
ple handling techniques are available, many of which are B. Infrared Spectra Coordinates
described in this article.
SomeexamplesofIRspectraaregiveninFigs.1through6.
Chemical group vibrations associated with spectral bands
are indicated. These are all liquids run in a 0.01-mm-thick
I. BASIC THEORY
NaCl cell. The horizontal coordinates for IR spectra are
usually either linear with wavelength (µm) or linear with
A. Electromagnetic Spectrum −1
wave number (cm ) with generally a factor of 2 scale
−1
Electromagnetic radiation can be characterized by its change at 2000 cm . An advantage of the wave number
wavelength λ, its frequency ν, or its wave number ¯ν.Inthe scale is that the wave number of the radiation is propor-
IR region the unit used for wavelength is the micrometer tional to its frequency and to photon energy, and these
(µm). The frequency unit is cycles per second or hertz properties are related to the frequencies and energies of
(Hz). The wave number unit is cycles per centimeter or molecular vibrations.
−1
−1
reciprocal centimeters (cm ). The wave number (cm ) The vertical coordinate in a single-beam spectrum is a
is the number of waves in a continuous wave sequence 1 measure of the intensity of the radiation of a given wave
cm long. The relationship between the units is given in number that has passed through a sample and reached the
Eq. (1): detector. Usually, this spectrum is ratioed with another
single-beam reference spectrum without a sample to give
1 10 4
−1 −1 a ratioed or double-beam spectrum. The transmittance T is
¯ ν (cm ) = ¯ ν (cm ) =
λ (cm) λ (µm) the intensity of the sample single-beam spectrum divided
(1)
ν (Hz) by the intensity of the reference single-beam spectrum
−1
¯ ν (cm ) = . at the same wave number. The vertical coordinate most
c (cm/sec)
commonly seen in an IR spectrum is linear with percent
−1
From this, one can see that wave number (cm ) is equal transmittance (%T ), which is transmittance T multiplied
by 100.
to the reciprocal of the wavelength (cm) or is equal to
4
10 times the reciprocal of the wavelength (µm). The wave
−1
number (cm ) in a vacuum is also equal to the frequency
(Hz) divided by c, the velocity of light in a vacuum given
in centimeters per second. This makes the wave number
proportional to the frequency.
The visible region of electromagnetic radiation extends
from about 0.38 to 0.78 µm. The IR region extends from
the end of the visible region at 0.78 µm to the microwave
region with a wavelength of ∼1 mm. The IR region is
usually divided into three sections. The section used most
by chemists is the mid-IR region extending from 2.5 µm,
−1
−1
or 4000 cm ,to ∼50 µm, or 200 cm . The division at FIGURE 1 Infrared spectrum of chloroform in a 0.01-mm NaCl
4000 cm −1 is the high wave numberlimit for fundamental cell.