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Introduction to Infrared Spectroscopy
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Chapter
16 Instrumental Analysis
Definition of Infrared
Spectroscopy
w The absorption of light, as it passes through
a medium, varies linearly with the distance the light travels and with
concentration of the absorbing medium.
Where a is the absorbance, the Greek lower-case letter epsilon is a
characteristic constant for each material at a given wavelength (known as the
extinction coefficient or absorption coefficient), c is concentration, and l is
the length of the light path, the absorption of light may be expressed by the
simple equation a= epsilon times c times l.
Infrared Spectroscopy
w Infrared spectroscopy is the measurement of
the wavelength and intensity of the absorption of mid-infrared light by a
sample. Mid-infrared is energetic enough to excite molecular vibrations to
higher energy levels.
w The wavelength of infrared absorption bands is
characteristic of specific types of chemical bonds, and infrared spectroscopy
finds its greatest utility for identification of organic and organometallic
molecules. The high selectivity of the method makes the estimation of an
analyte in a complex matrix possible.
Example of IR
Theory of Infrared Absorption
Spectroscopy
w For a molecule to absorb IR, the vibrations or
rotations within a molecule must cause a net change in the dipole moment of the
molecule. The alternating electrical field of the radiation (remember that
electromagnetic radiation consists of an oscillating electrical field and an
oscillating magnetic field, perpendicular to each other) interacts with
fluctuations in the dipole moment of the molecule.
w If the frequency of the radiation matches the
vibrational frequency of the molecule then radiation will be absorbed, causing
a change in the amplitude of molecular vibration.
Molecular Rotations
w Rotational transitions are of little use to
the spectroscopist. Rotational levels are quantized, and absorption of IR by
gases yields line spectra.
w However, in liquids or solids, these lines
broaden into a continuum due to molecular collisions and other interactions.
Molecular
Rotations (cont)
,
Vibrational-Rotational
Transitions
w
In general, a molecule
which is an excited vibrational state will have rotational energy and can lose
energy in a transition which alters both the vibrational and rotational energy
content of the molecule.
w The total energy content of the molecule is
given by the sum of the vibrational
and rotational energies. For a molecule in a specific vibrational and
rotational state, denoted by the pair of quantum numbers (v, J), we can write its
energy as: E(v, J)=Evib(v) + Erot(J)
Transitions (cont)
w The energies of these three transitions form a
very distinctive pattern. If we consider the lower vibrational state to be the
initial state, then we can label the absorption lines as follows.
w Transitions for which the J quantum number
decreases by 1 are called P-branch
transitions, those which increase by 1 are called R-branch transitions and those which are unchanged are called Q-branch transitions.
Molecular Vibrations
w In order to predict equilibrium stable-isotope
fractionations, it is necessary to know the characteristic frequencies of
molecular vibrations. It is also necessary to know how much each vibrational
frequency in a molecule changes when a heavy isotope is substituted for a light
one. Vibrational frequencies for isotopically substituted molecules are not
always known, so it is often necessary to use some type of force-field model to
predict them.
w Molecular vibrations are also important in
understanding infrared absorption and the mechanisms and kinetics of chemical
reactions. Frequencies are most commonly measured with infrared
or Raman spectroscopy. Rotational-vibrational spectroscopy, isotope
substitution, and many forms of force-field modeling are used to determine
characteristic atomic motions.
Vibrational Motion
w Subdivided into so-called normal modes of
vibration which rapidly increase with the number of atoms in the molecule. Each
of these normal vibrational modes contributes RT to the average molar energy of
the substance and is a primary reason why heat capacities increase with
molecular complexity.
w If there are Xvib modes of
vibration, then the vibrational energy contributes Xvib(RT) to the
average molar energy of the substance.
Stretching and Bending
Stretching Vibrations
Bending Vibrations
Quantum Treatment of
Vibrations
w Transitions in vibrational energy levels can
be brought about by absorption of radiation, provided the energy of the
radiation exactly matches the difference in energy levels between the
vibrational quantum states and provided also that the vibration causes a
fluctuation in dipole.
w Infrared measurements permit the evaluation of
the force constants for various types of chemical bonds.
Infrared Instruments
w An infrared spectrophotometer is an instrument
that passes infrared light through an organic molecule and produces a spectrum
that contains a plot of the amount of light transmitted on the vertical axis
against the wavelength of infrared radiation on the horizontal axis. In
infrared spectra the absorption peaks point downward because the vertical axis
is the percentage transmittance of the radiation through the sample.
w Absorption of radiation lowers the percentage
transmittance value. Since all bonds in an organic molecule interact with
infrared radiation, IR spectra provide a considerable amount of structural
data.
References
w http://www.cas.org
w http://www.chemcenter/org
w http://www.shu.ac.uk/schools/sci/chem/tutorials/molspec/irspec/.htm
