Advanced Chemistry: PART 15.2.1
Infrared Spectroscopy Theory
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Examples of the uses and applications of infrared
infrared spectra (on this page, and added links to
relevant organic section indexes)
Some simple NMR-IR
problem solving questions
15.2.1 The theory of infrared
Sub-index for this page
The basis of
infrared molecular spectroscopy
modes - deformations of molecules
infrared spectrometer works
(a) The basis of infrared
Which radiation causes
vibrational excitation and why?
One of the ways electromagnetic
radiation (emr) interacts with matter is to resonate with
the natural vibrational frequencies of the atoms and their
bonds e.g. in an organic molecule.
However, to absorb infrared photons,
the molecular vibrations must involve the oscillation of a
δ+ δ- dipole aspect of the molecule - this is needed to
produce an oscillating electromagnetic field which interacts
(resonates) with the incoming infrared electromagnetic
(distortions-deformations) of atoms-bond arrangements in
molecules are quantised and absorption of photons of the
appropriate frequency-energy will raise the vibrational
level to a higher level in the molecule.
These vibrational frequencies of
molecules coincide with the frequencies of the infrared
region of the emr spectrum.
~7 x 10-7 to 10-4 m, ~700 - ~105
nm; frequency ():
~3 x 1012 to ~4.3 x 1014 Hz
The wavelength range corresponds to ~7 x 10-5 to
It is normal to quote infrared
absorptions as their 'wavenumber'
(1/λ) which corresponds to the reciprocal of the absorption
wavelengths in cm-1.
Therefore the wavelength range of 7 x 10-5 to
~10-2 cm, corresponds to 14285 to 100 cm-1.
In practice, the region of 4000 to
400 cm-1 is typical for the recorded infrared
spectrum of organic molecules.
What causes variation in
the infrared absorption frequencies?
The energy needed to excite a
vibrational mode depends on bond length, the number of bonds
or atoms involved and also the mass of the atoms e.g.
for the simple diatomic molecules like
the hydrogen halides, as the bond enthalpy decreases, there
is a clear trend of decreasing wavenumber and decreasing
energy of infrared photon needed to excite the symmetrical
stretching vibration of the H-X bond (which is the only
vibrational mode here).
You can think of the two atoms
vibrating as if the covalent bond is acting as a spring.
The weaker the bond, 'the weaker
the spring', so less energy is needed to excite the
vibrational quantum level.
|Atomic mass of
absorption peak for symmetrical stretch/cm-1
decreasing wavenumber =====>
====== decreasing frequency
==== decreasing energy of photon
This is a very simple example,
with one vibration mode for a simple diatomic H-X molecule,
but even a simple triatomic molecule like water (H-O-H) has
several fundamental modes of vibration and there are extra
harmonic vibrations too (see below).
What causes variation in
the absorption intensity?
Absorption is usually described as
weak, moderate/medium or strong depending on the decrease in
% transmission (increase in absorption) - see (c) explaining
Generally speaking, the more polar the bond
the greater the intensity of absorption because of the
greater intensity of the dipole oscillation e.g. O-H, C-O
and C=O absorptions tend to be more intense than non-polar
C-H, C-C or C=C bond stretching vibrations.
Stretching vibrations (see section (b)
an A<=>B bond vibration) are often the most useful for
identifying structural features in a molecule, though many
are common to a large number of organic molecules e.g.
Strength of absorption key: weak W),
moderate/MEDIUM (M), strong (S) or variable (V).
||Examples and comments
on the stretching vibration
often S due to several of these groups
||-CH=CH-, W, M or S
||benzene ring hydrogens, can be S
due to several coincident vibrations
||saturated carbon chain
||W, M or S
||alcohols, carboxylic acids,
||aldehydes, amides, carboxylic
acids, esters, ketones
||aliphatic (W-M), aromatic (S)
||non-hydrogen bonded alcohols and
hydrogen bonded alcohols and phenols
hydrogen bonded carboxylic acids
||non-hydrogen bonded amines and
hydrogen bonded primary and secondary amines, amides
(1) Hydrogen bonding affects the O-H
and N-H stretching vibrations
Hydrogen bonded O-H or N-H tend to
give a broader peak compared to a sharper peak for
non-hydrogen bonded stretching vibrations e.g. the
difference between alcohol liquid and alcohol vapour -
where the molecules are not in close contact.
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(b) Vibrational modes
- deformations of molecules
The vibrations of molecules involve some
kind of distortion (deformation) of the bonding arrangement, but
no bond is broken.
There are lots of stretching and twisting
vibrations involving at least two atoms and one bond, but often
involving at least three or more atoms and two or more bonds.
Some of the possible vibrational modes
are illustrated below.
Three vibrational modes of the carbon
Some possible vibrations of an AX2 bond
arrangement in a more complex molecule.
Three basic vibrational modes of the water
The above diagram shows the C-H, C-O and
O-H stretching vibrations of the ethanol molecule.
There are lots of vibrational modes in
'modestly' complex molecules like ethanol including rocking,
scissoring, stretching, twisting and wagging vibrations, and,
combined with different bond strengths and harmonics, the
result is a very complex infrared spectrum.
Although the previous diagram illustrated
three stretching vibrations absorptions of ethanol, which are
pointed out in the above diagram of the complete infrared
spectrum of ethanol.
However there are other vibrations and
many harmonic vibrations derived from the fundamental
vibrations so the infrared spectra of multi-atom molecules
become very complex.
It is this complexity that actually
underlies the identification of an organic molecule from its
infrared fingerprint pattern.
Some good animation of various infrared
vibrations on the webpage
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(c) How an infrared
Below is a schematic diagram of one type
of infrared spectrometer
From the infrared energy source (e.g. a
heated filament), the beam
is split into two identical beams.
One beam passes through a reference cell
('control') and the other beam passes through the sample cell
and each beam is alternately sampled via the beam chopper.
The reference cell is needed to
eliminate absorption by any water vapour or carbon dioxide
in the air or other absorbing
molecules e.g. if a solvent is used.
In air, nitrogen and oxygen do not absorb
infrared photons because the vibration does not involve
oscillation of a dipole (no oscillating δ+ δ- aspect to the molecular
The sample in the cell can be liquid
or vapour and a thin film of a solid can be analysed when
compressed between potassium bromide discs (KBr doesn't
absorb infrared radiation).
The final signal is the difference between
the two beams and then passes onto the diffraction grating which
is rotated to sweep through the infrared frequencies.
The thermocouple detects the signal and
the output recorded on paper or computer screen.
The infrared spectrum is usually
graphically 'portrayed' as % transmittance (0 to 100 for the
vertical y axis) versus wavenumber (usually 4000 to 400 cm-1
on the horizontal x axis).
100% transmission means no absorption,
All Advanced Organic
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