Infrared Spectroscopy
Electromagnetic radiation spectrum
Infrared Spectroscopy
• Interaction between molecules of a sample and infrared radiation.
• The infrared radiation was discovered by Wilhelm Herschel during his
study of the heating ability of the visible region.
• He discovered that below the red colour, there was some form of
radiation which is not visible to the naked eye, and he termed it
“infra-red” – “below red”.
• Wavelengths of infrared radiation ranges from 780 nm to 1 mm in the
electromagnetic radiation spectrum.
Infrared spectroscopy
• When molecules are bombarded with a beam of infrared radiation
and absorb, they undergo vibrations and rotations.
• Two conditions to be met for IR radiation absorption to occur.
• Frequency of the absorbing molecule must equivalent to the frequency of
infrared radiation.
• Absorbing molecule must have a resultant dipole moment
• This means that homonuclear diatomic molecules such as O2, Cl2, N2
etc do not absorb IR radiation because they have zero dipole
moment.
• Quiz: Why does a molecule of H2O absorb IR radiation whereas CO2
does not?
Molecular vibrations
• In IR spectroscopy, when a polychromatic light (light having different
frequencies) is passed through a sample, the intensity of the
transmitted light is measured at each frequency.
• When molecules absorb IR radiation, bonds of molecules experience
various types of vibrations and rotations. This means transitions occur
from a ground vibrational state to an excited vibrational state.
• There are two forms of vibrations, namely, stretchings and bendings.
• Stretchings are classified as symmetric or asymmetric as shown in the
diagram below.
Molecular vibrations
Molecular vibrations
• Bendings are categorized as out of plane and in plane. Out of plane
bendings include
o Out of plane wagging
o Out of plane twisting
• In plane bendings include
o In plane rocking
o In plane scissoring
Molecular vibrations
Infrared spectroscopy spectrum
Infrared spectroscopy spectrum
Instrumentation
• Infrared instruments using a grating monochromator for wavelength
selection are constructed using double-beam optics.
• Double beam optics are preferred over single-beam optics because
they compensate for the weaker sources and less sensitive detectors
for infrared radiation which are less stable than that for UV/Vis
radiation.
• In addition, it is easier to correct for the absorption of infrared
radiation by atmospheric CO2 and H2O vapor when using doublebeam optics.
• Components of a grating IR spectrometer include source of IR,
monochromator, sample cell, detector and signal processor
Schematic diagram of a double beam grating
IR spectrometer
Schematic diagram of a double beam grating
IR spectrometer
• From the dispersive IR spectrometer schematic diagram, it can be
seen that the sample and reference sample compartments are placed
between source and monochromator.
• This is because IR radiation from source is not strong enough to
decompose the sample photochemically.
• Also, this arrangement allows the monochromator to block scattered
radiation and cell compartment IR emissions from reaching the
detector.
Fourier transformer infrared spectrometer
(FTIR)
• In the FTIR, a beam of IR from the source is directed towards the
wavelength selector, an interferometer in this case.
• The interferometer consists of a beam splitter and two mirrors, one is
movable and the other is stationary.
• When the parallel beam from the source reaches the beam splitter, it
is split into two halves, one half towards the movable mirror and the
other half directed towards stationary mirror.
• When the two parts of IR beam strike the mirrors, they are reflected
back to the beam splitter where they re-converge and directed
towards the sample and then a detector.
Schematic diagram of an FTIR
Schematic diagram of an FTIR
Schematic diagram of an FTIR
Double beam FTIR spectrometer
Double beam FTIR spectrometer
• In a double FTIR spectrometer, the beam emanating from the
interferometer is directed through the sample and reference cells by
mirror M1 oscillating in different positions.
• Mirror M2 which is synchronized to mirror M1 alternately directs the
reference beam and the sample beam to the transducer.
• As compared to the single beam FTIR, the double-beam design
compensates for source and detector drift.
Advantages of FTIR spectrometer
• Provides better speed and sensitivity in which a single scan is used to
obtain the entire spectrum
• Simple mechanical design in which few parts are moving, less wear
and tear
• Stray light and emission contributions are reduced due to FTIR
modulation.
Advantages of FTIR over Grating IR
spectrometer
• The FTIR has a greater throughput as compared to the dispersive or
grating IR spectrometer because in the grating IR there are slits which
restrict throughput.
• Signal to noise ratio of FTIR spectrometer is almost 10× greater than
the grating IR spectrometer.
• In the grating IR, analysis is done by successive wavelength
measurements whereas in the FTIR all the beam of radiation reaches
the interferometer, sample and detector simultaneously. This renders
the analysis quicker, more accurate and sensitive using an FTIR
compared to grating IR spectrometer.
Applications of FTIR spectrometers
• Used for identifying functional groups in a variety of samples
• Used for high resolution work
• Used for studying samples with high absorbance
• Used for kinetic studies
• Used for emission studies
Sources of IR radiation
• IR spectrometers require a continuum source of infrared radiation
and a sensitive infrared transducer, or detector.
• Infrared sources consist of an inert solid that is electrically heated to a
temperature between 1,500 and 2,200 K. The heated material will
then emit infra red radiation.
• Sources of IR radiation provide intensities in the different regions of
IR, namely, near IR, mid IR and far IR.
• These sources include the Nernst glower, the globar, incandescent
wire source, tungsten filament lamp, carbon dioxide laser source.
Sample cells
• Cells used for IR analysis are normally narrower (0.01 to 1 mm) than
those used for UV-Vis spectroscopy because of the tendency of
solvents to absorb IR radiation.
• Most commonly used cells are those of sodium chloride and
potassium bromide windows because of their cost.
• The cells should be handled with care because their surfaces absorb
moisture and become fogged. They also require frequent polishing to
restore their original using a buffing powder.
Sample preparation methods for IR analysis
• Solid samples
• Solid samples are prepared by grinding 2-5 mg into fine powder using
pestle and mortar.
• Fine powder is then mixed with one to two drops of nujol or liquid
paraffin to form a mull. The mull paste is then placed between KBr
plates for analysis.
• Solid samples can also be prepared my mixing 1 mg of finely
grounded sample with 100 mg KBr powder. The mixture is then
pressed into a disc or pellet. This method is called pelleting.
Sample preparation methods for IR analysis
• Liquid samples
• Very thin liquid films are prepared in a non-IR active solvents like
chloroform and placed between cells. Solvents such as water and
alcohols are not suitable for use because H2O absorbs strongly in the
IR region.
• Water and alcohol also attack alkali metal halides cells. Water
insoluble cells used include those made of barium fluoride material.
• Gases
• Special transparent holding cells are used for evaporating volatile
liquids and gaseous samples.
Quantitative analysis using IR spectrometer
• The IR spectroscopy is mostly used to identify an unknown sample. It
is not widely for determination of concentration or quantitative
analysis because of the following reasons:
• Often it does not obey the Beer’s law because IR bands are narrow.
Also because of the combination of weaker sources and less sensitive
transducers requires monochromators with wider slits and all this
leads to non-linearity.
• Complicated spectra may lead to overlapping peaks.
• Narrow cells are inconvenient to use and lead to uncertainties.
Quantitative analysis using IR spectrometer
• However, IR quantification can still be done using the Beer's law
which is written as:
• A = Ɛlc, where A is absorbance, Ɛ is molar absorptivity, l is the pathlength and
c is molar concentration
• From the Beer's Law, absorbance is directly proportional to the
concentration of the sample since the analytes have a particular
molar absorptivity at a particular wavelength.
• Therefore, we could use IR spectroscopy and Beer's Law to find the
concentration of substance or the components of mixture.
Quantitative analysis using IR spectrometer
• In case of qualitative analysis, group frequencies are used for
structural analysis to identify functional groups in a molecule.
• Information on vibrational frequencies of functional groups are then
matched with those stored in the library of the instrument for
identification.
• Conventionally the IR region is subdivided into three regions, near IR,
mid IR and far IR.
• Most of the IR vibration frequencies used originates from the mid IR
region.
Qualitative analysis using IR spectrometer
• The table below indicates the IR spectral regions
IR Region
Wavelength
Wavenumber
Frequency
Near IR
0.78 – 2.5 µm
12800 – 4000
3.8 x 1014 - 1.2 x 1014
Middle IR
2.5 – 50 µm
4000 – 200
3.8 x 1014 - 1.2 x 1014
Far IR
50 – 100 µm
200 – 10
3.8 x 1014 - 1.2 x 1014
Most Used
2.5 -15 µm
4000 - 670
3.8 x 1014 - 1.2 x 1014
Infrared vibration stretching frequencies
Infrared absorption bands
Infrared spectroscopy spectrum
Infrared spectroscopy spectrum
Infrared spectroscopy spectrum
Estimate wavelength and wavenumber region
of absorption of molecules
• The wavenumber, wavelength and frequency region of where
molecules absorb IR radiation can be calculated using formulae
below:
1
•  = wavenumber =
2𝑐
𝑘
= 5.3 × 10-12 s/cm
µ
𝑚1 𝑚2
• µ = reduced masses =
𝑚1+𝑚2
𝑘
µ
Estimate wavelength and wavenumber region
of absorption of molecules
• Quiz
Calculate the approximate wavenumber and wavelength of the
fundamental absorption due to the stretching vibration of a carbonyl
group C=O.
• Solution
 = wavenumber = 5.3 × 10-12 s/cm
𝑘
µ
𝑚1 𝑚2
µ = reduced masses =
, k is the force constant = 1 × 103 N/m
𝑚1+𝑚2
Estimate wavelength and wavenumber region
of absorption of molecules
𝑚1 𝑚2
• µ = reduced masses =
𝑚1+𝑚2
• 1 mole of C = 12 g
• 1 mole of C = 6.023 × 1023 atoms
• Therefore 6.023 × 1023 atoms = 12 g
•
Thus 1 atom =
12 𝑔
6.023 × 1023
• Thus 1 atom of C has a mass = 2.0 × 10-23 g
• Similarly, you calculate the mass of one atom of oxygen which will be
1 atom = 2.7 × 10-23 g
Estimate wavelength and wavenumber region
of absorption of molecules
𝑚1 𝑚2
• Thus, reduced mass =
𝑚1+𝑚2
(2.0 × 10−23 g)(2.7 × 10−23 g)
=
(2.0 × 10−23 g + 2.7 × 10−23 g)
= 1.1 × 10-23 g × 10-3
= 1.1 × 10-26 kg
Calculating for wavenumber = 5.3 × 10-12 s/cm
𝑘
µ
Estimate wavelength and wavenumber region
of absorption of molecules
• wavenumber = 5.3 × 10-12 s/cm
= 5.3 × 10-12 s/cm
= 1.6 × 103 cm-1
1 × 103 𝑁/𝑚
1.1 × 10−26 kg
1 × 103 𝑘𝑔𝑚𝑠 −2 /𝑚
1.1 × 10−26 kg
Interference fringes
• Fringes in IR spectroscopy refers to the interferences that introduce
distortions in spectra and affect the absorbance peaks and accuracy
of analysis.
• The number of interference fringes, ΔN, between two known
wavelengths 1 and 2 can be calculated using the formula below:
Δ𝑁
b=
2(1 − 2)
• where b is the thickness of IR cells, 1 and 2 are wavenumbers
between position points.