INFRARED SPECTROSCOPY

Priyankar Sen, Ph.D.
Centre for Bioseparation Technology.
VIT, Vellore 632014
Priyankar.sen@vit.ac.in
INFRARED (IR) SPECTROSCOPY

1/11/2018 4PRIYANKAR SEN
The portion of the infrared region most useful for analysis of organic
compounds is not immediately adjacent to the visible spectrum, but is that
having a wavelength range from 2,500 to 16,000 nm, with a corresponding
frequency range from 1.9*1013 to 1.2*1014 Hz.

The portion of the infrared region most useful for analysis of organic
compounds is not immediately adjacent to the visible spectrum, but is that
having a wavelength range from 2,500 to 16,000 nm, with a corresponding
frequency range from 1.9*1013 to 1.2*1014 Hz.

VIBRATION OF CO2
SYMMETRIC STRECHING ASYMMETRIC STRECHING

1/11/2018 PRIYANKAR SEN 14
vibrations

IR spectroscopy Day-2
Biological spectroscopy
BIT3005

Conventions:
Wavelength: it ranges from 780nm to 250,000nm. For
convenience it used to be written in micron (μm; 10-6m), thus
0.78 to 250 μm.
Wavenumber: number of wavelenghts per unit distance.
Therefore, 100cm-1 implies there are 100 wavelengths per
cm.wavenumber in cm-1 is given by:
What will the conversion formula if wavenumber is in cm-1 and
wavelength is in micron???

• Infrared region is usually divided into three regions:
near infrared, mid-infrared, and far infrared
• Mid-IR region (λ = 2.5 -25 μm; = 4000 – 400 cm-1) is the region of interest
for studying molecular vibrations.

Degrees of freedom and molecular vibrations
• At non-zero temperatures, i.e. temperatures
above 0 K, all the atoms in a molecule are in
motion. The molecule itself also is in
translational and rotation motion.
TRANSLATIONAL MOTION
MOLECULAR MOTION
ROTATIONAL MOTION

In a three dimensional space, an atom in isolation has 3 degrees of
freedom, corresponding to the motion along the three independent
coordinate axes. A molecule composed of N atoms has a total of 3N
degrees of freedom.
Degrees of rotational freedom for a diatomic (A) and a triatomic (B) molecule

 For a non-linear molecule, three of these 3N degrees of freedom
correspond to translational motion, three correspond to rotational
motion while rest 3N-6 are the vibrational degrees of freedom.
3N= 3 transl + 3 rotational + (3N-6) vib
 For a linear molecule, there are only two rotational degrees of
freedom that correspond to the rotation about the two orthogonal
axes perpendicular to the bond. A linear molecule, therefore, has
3N-5 vibrational degrees of freedom.
3N= 3 transl + 2 rotational + (3N-5) vib
BOND ANGLE 108° 180°

Let us have a look at the degrees of freedom of
a diatomic molecule.
 A diatomic molecule has a total of 3 × 2 = 6 degrees
of freedom.
 Three of these six degrees of freedom correspond to
translational motion of the molecule;
 two of them define rotational degrees of freedom;

• while one corresponds to the vibration of the atoms
along the bond. The 3N-6 vibrational degrees of
freedom (3N-5 for linear molecules) represent the
true/fundamental modes of vibration of a molecule.

The different types of vibrations are shown
Stretching and bending vibrations in molecules

• A vibrating bond can therefore be considered a spring with its
ends tethered to two atoms.
• If the masses of the atoms are m1 and m2, the frequency of
stretching vibration of the diatomic molecule can be given by
the Hooke’s law:
Express it in wavenumber?

• A vibrating bond can therefore be considered a spring with its
ends tethered to two atoms.
• If the masses of the atoms are m1 and m2, the frequency of
stretching vibration of the diatomic molecule can be given by
the ? law:
Frequency
of vibration
Spring constant
Reduced mass

Dividing equation by λ gives:
• The spring constant, k is the measure of the bond strength. The
stronger the bond, the higher the k , and consequently the higher is
the frequency of vibration.

QUESTION?
• An IR active vibration of a particular frequency
absorbs the IR radiation of same frequency.
Let us calculate the position of absorption
band for carbonyl stretching vibration
(frequency = 5.1 × 1013vibrations/second) in
acetone.

INSTRUMENTATION

WORKING DIAGRAM OF FTIR

ATR-FTIR

47
Infrared Sampling Methods
Transmission
detector source
Internal Reflection
Attenuated Total Reflection (ATR)

48
Transmission Mode Reflection Mode
•
•
sample
sample
radiation
radiation

49
• To obtain an IR spectrum in the
transmission mode, the sample
must be placed in a “container” or
cell that is transparent in the IR
region of the spectrum.
• Sodium chloride or salt plates are
a common means of placing the
sample in the light beam of the
instrument.
IR transparent Salt Plates
Transmission Mode

50
Dessicator
Water-free
Environment
for
Water-sensitive
Salt Plates.
Transmission Mode

51
Transmission Mode
Preparing a “Neat” IR liquid Sample

56
Light Path
(shown by red line)
Transmission Mode

57
Preparing a KBr Disk for solid samples
Transmission Mode
1 2 3
4
5

58
• Infrared energy propagates through an internal reflection crystal (IRE), usually ZnSe, Ge and KRS-5
(mixture of thallium iodide and bromide) and recently we use diamond as the internal reflection
element.
• Although the infrared radiation is “completely” internally reflected, an evanescent wave extends
beyond the surface of the crystal.
• Any sample that is in intimate contact with the crystal will interact with the infrared energy,
reducing the internal reflectance (attenuated).
Reflectance mode Attenuated Total Reflectance (ATR)
sample

59
FT-IR Sampling
 Traditional transmission experiment
 Employs alkali halide (salts) matrix
 True transmission spectrum is collected
 Attenuated Total Reflection
– Simple and faster to use.
– Minimal sample preparation (simply place the sample in
contact with the ATR element)
– Often 1 step process
– Easy to clean-up
– Reflectance-type spectrum is collected.
– solids/liquids/films.
– Spectral features are usually weaker than those of transmission
spectrum.

60
Traditional ATR Disadvantages
(using ZnSe as IRE)
• Chemically vulnerable
– Clean only with a non-corrosive solvent
• Physically susceptible
– Soft nature of internal reflectance element (IRE)
restricts the application of force
– Surface can be scratched
• Not suitable for:
– Corrosive liquids
– Harder powders

61
Diamond ATR Advantages
• Diamond ATR overcomes the limitations of traditional ATR
– Small ATR element size allows intimate contact with samples
and greatly reduces sample size requirements.
– Reproducible results are achievable because, by filling the ATR
element every time, path length and sample area are constant.
– Mechanical strength of the diamond ATR element allows for
compression of the sample for intimate contact.
– Resistant to all corrosive and abrasive solvents and samples.
– Cleans up easily because of the low friction coefficient of
diamond.
– Control sensitivity.

62
Diamond ATR Disadvantages
• Diamond interferes (absorbs IR) between 2200 cm-1 to 1900
cm-1
– Not major; however, some cyano group information could
be lost
• Spectrum usually weaker than traditional transmittance
experiments
– due to the small penetration depth of the evanescent
wave into the absorbing medium, typically around
10 µm in the infrared

63
4000 3000 2000 12001600 800 400
Infrared Spectra
(KCl matrix versus ATR)
Transmittance / Reflectance
Wavenumber (cm-1)
ATR
KBr matrix

INFRARED SPECTROSCOPY

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