Sample handling and interpretation of IR spectra – Instrumental Methods of Analysis B. Pharma 7th Semester

Sample handling and interpretation of IR spectra

Objectives

After this session
students will be able to

•       Recognise
that all three states of matter can be used for obtaining IR spectrum

•       Discuss
the sample handling techniques

•       Identify
the conditions for IR absorption  and
importance of wave numbers in IR spectroscopy

Sampling technique

•       Solid
Samples:

–                      Neat sample

–                      Cast films

–                      Pressed films

–                      KBr pellets

–                      Mull

•       Gas
Samples:

–                      Short path cell

–                      Long path cell

•       Liquid
Samples:

–                      Neat sample

–                      Diluted solution

–                      Liquid cell

•       Pressed
pellet techniques

•       Potassium
bromide (KBr) is probably the most widely used matrix material

–      Before
use, it is dried for 2 hours at 105˚C

–      Sample
+ 100 Times KBr (1:100)

–      Mixture
is transferred to a die that has a barrel diameter of 13 mm                       

–      Press
at least 25000 psi

–      Clear
glassy disk of about 1 mm thick obtained

–      Ready
for transmission

Sample
analysis procedure

1. Blank KBr  prepared

Sample
holders

Liquid Cell        

Fixed Cell

Application

•       Identification
of inorganic compounds and organic compounds

•       Identification
of components of an unknown mixture

•       Analysis
of solids, liquids, and gasses

•       In
remote sensing

•       In
measurement and analysis of Atmospheric Spectra

–      Solar
irradiance at any point on earth

–      Longwave/terrestrial
radiation spectra

•       Used
on satellites to probe the space

Interpretation

•       Infrared
Spectroscopy or Vibrational spectroscopy is concerned with the study of
absorption of IR radiation, which results in Vibrational transition

•       IR
spectra is mainly used in structure elucidation to determine functional group

•       Energy
of molecule = Electronic energy + Vibrational energy + Rotational energy

•       IR
spectroscopy – changes in vibration of molecule or absorption of energy due to
vibration

•       Wavelengths
usually used is 2.5-25 µm

•       IR
spectra, we use wave numbers(cm^-1),the reciprocal of the wavelength
in centimeters (4000-400cm^-1)

•       Because
wave numbers have larger values  &
easy to handle than wavelengths which will show only small differences between
functional groups

•       Wave
numbers -the number of waves present per cm, which can be calculated from the
wavelength.                                                                   ϑ = 1 / λ

•       Wave
numbers are proportional to frequency and energy

•       Every
bond or portion of a molecule requires different frequency for absorption

•       Hence
characteristic peak is observed for every functional group or part of molecule

•       IR
spectrum is  a finger print of a molecule

•       Criteria
for compound to absorb IR radiation:

•       Absorption
of IR should cause change in dipole moment

•       Applied
IR frequency should be equal to the natural frequency of radiation-Otherwise
compound do not give IR peak

•       No
net change in dipole moment occurs during the vibration or rotation of homo
nuclear species  such as O₂ ,N₂
,Cl₂; such compounds cannot absorb IR radiation

•       Infrared
regions may be categorized into three distinct zones based on their wave
numbers and wavelengths as under:

Region of IR	Wave length(µm)	Wave number(cm-1)
Near IR (Overtone region)	0.8-2.5	12,500-4000
Mid IR (Vibration – rotation region)	2.5-50	4000-200
Far IR(Rotation region)	50-1000	200-10
Most used	2.5-25	4000-400

 

•       Principle
of IR spectroscopy is based upon the molecular vibrations – composed of
stretching and bending vibrations of a molecule

•       In
any molecule, atoms or groups of atoms are connected by bonds

•       These
bonds are in  a continuous motion in a
molecule- as a result, they maintain some vibrations with some frequency, which
is characteristic to every portion of molecule

•       This
is called natural frequency of vibration

Stretching
Vibrations of a CH₂ Group

Bending Vibrations
of a CH₂ Group

Bending Vibrations
of a CH₂ Group

Molecular vibrations

Vibrations	Type
Stretching Vibration (change in bond length & bond angle remains the same)	Symmetrical
	Unsymmetrical
Bending Vibration (bond length remains the same & bond angle may or may not change)	Inplane •       Scissoring •       Rocking
	Outplane •       Wagging •       Twisting

•       Vibrations
seen before are called fundamental absorption

•       They
arise from excitation from the ground state to the lowest energy excited state

•       Usually
spectra are complicated because of the presence of weak overtone, combination
and difference bands

•       Overtones
(multiples of given frequency) results from excitation from ground state to
higher energy states

Summary

•       IR
spectrum is obtained by plotting transmittance against wave numbers

•       IR
spectrum can be obtained for solids, liquids and gases

•       The
path lengths for solids and liquids are as small as fraction of a millimeter
while for gases the normal path length is 10 cm

 

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