2. What is Spectroscopy?
• It is study of interaction of electromagnetic radiation with Matter
• Spectroscopy is a technique which helps to determine structure, to identify
compounds etc.
• It destroys no or little sample.
What is Electromagnetic Radiation?
• Radiation or light is the process of heat transfer which do not requires any medium.
• A Radiation with electric and magnetic field component is called as EMR for ex The
portion of the EM spectrum from 400-800 nm is observable to humans i.e. Visible
Light
Violet 400-420 Indigo 420-440 Blue 440-490 Green 490-570
Yellow 570-585 Orange 585-620 Red 620-780
3. Wave Parameters
• Wavelength (λ) : The Distance between two successive crusts or troughs in a
wave.
Units : 1A0 = 10-8cm
1nm = 1mμ=10-7cm
• Wave Number (ν ) : Reciprocal of wavelength (1/ λ)
Unit : cm-1
• Frequency (ν ) : Total number of waves passing through a point in one second
(ν = C/ λ , Where C = 3X1010cm/S)
Units : 1CPS = 1Hz , 106 Hz = 1MHz
• Energy (E) : Entity to do work. (E= hν where h=6.626X10-36JS)
Thus E = hν = hC/ λ = hC ν
4. What is Matter?
• Matter is anything which we feel through a sense
• There are three states of Matter
• Solid , Liquid and Gases
About 99% of mammal's mass are the elements carbon, nitrogen, calcium,
sodium, chlorine, potassium, hydrogen, phosphorus, oxygen and sulfur.
Proteins, lipids and carbohydrates contain the majority of the carbon and
nitrogen and most of the oxygen and hydrogen is present as water.
5. Spectroscopy
When continuous wave radiation passes through a transparent
material (solid or liquid) some of the radiation might be
absorbed by that material.it will produce a light spectrum that
has gaps in it (caused by the absorption of radiation by the
transparent material through which is passed).
When continuous wave radiation is passed
through a prism a diffraction pattern is produced
(called a spectrum) made up of all the
wavelengths associated with the incident
radiation.
Radiation source
Diffraction prism
Spectrum
Transparent material that
absorbs some radiation
Spectrum with ‘gaps’ in it
With uv and visible spectroscopy The effect of absorption of radiation on the transparent
material is to change is from a low energy state (called the ground state) to a higher energy
state (called the excited state). Where as with infra red spectroscopy the low energy
radiation simply causes bonds to bend and stretch when a molecule absorbs the radiation.
7. IR Spectroscopy
Near Infra-red Region 0.8-2.5 m
Ordinary Infra-red region 2.5-15 m
Far Infra-red region 15-200 m
IR Region (Just below red in the visible region)
Absorbed light energy is utilized for vibrational motion .
Mostly IR spectra are plotted as % Transmittance against
wave number
8. IR Spectroscopy
• Stretching Vibration : The distance between two atoms
increases or decreases but atoms remains in same bond axis.
1. Symmetrical stretching : Movement of atoms with respect to particular
atom is in same direction
2. Asymmetrical stretching : One atom approaches the central atom while
the other departs from it
Types of Stretching Vibration :
Absorbed IR radiation is utilise to perform various kinds of vibrations
9. IR Spectroscopy
Bending Vibration : Position of atoms changes with respect to original
bond axis
1. Scissoring : Two atoms approach each other
2. Twisting : One of the atoms moves up the plane while other moves down
the plane w.r.to central atom
3. Wagging : Two atoms moves up and below the plane w.r.to central atom
Types of Bending Vibration :
10. 4. Rocking : Movement of atoms takes place in the same direction
Bending vibrations requires lesser energy and hence occurs at lower wave
number than stretching vibrations.
What is Difference between
Scissoring and Twisting
11. Vibrational Frequency :
Bonds are not rigid but behave like a spring with a mass at
either end. So it is calculated by application of Hook’s law
This gives rise to a characteristic frequency for the vibration:
2
1
2
1
_
m
m
m
m
mass
reduced
mass
reduced
k
_
2
1
n = n
c
m1 and m2 are masses of atoms in grams
k is force constant related to strength of bond. For single bond it is
approximately 5 X 105 gm sec -2. It becomes double or triple for double
and triple bond
c is velocity of light (2.998 X1010 cm sec-1)
12. Calculate wave no for C-H stretching vibration
Mass of carbon atom = 20 X 10-24 gm
Mass of hydrogen atom = 1.6 X 10-24 gm
k = 5 X 105 gm sec -2
c = 2.998 X1010 cm sec-1
Ans: Wave Number = 3032 cm-1
SOLVE
n Experimental 3000 cm-1
13. Calculating stretching frequencies
C=C K = 10* 105 dynes/cm
n = 4.12
K
m
m
M1 M2
M1 + M2
(12)(12)
12 + 12
6 n= 4.12
10* 105
6
= 1682 cm-1
n Experimental 1650 cm-1
C—D K = 5* 105 dynes/cm
n= 4.12
5* 105
.923
= 2228 cm-1
m
M1 M2
M1 + M2
(12)(2)
12 + 2
1.71
n Experimental 2206 cm-1
14. Fundamental Bands and Overtones :
Evib = (V + ½) hv
h is Plank’s constant
V is vibrational quantum number
v is vibrational frequency
Transition from ground state (V =0) to first excited state (V = 1) absorbs strongly and
gives rise an intense band known as Fundamental band ΔEvib = hv
Transition from ground state (V =0) to second excited state (V = 2) gives rise weak
band known as overtone (Energy of first overtone is ΔEvib = 2hv)
15. Degrees of Freedom : Sum of number of cartesian coordinates required to locate
position of point
Example No of degrees of freedom = 1
Example No of degrees of freedom = 2
Example No of degrees of freedom = 3
(x,y )
(x,y,z)
16. Molecule has always three translational degrees of freedom
3n degrees of freedom = Translational + Rotational + Vibrational
Where n is number of atoms
Vibrational degrees of freedom = 3n degrees of freedom–Translational degrees
of freedom – Rotational degrees of freedom
Where n is number of atoms
For linear molecule there are 2 roatational degrees of freedom
For non-linear molecule there are 3 roatational degrees of freedom
For linear molecule number of fundamental bands are
Vibrational degrees of freedom = 3n -3 – 2 = 3n-5
For non-linear molecule number of fundamental bands are
Vibrational degrees of freedom = 3n -3 – 3 = 3n-6
Number of fundamental bands = Vibrational degrees of freedom
Calculate number of fundamental bands for CO2 and C6H6
18. How to Analyze an IR Spectrum
• Pay the most attention to the strongest absorptions:
– -C=O
– -OH
– -NH2
– -C≡N
– -NO2
• Pay more attention to the peaks to the left of the
fingerprint region (>1250 cm-1).
23. Instrumentation
Solid : The sample is grind with KBr and is made into disc by
application of heat and pressure where as pure KBr disc is used as
reference disc
Liquids: A thin film (0.1 to 0.3mm) of sample squeezed between
two NaCl plates
Gases: A gas sample is introduced into 10 cm long gas
cell made up of NaCl
Solution : Solvents used are CHCl3, CCl4 , CS2
A = log10 (I0/I)
%T = (I/I0) X 100
26. Applications of IR Spectroscopy
• Chemical Analysis: Match spectra to known databases
(Identifying an unknown compound, Forensics, etc. )
Monitor chemical reactions
• Structural ideas: Can determine what chemical groups are in a
specific compound
• Detection and Identification of impurities :
If the compound have been characterized before, any bands
that are not found in the pure sample can be assigned to the
impurity
(provided that the 2 spectrum are recorded with identical
conditions: Phase, Temperature, Concentration)
Quantitative Analysis of mixture
