CY1002

Chemistry of Materials for Energy

Applications

Branch: MM/MN
 Books

1. P. W. Atkins, Elements of Physical Chemistry, Oxford University

Press, 2007.

2. M. S. Bhatnagar, A Text book of Polymer Chemistry

3. C. N. Banwell, E. N. McCash, Fundamentals of Molecular

Spectroscopy, tata McGraw-Hill Education, 1994.

4. A. R. West, Solid State Chemistry and its applications, second

edition, John Wiley & Sons Ltd., 2014.

2
 Molecular Spectroscopy
in
Microwave to UV-visible range

Books:

1. Banwell & McCash, Fundamentals of Molecular Spectroscopy

2. J. M. Hollas, Basic Atomic and Molecular Spectroscopy
3. D. N. Sathyanarayana, Handbook of Molecular Spectroscopy
 Introduction
What is spectroscopy?
Interaction of molecules with electromagnetic radiation.
Absorption, emission or scattering of electromagnetic radiation by
atoms or molecules

Purpose?

• It gives us structural information of molecules.

The number, position, band width and intensity of the absorption
or emission bands can be correlated with the electronic and
molecular structure and bonding.
• Helps to identify a molecule.
• Strongest analytical tool in chemistry.
 Introduction

Why molecules interact with electromagnetic radiation?

Molecule and its components (electron cloud, proton) are under constant periodic
motion. This motion creates a local electric/magnetic field.

Electromagnetic Radiation
Electromagnetic wave follows:
y
y = A sin(ωt) t

At t = π/2ω, 5π/2ω, 9π/2ω ...etc, y = A (maximum value)

At t = π/ω, 2π/ω, 3π/ω ...etc, y = 0

And at θ = 3 π/2ω, 7 π/2ω, 11 π/2ω ...etc,

y = - A (minimum value)
 Introduction

Two sinusoidal waves consist of oscillating electric and magnetic fields directed
perpendicular to each other and the direction of propagation of the wave.
EMR
EMR is consisting of a stream of photons or quanta travelling in the direction of
propagation of the beam with the velocity of light.
Dual nature of light
hc
E  h   hc  mc 2

Electromagnetic Spectrum
Arrangement of EM radiations in order of their wavelengths or frequencies
 Interaction of radiation with matter
When a beam of light interacts with matter, numerous changes occur in
both light and matter. These changes are the basis for several research
tools
EMR is characterised by ,, I and direction, so the change can be in
any of these characteristics

Change in direction: Reflection and Refraction, Diffraction

Scattering: Elastic (Rayleigh) and Inelastic (Raman)
If the beam is plane polarised and the direction is rotated by passing
through the compound: Optical rotation (Polarimetry)
Change in Intensity: Absorption
Emission of light (different  than the absorbed one)
 Interaction of radiation with matter

For a molecule to interact with EMR and to absorb or emit a photon

of frequency , it must exhibit an oscillating dipole at that frequency.
This condition imposes certain restrictions on the system and are
known as gross selection rules of a particular spectroscopy

Exactly how the radiation interacts with matter is directly dependent on

the energy of the radiation.
 Rotational motion
Magnitude of dipole moment vector in y direction in each case is shown:

+
-

-
+

- +
+ -

-
+
0 α 0 -α 0

Molecule rotates anti-clockwise manner

Dipole moment along y axis

α α

0 0
0 0
time
-α
-α

Periodic fluctuation or alteration of dipole moment

 Vibrational motion
y

δ-O

δ + +C

δ-O
Net dipole moment = 0
 Interaction of radiation with matter
Rotational motion
Fluctuating dipole moment interacts with periodically fluctuating
electric field of electromagnetic radiation Rotational spectroscopy

Vibrational motion
Fluctuating dipole moment interacts with periodically fluctuating
electric field of electromagnetic radiation vibrational spectroscopy

….and similarly,

Electronic motion
The excitation of a valence electron involves the moving of electric charges in
a molecule creating changes in dipole moment, which interacts with undulatory
electric field of electromagnetic radiation electronic spectroscopy
 Region of spectrum

E = hν

ν = c/λ, or c = νλ,

E = hc/λ or, E = hcλ-1 = hc𝜈ҧ

[ where 𝜈ҧ stands for ‘nu bar’]
What happens after a molecule interacts with
electromagnetic wave?
Excited state
Excited state energy

hν hν ΔE

molecule
Ground state energy
Ground state
 Change in

structure
nuclear
in spin orientation of nuclei in molecule

vibrational motion in molecule

jump in molecular
Due to electronic
Energy gap created due to difference

electronic

in atomic
rotational motion in molecule

Energy gap created due to

orbital
Excited

jump
Energy gap created due to
state 6

orbital
Excited
state 5

Excited
state 4 ΔE6
ΔE5
Excited
state 3 C
Excited
Excited
state 2 ΔE3
state 1
ΔE2
ΔE1
Ground state
Radio Micro Infra red Visible & X-ray γ-ray
wave wave Ultra violet
Δ E 1 = hcλ1−1 Δ E 2 = hcλ2−1 Δ E 3 = hcλ3−1 Δ E 4 = hcλ4-1 Δ E 5 = hcλ5−1 Δ E 6 = hcλ6−1
λ1 =10 m – 1 λ2 = 1 cm - 100 λ3 = 100 μm – λ4 = 0.8 μm – 10 λ5 = 10 nm – 100 λ6 = 100 pm - -
cm μm 0.8 μm nm pm
 Interaction of EMR with matter
Change of spin Change of Change of Change of electron distribution Change of
orientation configuration nuclear
configuration
radiowave Microwave Infra-red Visible and UV X-ray - ray
NMR ESR

Or

͞ in cm-1 10-2 1 100 104 106 108

λ 10m 100cm 1cm 100μm 1μm 10nm 100pm

 in Hz 3 x 106 3 x108 3x 1010 3 x 1012 3x 1014 3 x 1016 3 x 1018

E in J/mol 10-3 10-1 10 103 105 107 109

•NMR spectroscopy deals with nuclear spin motion

•Microwave spectroscopy deals with rotational motion
•IR spectroscopy deals with vibrational motion
•uv-visible spectroscopy deals with electronic motion (transition).
 Spectrum
What is a Spectrum
The data that is obtained from spectroscopy is called a spectrum.
A spectrum is a plot of the intensity detected versus energy (wavelength, frequency,
wavenumber etc.).

What information is Obtained

• A spectrum can be used to obtain information about atomic and molecular
energy levels, molecular geometries, chemical bonds, interactions of
molecules, and related processes.

• Often, spectra are also used to identify the components of a sample

(qualitative analysis). Spectra may also be used to measure the amount of
material in a sample (quantitative analysis).
 Spectrum

A spectrum is characterised by its position (, ), intensity (I), shape

and width
1. Spectral Position: Band position relates to the appropriate energy
levels and difference in energy thus it is the easiest parameter to
measure
2. Spectral Intensity: The intensity depends on three factors
(i) Transition Probability: allowed transitions generate intense
bands than the forbidden transitions
(ii) Population: Higher the population greater is the intensity
(iii) Concentration (and path-length): Beer-Lamberts’law can be used
to express the relationship between concentration, path-length
and intensity of radiation
 Population of energy states
• The continuous thermal agitation that molecules experience at any temperature
ensures that they are distributed over all possible energy levels.
• Population of a state = the average number of molecules in a state at any given
time.

The mathematical formulation of how to calculate the population

of a state was provided by Ludwig Boltzmann in the late 19th
century.
The Boltzmann distribution
The Boltzmann distribution defines the relative population of energy
states (usually the ratio of excited states to ground state).

There is always a higher population in a state of

lower energy than in one of higher energy.

Effect of temperature
• At lower temperatures, the lower energy states are more populated.
• At higher temperatures, higher energy states are also populated
kBT ~ 2.5 kJ mol-1 at 300 K.
Examples of molecular spectra:

Rotational/microwave spectra
Examples of molecular spectra:

Vibrational/IR spectra

cm-1
 electronic/uv-visible spectra
Absorbance

Abs
What are common in the last 3 examples?

•There are peaks of curves as well as sharp lines.

•In the ‘y’ axis, either ‘absorbance’ or ‘transmittance’ is plotted

•in ‘x’ axis parameters of an electromagnetic radiation

(wave numbers or wavelength) have been plotted.
Summary:
Event Spectroscopy Range
Change in magnetic NMR/ESR Radiowave
dipole of
nucleus/electron
Change in dipole Rotational Microwave (mostly)/IR
moment due to rotation
Change in dipole Vibrational IR our study
moment due to vibration topics
Change in dipole Electronic UV-visible
moment due to
electronic transition
Change in electrical Raman IR (mostly)
polarizability due to
rotation/vibration