1. North eastern Hill University Shillong
CENTRE FOR ADVANCED STUDIES IN CHEMISTRY
North eastern Hill University Shillong
SPECTROSCOPY : An explalnation of SPECTRUM
This Presentation Explains by an elementary descriptions what spectroscopy entails.
Water molecule is used to illustrate what the Molecular Orbitals are and the way the quantized levels and the electrons are distributed in the molecule. Without a rigorous reference to water molecule system, what happens in
Presence of electro magnetic radiation is explained; as a consequence the definition of spectrum is elucidated.
S. Aravamudhan
10/19/2015 7:03:16 AM
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Dr. S. Aravamudhan

2. From the linear combinations of
The 4 oxygen and 2 hydrogen atomic orbital, the 6 molecular orbital can be constructed
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Dr. S. Aravamudhan

3. Energy values on the yaxis are in au
Energy values on the yaxis are in au
Most Stable MO
HOMO
LUMO 1 2 3 4 5 6
Energy level ordering of the 6 molecular orbital.
These calculations on water are merely to serve the purpose of basic approaches and not rigorous methods.
It swell known that water is completely transparent inth UV-visible region. Experimentally certain absorption features have been reported for water in the Vaccuum UV regio, which falls in the wavwlength range of. The order of nanometers.
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Dr. S. Aravamudhan

4. Shapes of the molecular orbital illustrated by the surface plots on the LHS
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Dr. S. Aravamudhan

5. Illustration of Spectroscopic Transition
The electron has been pomoted to a different obital and hence electronic charge distribution in the molecule undergoes a change.
Illustration of Spectroscopic Transition
Energy values on the yaxis are in au
Most Stable MO
HOMO
LUMO
E= au = h.Hz
A photon of the electromagntic radiation at the appropriate frequency can interact with the electron to cause a transition
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Dr. S. Aravamudhan

6. Hz Spectrum 1 photon energy atHz E=0.6204 au = h.Hz
Amount of energy absorbed Vs frequency of radiation
1 photon energy atHz
Amount of energy absorbed
Hz
E= au = h.Hz
UV-Visible range
Quantization of Energy requires That the Energy difference betweeen the given quantized levels of the system should exactly correspond to the frequency of electromagnetic radiation
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Dr. S. Aravamudhan

7. Molecular Spectroscopy
Till now, the considerations have been electronic energy levels in molecules. The electronic energy level separations are of such magnitude that the energy differences correspond to the energy of Photon in the UV-Visible range of the electromagnetic radiation.
Molecular vibrations and rotations are also characterized by well defined quantized energy levels and spectra corresponding to these motions in molecules . Each kind of motion has quantized levels, which differ in their energy values which correspond to different range of the photon energy. This energy level differences correspond to the characteristic frequency range of the electro magnetic spectrum
Vibration of molecules can be charcterized by the spectrum in the Infrared range of the E-M spectrum. And, molecular rotations are characterized by spectra in the Microwave range.
Such variety of photon energy ranges are better appreciated in terms of the Electromagnetic Spectrum..
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Dr. S. Aravamudhan

8. E < ~104 < ~104 12-09-2019 17:25:07 Dr. S. Aravamudhan
Nano meters
E < ~ < ~104
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Dr. S. Aravamudhan

9. ΔE e Electronic spectrum Vibrational Spectrum Rotation Spectrum
Lower energy state
Ground state
Higher energy state
Excited state ΔE
Rotation Spectrum
Vibrational transition occurred for which the ΔE = 10-4 times that of Electronic transition
Rotational excited state : rotation faster
Ground electronic state
Rotational levels
Emission
absorption
UV Visible
Photon e
electron
Vibrational states of electronic ground state
Vibrating Molecule
Rotating molecule
The moeclue vibrates with larger amoplitude hence is in excited state. Requency of vibration remains the same.
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Dr. S. Aravamudhan

10. Photon-2 causes Transition
Absorbed Photon emiited
absorbed
Photon-1 causes Transition
No net energy absorbed or emitted from the radiation because the absorbed and emiited photons are of the same frequency
Emitted radiation
Frequency higher than incident photon
Frequency less than incident photon
There are additional characteristics of the emitted radiation mark the Raman spectroscopic lines
Incident Photons corrresponding to ΔE cause the ransitions between the electronic stats.
The transitions actually take place including certain vibrational levels also.
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Dr. S. Aravamudhan

11. in menu bar click on "Videos"
In addition to the Molecular Spectroscopic Techniques described in this presentation, Magnetic resonance Spectroscopy is an important spectroscopic tool for molecular structure determination.
Links for introductory maternal on Magnetic Resonance are as below:
Display the "My Channel" of Sankarampadi Aravamudhan from
in menu bar click on "Videos"
Then at the end of te page click on "Load more"
In the 42 video uploads look for the file name
esr_display_xpversion.WMV
for which the link URL is s follows
Display the My Channel of Sankarampadi Aravamudhan
from
In the menu bar Click on "playlists"
In the play list look for the list name "NMR Videos"
open the playlist. NMR Playlists
The link URL for this listas follows
Download a FTNMR file at
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Dr. S. Aravamudhan