1. Microwave Spectroscopy Rotational Spectroscopyor
Rotational Spectroscopy
PHYSICAL Chemistry
Course: CH-361
Dr.P.R.PATIL

2. Regions of Electromagnetic Radiation
Frequency ()
Wavelength ()
Radio-waves
Region
Microwaves
Infra-red
Visible
Ultra-violet
X-ray Region
-ray Region
Frequency (HZ)
Wavelength
10m – 1 cm
1 cm – 100µm
100µm – 1µm
700 – 400 nm nm
10nm – 100pm
100pm –
1 pm
NMR, ESR
Rotational
Spectroscopy
Vibrational spectroscopy
Electronic
Spec.
Energy
0.001 – 10 J/mole
Order of some 100 J/mole
Some 104 J/mole
Some 100
kJ/mole
Some 100s
J/mole
UHF TV, cellular, telephones.
(300 MHz and 3 GHz)
Table lamp,
Tube light (400 nm -800 nm or 400–790 THz)
Sun Lamp
X-ray
FM Radio, VHF TV.
AM Radio
Microwave ovens,
Police radar, satellite stations-- (3 to 30 GHz)

3. Electromagnetic Radiation

4. Electromagnetic Wave

5. Regions of Electromagnetic Radiation

6. Absorption of Electromagnetic Radiation - The Coupling Mechanism
An electromagnetic wave is an oscillating electrical field and interacts only with molecules that can undergo a change in dipole moment.
The oscillating dipole can be provided by the rotation of a permanent dipole like for example HCl. This type of interaction leads to microwave spectra
HCl
Fig. The rotation of a polar diatomic molecule, showing the fluctuation in the dipole moment measured in a particular direction

7. Microwave Spectroscopy
Incident electromagnetic waves can excite the rotational levels of molecules provided they have an electric dipole moment. The electromagnetic field exerts a torque on the molecule.
The spectra for rotational transitions of molecules is typically in the microwave region of the electromagnetic spectrum.
Absorption of microwave radiation causes heating due to increased molecular rotational activity....

8. Microwave Spectroscopy
Incident electromagnetic waves can excite the rotational levels of molecules provided they have an electric dipole moment. The electromagnetic field exerts a torque on the molecule.
Homonuclear diatomic molecules (such as H2, O2, N2 , Cl2) – have zero dipole (non polar) -- have zero change of dipole during the rotation – hence NO interaction with radiation -- hence homonuclear diatomic molecules are microwave inactive
Heteronuclear diatomic molecules (such as HCl, HF, CO) – have permanent dipolemoment (polar compound) -- change of dipole occurs during the rotation – hence interaction with radiation takes place – Therefore, heteronuclear diatomic molecules are microwave active.

9. RIGID ROTOR
For simplicity, we can consider only rotational motion of rigid diatomic molecule,
A diatomic molecule can rotate around a vertical axis. The rotational energy is quantized.
Assume a rigid (not elastic) bond
  r0 = r1 + r2
For rotation about center of gravity, C :
  m1r1 = m2r2
= m2 (r0 - r1)

10. RIGID ROTOR Moment of inertia about C:
IC = m1r12 + m2r22 = m2r2r1 + m1r1r2 = r1r2 (m1 + m2)
 = reduced mass,
A diatomic molecule can rotate around a vertical axis. The rotational energy is quantized. By the using Schrödinger equation, the rotational energy levels allowed to the rigid diatomic molecule are given by,
J = Rotational quantum number (J = 0, 1, 2, …)
I = Moment of inertia = mr2
= reduced mass = m1m2 / (m1 + m2)
r = internuclear distance

11. Energy is quantized
Planck suggests that radiation (light, energy) can only come in quantized packets that are of size hν.
Planck’s constant
h = × J·s
Energy (J)
Planck, 1900
Frequency (s-1)
Note that we can specify the energy by specifying any one of the following:
1. The frequency, n (units: Hz or s-1):
2. The wavelength, λ, (units: m or cm or mm):
Recall:
3. The wavenumber,
(units: cm-1 or m-1)
Recall:

12. Rotational Spectra of Rigid Diatomic molecule
Rotational Energy Levels for rigid rotor:
Where

13. Rotational Spectra of Rigid Diatomic molecule
For rigid rotor, J  J + 1,
The allowed rotational energy levels of a rigid diatomic molecule
Allowed transitions between the energy levels of a rigid diatomic molecule and the spectrum

14. Rotational Spectra of Rigid Rotor
Selection Rule: Apart from Specific rule, DJ= 1, Gross rule- the molecule should have a permanent electric dipole moment, m . Thus, homonuclear diatomic molecules do not have a pure rotational spectrum. Heteronuclear diatomic molecules do have rotational spectra.

15. Rotational Energy levels
The spectra for rotational transitions of molecules is typically in the microwave region of the electromagnetic spectrum.