Molecular spectroscopy basics

1. Online lectures for
B.Sc. Part III sem 5
(5S chemistry)
Physical Chemistry
Unit VI
Molecular Spectroscopy
Dr. Dewal S. Deshmukh
Assistant Professor
Department of Chemistry,
Shri Shivaji College of Arts, Commerce and
Science, Akola

2. Unit VI: Molecular Spectroscopy
Marks weightage: 12 Marks (minimum) Lectures allotted: 14 Lectures
 Electromagnetic radiation, characteristics of electromagnetic radiation in terms of
wavelength, wave number, frequency and energy of photon. Spectrum of electromagnetic
radiation.
 Types of spectra - Emission and absorption spectra, atomic and molecular spectra, line
and band spectra
 Translational, vibrational, rotational and electronic motion. The degree of freedom in each
motion.
 Energy level diagram of a molecule indicating electronic, vibrational and rotational
transitions.
 Condition for pure rotational spectrum (i.e. microwave active molecules), selection rule for
rotational transition. Derivation of expression for moment of inertia of a diatomic rigid
rotor. Isotope effect. Applications of microwave spectroscopy for the determination of
moment of inertia and bonding.
 Condition for exhibiting vibrational spectra (i.e. IR active molecule), selection rule for
vibrational transition. Vibrational energy levels of a simple harmonic oscillator. Zero point
energy, position of a spectral line. Determination of force constant of a covalent bond.
 Raman effect - Raman’s spectrum of a molecule. Condition for exhibiting Raman spectrum
(i.e. Raman active molecule), selection rule for rotational transitions. Pure rotational
spectrum of diatomic molecule, vibrational Raman spectrum of a diatomic molecule.
 Numericals.

3. Identification of compounds:
1. Classical methods
2. Modern Methods (Instrumental)
Qualitative- Identification by colour, indicators, boiling points,
odours
Quantitative- Mass or volume (e.g. gravimetric, volumetric)
Qualitative- Chromatography, electrophoresis and identification by
measuring physical property (e.g. spectroscopy,
electrode potential)
Quantitative- measuring property and determining relationship to
concentration (e.g. spectrophotometry, mass
spectrometry). Often same instrumentation used for
qualitative and quantitative analysis.

5. The study of the interaction between ELECTROMAGNETIC (EM)
RADIATION and MATTER
5
WHAT IS
SPECTROSCOPY ?

6. Spectroscopy Introduction:
Interaction of EMR with matter

9. General terminologies in spectroscopy:
Spectrum:
A plot of the response as a function of wavelength or more commonly
frequency is referred to as a spectrum.
Spectrometry:
It is the measurement of these responses and an instrument which performs
such measurements is a spectrometer or spectrograph, although these terms
are more limited in use to the original field of optics from which the concept
sprang.
Spectroscopy is often used in physical and analytical chemistry for the
identification of substances through the spectrum emitted from or absorbed
by them. Spectroscopy is also heavily used in astronomy and remote sensing.
Most large telescopes have spectrometers, which are used either to measure
the chemical composition and physical properties of astronomical objects or
to measure their velocities from the Doppler Shift of their spectral lines.

11. According to the wave theory, light travels in the form of wave. It was believed that
radiant energy is emitted by fluctuation of electrical charge and magnetic field.
Like light, there are various forms of electromagnetic radiation such as Ultra-violet,
Infra-red, X-rays, Radio-waves etc.
E
M
C
Planes of Electromagnetic waves

14. Few of the important characteristics of the electromagnetic radiation are:
 These are produced by the oscillation of electric charge and magnetic field residing on the atom . The
electric and magnetic components are mutually perpendicular to each other and are coplanar.
 These are characterized by their wavelength of frequency of wavenumber.
 The energy carried by the an electromagnetic radiation is directly proportional to its frequency. The
emission or absorption of radiation is quantized and each quantum of radiation is called a photon.
 All type of radiations travel with the same velocity and no medium is required for their propagation. They
can travel through vacuum.
 When visible light (a group of electromagnetic radiation ) is passed through a prism, it is split up into seven
colors which correspond to define wavelength to definite wavelength. This phenomenon is called dispersion.

15. Electromagnetic Waves
• Transverse waves without a medium!
• They can travel through empty space)