Download Introduction to Infrared Spectroscopy

Introduction to Infrared
Spectroscopy
How IR energy is absorbed by
molecules
"The Introduction to Infrared Spectroscopy PowerPoint” is a modified version
of the one my mentor at Lawrence Berkeley National Laboratory, Dr. Michael
Martin, uses to teach visiting interns about the use of spectroscopy.
The EM Spectrum
Atomic Emission Spectra
Electrons are in quantized energy states
(orbits) in an atom.
Absorbing a photon can promote an
electron to a higher energy orbital, then
this excited state can decay via emission
of a photon.
Atomic Absorption Spectra
Emission Spectra
Input spectra
(like the sun)
Absorption spectra
(like from a gas between the sun and us)
This technique is so reliable that scientists can tell what elements they are looking at just
by reading the lines. Spectroscopy is the science of using spectral lines to figure out what
something is made of. That's how we know the composition of distant stars!
What can we learn from IR
spectroscopy?
• Atoms vibrate with frequencies in the IR
range
• Chemical Analysis:
• Match spectra to known databases
– Identifying an unknown compound, Forensics, etc.
• Monitor chemical reactions in-situ
• Structural ideas:
• Can determine what chemical groups are in a
specific compound
• Electronic Information:
• Measure optical conductivity
– Determine if Metal, Insulator, Superconductor,
Semiconductor
A Simple Oscillator
k
m
Imagine the mass is an atom, carrying a charge.
This will then couple to the electric field in light.
Ball & Spring equation: F = -k Dx
Spring constants for chemical bonds:
• k = 40 – 400 N/m for covalent bonds
• k = 20 – 200 N/m for ionic bonds
• k = 0.5 – 4 N/m for Van der Waals bonds
Solution to resonance frequency for ball & spring:
Ideal : mx  kx,  0 
k
m
it
x  kx  x  E (t ),   damping , E-27
General
:
m
(
t
)

qE
e
0
A hydrogen atom has a mass of 1.6735 x 10 kg
X 10-26 kg
An oxygen atom has a mass
of
2.6561


qE
qE / m
Solution : x  
0
2
e it  
0



e it
An Example: CO2
IR Absorbence
O
C
O
Symmetric Stretch
(Dipole moment = 0 so not IR active)
Asymmetric Stretch
(Has dipole moment so IR active)
Wavenumber (cm-1)
A Dipole Moment = charge imbalance in the molecule
Bending Mode
(Has dipole moment so IR active)
A more complex example:
Toluene
From: http://www.cem.msu.edu/~parrill/AIRS/