Download lecture 5 infrared spectrometry

Lecture 3 INFRARED SPECTROMETRY
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INFRARED SPECTRAL REGIONS
INTRODUCTION TO INFRARED SPECTROMETRY
INFRARED ABSORPTION SPECTRUM
Energy of IR photon insufficient to cause electronic excitation but can
cause vibrational or rotational excitation
Molecule electric field (dipole moment) interacts with IR photon
electric field (both dynamic)
Magnitude of dipole moment determined by
(i) charge
(ii) separation of charge
INTRODUCTION TO INFRARED SPECTROMETRY
Vibration or rotation causes varying separation
INTRODUCTION TO INFRARED SPECTROMETRY
Molecule must have change in dipole moment due to vibration or
rotation to absorb IR radiation!
Absorption causes increase in vibration amplitude/rotation frequency
INTRODUCTION TO INFRARED SPECTROMETRY
Molecules with permanent dipole moments (μ) are IR active!
INTRODUCTION TO INFRARED SPECTROMETRY
INTRODUCTION TO INFRARED SPECTROMETRY
Types of Molecular Vibrations:
Stretch - change in bond length
symmetric
assymetric
Bend – change in bond angle
scissoring
wagging
rocking
twisting/tortion
INTRODUCTION TO INFRARED SPECTROMETRY
STRETCH
INTRODUCTION TO INFRARED SPECTROMETRY
BEND
GENERAL DESIGN OF OPTICAL INSTRUMENTS
Only some modes may be IR active:
Example CO2
O=C=O
linear
GENERAL DESIGN OF OPTICAL INSTRUMENTS
GENERAL DESIGN OF OPTICAL INSTRUMENTS
GENERAL DESIGN OF OPTICAL INSTRUMENTS
Classical vibrational motion:
GENERAL DESIGN OF OPTICAL INSTRUMENTS
GENERAL DESIGN OF OPTICAL INSTRUMENTS
CLASSICAL VIBRATIONAL FREQUENCY
What about quantum mechanics? 
INFRARED SPECTROSCOPY
SAMPLE PROBLEM
The force constant for a typical triple bond is 1.91 x 103 N/m.
Calculate the approximate frequency of the main absorption peak
due to vibration of CO.
INFRARED SPECTROSCOPY
 = ±1
Vibrational Selection Rule
Since levels equally spaced - should see one absorption frequency.
SELECTION RULE
Anharmonic oscillator:
Must modify harmonic oscillator potential for
(i) electron repulsion (steeper at small distances)
(ii) dissociation (bond breaks at large distances)
GENERAL DESIGN OF OPTICAL INSTRUMENTS
New E-y curve:
IR SPECTROSCOPY
IR SPECTROSCOPY
VIBRATIONAL MODES
How many vibrational mode?
HCN, H2CO3
Coupling of different vibrations shifts frequencies
Coupling likely when:
(1) common atom in stretching modes
(2) common bond in bending modes
(3) common bond in bending + stretching modes
(4) similar vibrational frequencies
Coupling not likely when
(1) atoms separated by two or more bonds
(2) symmetry inappropriate
IR SPECTROSCOPY
IR SPECTROSCOPY
SOURCES
IR SPECTROSCOPY - INSTRUMENTATION
TRANSDUCERS
IR SPECTROSCOPY - INSTRUMENTATION
IR SPECTROSCOPY - INSTRUMENTATION
IR SPECTROSCOPY - INSTRUMENTATION
FOURIER TRANSFORM INFRARED SPECTROMETER
A photodiode array can measure an entire spectrum at once. The
spectrum is spread into its component wavelength and each
wavelength is directed onto one detector element.
Fourier Analysis is a procedure in which a spectrum is decomposed
into a sum of sine and cosine terms called a Fourier series.
Fourier Transform
Instruments have
two advantages:
IR SPECTROSCOPY - INSTRUMENTATION
FOURIER TRANSFORM
FOURIER TRANSFORM
Unfortunately, no detector can respond on 10-14 s time scale
Use Michelson interferometer to measure signal proportional to time
varying signal
GENERAL DESIGN OF OPTICAL INSTRUMENTS
IR SPECTROSCOPY - INSTRUMENTATION
• If moving mirror moves 1/4  (1/2  round-trip) waves are out of
phase at beam-splitting mirror - no signal
• If moving mirror moves 1/2  (1  round-trip) waves are in phase at
beam-splitting mirror - signal
GENERAL DESIGN OF OPTICAL INSTRUMENTS
• Difference in pathlength called retardation 
• Plot  vs. signal - cosine wave with frequency proportional to light
frequency but signal varies at much lower frequency
• One full cycle when mirror moves distance  /2 (round-trip = )
velocity of moving mirror
GENERAL DESIGN OF OPTICAL INSTRUMENTS
GENERAL DESIGN OF OPTICAL INSTRUMENTS
GENERAL DESIGN OF OPTICAL INSTRUMENTS
GENERAL DESIGN OF OPTICAL INSTRUMENTS
GENERAL DESIGN OF OPTICAL INSTRUMENTS
GENERAL DESIGN OF OPTICAL INSTRUMENTS
APPLICATIONS:
FTIR single-beam, dispersive IR double-beam, but FTIR advantages
include
• High S/N ratios - high throughput
• Rapid (<10 s)
• Reproducible
• High resolution (<0.1 cm-1)
• Inexpensive
GENERAL DESIGN OF OPTICAL INSTRUMENTS
IR (especially FTIR) very widely used for
• qualitative
• quantitative
analysis of
• gases
• liquids
• solids
GENERAL DESIGN OF OPTICAL INSTRUMENTS
Most time-consuming part is sample preparation
Gases fill gas cell
(a) transparent windows (NaCl/KBr)
(b) long pathlength (10 cm) - few molecules
Liquids fill liquid cell
(a) solute in transparent solvent - not water (attacks windows)
(b) short pathlength (0.015-1 mm) - solvents absorb
GENERAL DESIGN OF OPTICAL INSTRUMENTS
GENERAL DESIGN OF OPTICAL INSTRUMENTS
GENERAL DESIGN OF OPTICAL INSTRUMENTS
TRANSFORMATION OF ALKENES AND ALKYNES
SAMPLE
SAMPLE
Group Frequencies:
• Approximately calculated from masses and spring constants
• Variations due to coupling
• Compared to correlation charts/databases
Quantitative Analysis:
IR more difficult than UV-Vis because
• narrow bands (variation in e)
• complex spectra
• weak incident beam
• low transducer sensitivity
• solvent absorption
IR mostly used for rapid qualitative but not quantitative analysis