Infrared Spectrometry

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Infrared Spectrometry

• Principle
• Instrument
• Qualitative Analysis
• Quantitative Analysis

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Infrared Light
Energy of IR photon insufficient to cause
electronic excitation but can cause vibrational
or rotational excitation
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The potential curve for a diatomic molecule
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Morse energy curve for a diatomic molecule.
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Vibrational Spectroscopy
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Selection Rule of Infrared Spectrum

• Molecule must have change in dipole moment due to
  vibration or rotation to absorb IR radiation.
• Homonuclear diatomic molecules will have no IR
  spectrum.
• Molecule dipole moment interacts with IR photon
  electric field.
• Absorption causes increase in vibration
  amplitude/rotation frequency.

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Molecules with permanent dipole moments (µ) are
IR active
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Types of Molecular VibrationsStretch-change in
bond length

• symmetric stretching

• asymmetric stretching

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Types of Molecular Vibrations Bend-change in
bond angle
scissoring
wagging
rocking
twisting/torsion
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Normal Modes of Vibration

• Linear molecule of N atoms normal modes 3N -
  5  
• Nonlinear molecule of N atoms normal modes 3N
  - 6

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Only some modes may be IR active
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The three fundamental vibrations for sulfur
dioxide
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How many vibrational modes?

• 2 atoms (H2) - 1 vibration
• 3 atoms (H2O) - 3 vibrations
• 3 atoms (CO2) - 4 vibrations
• 4 atoms (H2CO) - 6 vibrations

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Instrumentation Sources
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Instrumentation Transducers
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Dispersive Grating IR Instruments

• Similar to UV-Vis spectrophotometer
• Grating 10-500 lines per mm
• Single beam and double beam
• eliminates atmospheric gas interference

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Fourier Transform Instruments
Michelson interferometer
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Fourier Transform
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Jaquinot advantage

• high intensity
• improve resolution with decreaseing slit width
• signal to noise ratio (S/N ) improves with more
  scans

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Fellget advantage

• Simultaneously measure all spectrum at once saves
  time

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FTIR advantages

• High S/N ratios
• Rapid (lt10 s)
• Reproducible
• High resolution (lt0.1 cm-1)
• Inexpensive

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Qualitative and Quantitative Analysis

• 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
• (b) short pathlength (0.015-1 mm)
• Solids-make semi-transparent pellet with KBr

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Qualitative Analysis

• Step One Identify functional groups
• Step Two Compare with standard spectra

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Quantitative Analysis

• IR more difficult than UV-VIS
• complex spectra
• weak incident beam
• low sensitivity
• solvent absorption
• IR mostly used for rapid qualitative but not
• quantitative analysis