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Infrared Spectroscopy Tutorial

Key Concepts

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Infrared Spectrometer

The Infrared (ir) Spectrometer

  1. infrared radiation is produced by electrically heating a filament which is divided by mirrors into 2 beams, a reference beam and a sample beam.
  2. In the sampling area, a segmented rotating disk allows each beam to pass through alternately.
  3. The reference beam and the sample beam are combined into a beam of alternating segments.
  4. The detector measures the heat energy and the recorder records the results as a plot of percent absorption (or transmittance) as a function of wavenumber (cm-1) or wavelength (μm).

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Infrared (ir) Spectra

Particular chemical bonds absorb energy at a particular wavelength (wavenumber) regardless of the compound the bond is in.

Some examples are given below:

molecular vibration wavenumber (cm-1) wavelength (μm)
C-Cl 700-800 14.3-12.5
C-C 750-1100 13.3-9.1
C-O 1000-1300 10-7.7
C=C 1610-1680 6.2-6.0
C=O 1670-1750 6.0-5.7
O-H (acids) 2500-3300 4.0-3.0
C-H 2850-3300 3.5-3.0
O-H (alcohols) 3200-3550 3.1-2.8
N-H (primary amines) 3350-3500 3.0-2.9

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Examples of Infrared (ir) Spectra

The infrared (ir) spectrum of hexane, CH3CH2CH2CH2CH2CH3.
Bond types present
  • C-H stretches
    (2850-3300 cm-1)
The infrared (ir) spectrum of hex-1-ene (1-hexene), H2C=CHCH2CH2CH2CH3.
Bond types present
  • C-H stretches
    (2850-3300 cm-1)

  • C=C stretches
    (1610-1680 cm-1)
The infrared (ir) spectrum of ethanol, CH3CH2OH.
Bond types present
  • O-H (alcohol) stretches
    (3200-3550 cm-1)

  • C-H stretches
    (2850-3300 cm-1)
The infrared (ir) spectrum of octanal,  CH3CH2CH2CH2CH2CH2CH2O
||
C 
H 
Bond types present
  • C-H stretches
    (2850-3300 cm-1)

  • C=O stretches
    (1670-1750 cm-1)
The infrared (ir) spectrum of pentan-2-one (2-pentanone),   CH3CH2O
||
C 
CH2CH3
Bond types present
  • C-H stretches
    (2850-3300 cm-1)

  • C=O stretches
    (1670-1750 cm-1)

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