Infrared Spectroscopy

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Infrared Spectroscopy

• The structure of new compounds that are isolated
  from natural sources or prepared in the lab must
  be determined (and/or verified).
• Chemical analysis
• Spectroscopy
• Spectroscopic techniques are non-destructive and
  generally require small amounts of sample

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Infrared Spectroscopy

• Four common spectroscopic techniques used to
  determine structure
• Infrared Spectroscopy (IR)
• Mass Spectrometry (MS or Mass Spec)
• Nuclear Magnetic Resonance Spectroscopy (NMR)
• Ultraviolet Spectroscopy

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Infrared Spectroscopy

• Infrared spectroscopy
• Used to determine the functional groups present
  (or absent) in a molecule

http//riodb01.ibase.aist.go.jp/sdbs/ (National
Institute of Advanced Industrial Science and
Technology, 1/5/11)
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Infrared Spectroscopy

• Infrared spectroscopy is a type of absorption
  spectroscopy
• Any technique that measures the amount of light
  absorbed by (or transmitted through) a compound
  as a function of the wavelength of light
• Sample is irradiated by a light source
• Amount of light transmitted (or absorbed) at
  various wavelengths is measured by a detector
• A spectrum is obtained.
• Graph of light transmitted (or absorbed) as a
  function of wavelength

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Infrared Spectroscopy

• Infrared spectroscopy uses light from the
  infrared region of the electromagnetic spectrum.
• The absorption of IR radiation leads to
  absorption bands in the IR spectrum.
• The position of a band is reported in wavenumbers
  ( u )
• the number of wavelengths per cm
• u 10,000 where l mm
• l
• Directly proportional to energy

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Infrared Spectroscopy

• The atoms in a molecule are in constant motion.
• The covalent bond between two atoms acts like a
  spring, allowing the atoms to vibrate (stretch
  and bend) relative to each other.

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Infrared Spectroscopy

• The absorption of IR radiation increases the
  amplitude of the various types of bond
  vibrations.
• Stretching
• Symmetric
• Asymmetric
• Bending

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Infrared Spectroscopy

• Since energy is quantized, covalent bonds can
  vibrate/stretch only at certain allowed
  frequencies.
• The position of an absorption band correlates
  with the type of chemical bond.

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Infrared Spectroscopy

• The frequency of an absorption band in an IR
  spectrum depends primarily on
• Type of vibration
• Stretching vibrations higher frequency
• Bending vibrations lower frequency
• Masses of the atoms in a bond
• Strength of the bond or bond order

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Infrared Spectroscopy

• The polarity of a bond has a significant impact
  on the intensity of an IR absorption band.
• Vibrations that cause a significant change in the
  dipole moment of a chemical bond lead to strong
  absorption bands.
• Vibrations that result in no change/very little
  change in dipole moment lead to very weak or no
  absorption band.
• Symmetrical bonds often exhibit very weak or no
  absorption band.

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Infrared Spectroscopy

• Each molecule has a unique IR spectrum.
• The IR spectrum is a fingerprint for the
  molecule.
• IR spectrum results from a combination of all
  possible stretching and/or bending vibrations of
  the individual bonds and the whole molecule.
• Simple stretching 1600-4000 cm-1.
• Complex vibrations 600-1400 cm-1, called the
  fingerprint region.

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Infrared Spectroscopy

• IR Spectrum of n-octane

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Infrared Spectroscopy

• An IR spectrum is used to identify functional
  groups that are present (or absent).
• Cannot conclusively identify a structure by IR
  alone unless an IR spectrum of an authentic
  (known) sample of the compound is available.
• Absorptions from specific functional groups are
  found in certain regions of the IR spectrum.

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Carbon-Carbon Bonds

• Increasing bond order leads to higher
  frequencies
• C-C 1200 cm-1 (fingerprint region)
• CC 1600 - 1680 cm-1
• C?C 2200 cm-1 (weak or absent if internal)
• Conjugation lowers the frequency
• isolated CC 1640-1680 cm-1
• conjugated CC 1620-1640 cm-1
• aromatic CC approx. 1600 cm-1
• CC peaks are generally weak to moderate in
  intensity.

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Carbon-Hydrogen Bonds

• Bonds with more s character absorb at a higher
  frequency.
• sp3 (alkane) C-H
• just below 3000 cm-1 (to the right)
• sp2 (alkene or aromatic hydrocarbon) C-H
• just above 3000 cm-1 (to the left)
• sp (alkyne) C-H
• at 3300 cm-1

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O-H and N-H Bonds

• Both O-H and N-H stretches appear around 3300
  cm-1, but they look different.
• Alcohol O-H
• broad with rounded tip when hydrogen bonding is
  present (sharp in the absence of hydrogen
  bonding)
• Secondary amine (R2NH)
• Broad (usually) with one sharp spike
• Primary amine (RNH2)
• Broad (usually) with two sharp spikes.
• No signal for a tertiary amine (R3N)

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NH Bend

• A broad, round peak may be observed around 1600
  cm-1 for the N H bend, especially with primary
  amines.

NH2 stretch
N-H bend
N-H bend has a different shape than an aromatic
ring or CC
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Carbonyls

• Carbonyl stretches are generally strong
• Aldehyde 1710 cm-1
• Ketone 1710 cm-1
• Carboxylic acid 1710 cm-1
• Ester 1730 - 1740 cm-1
• Amide 1640-1680 cm-1
• Conjugation shifts all carbonyls to lower
  frequencies.
• Ring strain shifts carbonyls to higher
  frequencies.

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Aldehydes
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Carboxylic Acids
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Ketones
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Esters

• CO stretch at 1730-1740 cm-1
• and
• C-O stretch at 1000-1300 cm-1 (broad)
• (Note other functional groups may have peaks
  in the 1000-1300 cm-1 region too!)

strong
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Amides

• CO stretch at 1640-1680 cm-1 (sometimes a double
  peak)
• N-H stretch (if 1o or 2o) around 3300 cm-1

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Nitriles

• C ? N absorbs just above 2200 cm-1 (med
  strong)
• The alkyne C ? C signal is much weaker and is
  just below 2200 cm-1

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IR Spectroscopy

• Example Interpret the following IR spectrum by
  assigning each of the major peaks. Identify what
  functional group(s) are present.

3403 cm-1
1604 cm-1
http//riodb01.ibase.aist.go.jp/sdbs/ (National
Institute of Advanced Industrial Science and
Technology, 12/30/09)
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IR Spectroscopy

• Example Interpret the following IR spectrum by
  assigning each of the major peaks. Identify what
  functional group(s) are present.

2733 cm-1
2814 cm-1
1642 cm-1
1691 cm-1
http//riodb01.ibase.aist.go.jp/sdbs/ (National
Institute of Advanced Industrial Science and
Technology, 12/30/09)
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Infrared Spectroscopy

• Example Which one of the following compounds is
  the most reasonable structure for the IR spectrum
  shown below?

http//riodb01.ibase.aist.go.jp/sdbs/ (National
Institute of Advanced Industrial Science and
Technology, 12/30/09)
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Infrared Spectroscopy

• Example Which of the following compounds is the
  most reasonable structure for the IR spectrum
  shown below?

http//riodb01.ibase.aist.go.jp/sdbs/ (National
Institute of Advanced Industrial Science and
Technology, 12/30/09)