INFRARED SPECTROSCOPY

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INFRARED SPECTROSCOPY
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Infrared Spectrometer
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Molecular vibrations

• IR-Tutor available in CB 280 is great!
• Basically two types
• stretching vibrations
• bending vibrations

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IR TUTOR

• Select Chemistry Applications icon
• Select Structure Drawing icon
• Select Spectroscopy icon
• Select IR Tutor icon

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Typical Infrared AbsorptionRegions
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C-H stretching region

• Alkanes C-H sp3 stretch lt 3000 cm-1
• Alkenes C-H sp2 stretch gt 3000 cm-1
• Alkynes C-H sp stretch 3300 cm-1

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C-H bending region

• CH2 bending 1460 cm-1
• CH3 bending (asym) appears near the same value
• CH3 bending (sym) 1380 cm-1

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Hexane
CH3 bend
CH2 bend
C-H stretch
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How do you think the infrared spectrum of
polyethylene would differ from the spectrum of
hexane?????
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1650 - 1450 cm-1

• The CC is found at about 1650 cm-1. Monomers
  would be expected to show the CC bond, but in
  polymers the double bond will be absent, unless
  there is a benzene ring.
• The double bonds in a benzene ring are found in
  the range of about 1600 to 1450 cm-1

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1-Hexene
sp2 C-H
CC stretch
out of plane bendings (oops)
sp3 C-H stretch
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Toluene
sp2 C-H
sp3 C-H
aromatic CC
aromatic oops
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O-H stretching region

• O-H 3300 cm-1 (alcohol). Common polymers with
  O-H cellulose and PVA
• O-H 3300 cm-1 (acid, broad and ugly).
  Dicarboxylic acids are monomers. When the
  carboxylic acid is polymerized with an diol, you
  get an polyester.

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Cyclohexanol
bending
O-H stretch
C-O stretch
sp3 C-H stretch
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Butanoic Acid
O-H stretch
CO stretch
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2250 cm-1

• CN 2250 cm-1

The only common polymers with this
functional group polyacrylonitrile and ABS
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Propanenitrile
sp3 C-H
CN
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N-H stretching region

• 3300 - 3400 cm-1
• NH2 Primary amines give a pair of peaks. You
  dont see primary amines in polymers.
• -NH Secondary amines give a single peak
• The N-H in polyamides and polyurethanes show a
  very prominent singlet peak!

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1-Butanamine
N-H bend
CH2 CH3 bend
N-H stretch doublet
sp3 C-H stretch
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3-Methylbenzenamine
sp2 sp3 C-H
N-H stretch
Aromatic CC
N-H bend
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N-Ethylbenzenamine
N-H stretch
sp2 sp3 C-H
Aromatic CC
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1770 - 1670 cm-1

• This is the carbonyl region!! All bands are very
  strong!!
• The following polymers would be expected to show
  prominent CO peaks
• Polyesters
• Acrylics
• Polyamides and polyurethanes
• Poly ether ketone (PEEK)

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CO Stretching in monomers and polymers
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2-Butanone
sp3 C-H stretch
CO stretch
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4-Methyl-2-pentanoneC-H lt 3000, CO _at_ 1715 cm-1
C-H stretch
CO stretch
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Ethyl Butanoate
C-O stretch
sp3 C-H
CO stretch
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Butanoic Acid
O-H stretch
CO stretch
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Propanamide
sp3 C-H
CO and
N-H stretch (pair)
N-H bend
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Conjugation of CO with CC

• Conjugation of a carbonyl with a CC bond shifts
  values to lower frequencies
• For ketones and esters, subtract about 30 cm-1
  for conjugation with CO
• Conjugated ketone 1690 to 1680 cm-1
• Conjugated ester 1710 to 1700 cm-1
• CC becomes quite strong!!

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4-Methyl-3-penten-2-one
C-H stretch
CO stretch
CC stretch
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Acetophenone
C-H stretch
aromatic CC
conj CO
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C-X stretching region

• C-Cl 785 to 540 cm-1, often hard to find amongst
  the fingerprint bands!!
• C-Br and C-I appear outside the useful range of
  infrared spectroscopy.
• C-F can be found easily, but arent that common -
  exception! polytetrafluoroethylene

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Carbon Tetrachloride
no C-H!
C-Cl stretch
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