infrared spectroscopy - iut.ac.irivut.iut.ac.ir/content/849/6896.infrared_spectroscopy.pdf · ir...
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Infrared Spectroscopy
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IR Spectroscopy
• Used to identify organic compounds – IR spectroscopy provides a 100% identification if the
spectrum is matched. – If not, IR at least provides information about the types of
bonds present.
• Easy to use – liquids analyzed between salt plates – solids in a KBr pellet – small amounts of unknowns via an FTIR microscope – analysis time typically < 10 minutes
• Inexpensive – FTIR spectrophotometers are found in most labs.
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IR Spectroscopy
• IR is used to measure the vibrational frequencies of bonds in the molecule.
– Bonds are not rigid. A bond can be viewed as a spring with a weight at each end.
– Each bond has a characteristic frequency.
– The IR scans a range of frequencies (in the infrared part of the electromagnetic spectrum). Any frequency which matches the characteristic frequency of a bond will be absorbed.
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IR Spectrophotometers
• In the old days, the frequencies were stepped through, one at a time, and the absorption measured.
• This means of data collection was very slow.
• Laser technology and computers have made available a much faster means of data collection.
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FTIR Spectrophotometers
• Fourier Transform Infrared Spectrophotometer – Uses an interferometer and polychromatic light (all
frequencies used at one time, instead of one at a time) to generate an interferogram.
– The interferogram is then mathematically decoded by a Fourier transformation.
• interferogram – intensity vs time
• after the Fourier transformation – intensity vs frequency…an IR spectrum
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An IR Spectrum
• A plot of % transmittance vs vibrational frequency in wavenumbers (cm-1)
c
1c
)(
100001
mwavenumber
λ = wavelength
υ = frequency
c = speed of light in a vacuum
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An IR Spectrum
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An IR Spectrum
• The wavelength of IR radiation is in the 2.5-25 micron range (compare to visible light in the 400-700 nm range).
• The frequencies of IR radiation are more conveniently expressed by a wavenumber (cycles per cm), than by υ (cycles per 3 x 1010 cm).
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Characteristic Vibrational Frequencies of Bonds
• Bonds are not rigid but behave like a spring with a mass at either end.
– Obey Hooke’s Law: F = -kx
– This gives rise to a characteristic frequency for the vibration:
massreduced
k
_2
1
21
21_mm
mmmassreduced
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Characteristic Vibrational Frequencies of Bonds
• Characteristic frequency for the vibration:
• The frequency is affected by
– the masses of the atoms in the bond
– the strength of the bond
massreduced
k
_2
1
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Characteristic Vibrational Frequencies of Bonds
• The lower the mass, the higher the vibrational frequency.
– Stretching frequencies for bonds to carbon: C-H > C-C > C-N > C-O
massreduced
k
_2
1
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Characteristic Vibrational Frequencies of Bonds
• The stronger the bond, the higher the vibrational frequency.
– Stretching frequencies
• C≡C > C=C > C-C
• C≡N > C=N > C-N
• C≡O > C=O > C-O
• C(sp)-H > C(sp2)-H > C(sp3)-H
massreduced
k
_2
1
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Number of Vibrational Frequencies in a Molecule
• There are 3n-6 possible vibrational modes in a nonlinear molecule with no symmetry
– Symmetry reduces the number of possible vibrational modes.
• Water has 3 possible vibrational modes.
• Formaldehyde has 6.
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The Fingerprint Region is Unique to the Molecule
• In addition, the vibration of a particular bond in a molecule affects the whole molecule. – The various harmonics of a bond vibration can
combine and lead to a number of combinational bands. • The intensity of these bands is usually 1/100 the
intensity of the main vibrational absorptions.
• These make up the “fingerprint region.” (occur at <1250 cm-1)
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Intensity of IR Absorptions
• In order for a vibration mode to absorb in the infrared, the vibrational motion must cause a change in the dipole moment of the bond.
• The intensity of the IR “peaks” is proportional to the change in dipole moment that a bond undergoes during a vibration.
– C=O bonds absorb strongly.
– C=C bonds generally absorb much less.
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How to Analyze an IR Spectrum
• Pay the most attention to the strongest absorptions:
– -C=O
– -OH
– -NH2
– -C≡N
– -NO2
• Pay more attention to the peaks to the left of the fingerprint region (>1250 cm-1).
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How to Analyze an IR Spectrum
• Pay the most attention to the strongest absorptions.
• Pay more attention to the peaks to the left of the fingerprint region (>1250 cm-1).
• Note the absence of certain peaks.
• Be wary of O-H peaks, water is a common contaminant.
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Characteristic IR Wavenumbers
Functional group wavenumber (cm-1)
sp3 C-H str ~2800-3000
sp2 C-H str ~3000-3100
sp C-H str ~3300
O-H str ~3300 (broad*)
O-H str in COOH ~3000 (broad*)
N-H str ~3300 (broad*)
aldehyde C-H str ~2700, ~2800
*The peak is broad when H bonding is extensive. Otherwise, the peak can be sharp.
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Characteristic IR Wavenumbers
Functional group wavenumber (cm-1)
C=C isolated ~1640-1680
C=C conjugated ~1620-1640
C=C aromatic ~1600
C≡N just above 2200
C≡C just below 2200
C=O ester ~1730-1740
C=O aldehyde, ketone,
or acid
~1710 (aldehyde can run 1725)
C=O amide ~1640-1680
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How to Analyze an IR Spectrum
• Look for what’s there and what’s not there.
• C-H absorption
– The wavenumber will tell you sp3(C-C), sp2(C=C), sp (C≡C) and perhaps aldehyde.
• Carbonyl (C=O) absorption
– Its presence means the compound is an aldehyde, ketone, carboxylic acid, ester, amide, anhydride or acyl halide.
– Its absence means the compound cannot be any of the carbonyl-containing compounds.
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How to Analyze an IR Spectrum
• O-H or N-H absorption – This indicates either an alcohol, N-H containing
amine or amide, or carboxylic acid.
• C≡C and C≡N absorptions – Be careful: internal triple bonds often do not show
up in IR spectra.
• C=C absorption – Can indicate whether compound is alkene or
aromatic.
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How to Analyze an IR Spectrum
• N-O of NO2 absorption
– This is a distinctive, strong doublet that it pays to know (1515-1560 & 1345-1385 cm-1).
• Read the scale for the value of the wavenumbers (be able to interpolate), or
• Read the wavenumbers in the table provided.
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sp3 C-H str
no O-H str
no N-H str
no sp or sp2 C-H str
no C=O str
no C=C str
C-H bend
This is an alkane.
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This is an alkene.
sp3 C-H str
sp2 C-H str
C=C str
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IR Spectra - Examples
This is a terminal alkyne.
C≡C str at 2120 cm-1
sp3 C-H str sp C-H str
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sp3 C-H str
C≡N str at 2260 cm-1
This is a nitrile.
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sp3 C-H str
alc. O-H str
C-O str
This is an alcohol.
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How to Analyze an IR Spectrum
• O-H absorption, peak shape – Peak shapes are influenced by hydrogen
bonding.
– Lots of H-bonding, broad peak around 3300 cm-1.
– In a dilute solution, there is little H bonding and the O-H peak is sharper and around 3500 cm-1.
• This can happen to N-H and the acid O-H as well .
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Cyclohexanol, neat
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Cyclohexanol in CCl4
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sp3 C-H str acid C=O str
acid O-H str
This is a carboxylic acid.
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aldehyde C=O str
C-H str doublet: 2826 cm-1 and 2728 cm-1
sp2 C-H str aromatic C=C str
This compound has two functional groups: a benzene ring and an aldehyde.
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sp3 C-H str
C-O str
ester C=O str
This is an alkyl ester. Esters and ketones have fairly similar spectra.
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Effects of Conjugation
Conjugation will lower typical absorption frequencies of double bonds due to the presence of some single bond character.
– C=C
– C=O
• Ring strain in cyclic compounds goes the other way and increases frequencies.
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IR Spectra – Effect of Conjugation
methyl methacrylate
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What functional group(s) is (are) present?
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sp3 C-H str N-H str 1° amine
This is a primary alkyl amine.
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Identify all major absorptions and functional groups. (acetanilide)
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aromatic hydrocarbon, toluene, There are both the aromatic and aliphatic CH stretches, and two absorptions for the aromatic C=C stretches
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Acetone (2-propanone) is the "classic" carbonyl containing compound with the obvious C=O stretch in the middle of the spectra. Peak has a very strong absorption. Compare it with the C=C in the previous case which are weaker and sharper.
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The characteristic absorption of the alcohol, 2-propanol, is the broad band due to the hydrogen bonded -OH group.
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Carboxylic acids contain both C=O and OH groups. the C=O is typically at slightly lower frequency than that of a ketone.
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An ester has the following key absorptions, the C=O and typically two bands for the C-O (not always easy to identify) since there are sp3 C-O and sp2 C-O bonds.
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3500-3200 cm-1
1790-1720 cm-1
1610-1590
1600-1580 cm-1
1510-1490
1680-1630 cm-1 1550-1530 cm-1
Polyamid
epoxies, polycarbonate, alkyd resins, polyesters,
cellulose-ether, PVC
poly(vinyl acetate), PVC-copoly., cellulose ester, PU
acryl polymers
Phenol resins, epoxies, aryl polymers
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1790-1720 cm-1
modified epoxides, polycarbonate, Alkyd resins, polyester, cellulose ester, cellulose ether, PVC (plast), PVAc, PVC-copolym., PU, acrylics
1610-1590 1600-1580 cm-1
1510-1490
modified epoxides, polycarbonate, Alkyd resins, polyester, cellulose ester, cellulose ether, PVC (plast)
820-840 cm-1
modified epoxies, polycarbonate polycarbonate
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?
polycarbonate
1610-1590 1600-1580 cm-1
1510-1490 1450-1410 cm-1
1100-1000 cm-1
typical pattern of normal PC
typical pattern of PU
C-O-C-ether region
Poly (ether urethane)
? cellulose ester or polyurethane ?
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Infrared Spectroscopy: 760 nm….1mm
near infrared “quartz-infrared”
~ 10,000…4,000 cm-1
NIR
middle infrared “conventional” infrared ~4,000…250 cm-1
far infrared < 250 cm-1
use of quartz cuvettes and light pipes
higher order absorptions (lower intensity) liquids can be measured in thicker layers