CI 6.4(Ir Spectra)
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Transcript of CI 6.4(Ir Spectra)
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I.R. Spectroscopy
C.I. 6.4
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Infrared Spectroscopy
Substances exposed to radiation fromfrequency range 1014 Hz to 1013 Hz
(wavelengths 2.5m -15m)
causing vibrational energy changes in themolecule
these absorb infrared radiation ofspecificfrequencies.
point is to identify functional groups inthe molecule
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the reciprocal is described as the
wa
venumberit is the wavenumber, measured in cm-1 that
is recorded on an infrared spectrum
wavenumber (1/) / cm-1
wavelength ()/
m
frequency (v) / Hz
1000200030004000
102.5
2.5 x 10131.0 x 1014
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Bond deformation
SIMPLE diatomic molecules can only vibrate one way,by stretching.
H Br
For these molecules there is only one vibrationalinfrared absorption.
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Bond deformation
More complex molecules have more possibledeformations
O C O
symmetric stretch
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Bond deformation
O C O
O C O
asymmetric stretch
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Bond deformation
O C O
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Bond deformation
O C O
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Bond deformation
O C O
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Bond deformation
O C O
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Bond deformation
O C O
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Bond deformation
O C O
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Bond deformation
O C O
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Bond deformation
O C O
bending
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Bond deformation
Frequencies are different
for each molecule
Energy required forvibration depends on
strength of bond
Weaker bonds requiringless energy.
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Simple version ample placed in ir spectrometer
Subjected to ir radiation
Molecule absorbs energy
Molecule bonds starts to undergo
different types of vibration
(stretching, bending etc.)
This produces different signals
that the detector records as
peaks on the spectrum.
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Important When an ir spectrum is obtained we do not try to explain thewhole thing, simply look for one or two signals that arecharacteristic of different bonds.
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O-H bond
stretch
C-H bond
stretchC-O bond
stretch
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OCC
H
H
H HH
H
O-H bond
stretch
3670 cm-1
C-O bond
stretch
1050 cm-1
C-H bond
stretch
3010 -2850 cm-1
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Interpreting the spectra!
Usually match a particular bond to a
particular absorptionregion.
The precise position
ofthe peak depe
ndson the bond environment, so only
wavenumberregions can be quoted.
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absorption intensity
The strongest (more intense) absorptionsoccur when a large change in bondpolarity associated with the vibration.
e.g. C=O bonds will give more intenseabsorptions than C=C bonds.
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Some typical absorptions
Below 1500cm-1 the ir spectrum can be quite complex
This region is characteristic ofa particular molecule
Hence known as fingerprint region
Absorption range /cm-1 Bonds responsible Examples
4000-2500 Single bonds to H O-H, C-H, N-H
2500-2000 Triple bonds CC, CN
2000-1500 Double bonds C=C, C=O
Below 1500 various C-O, C-X