CI 6.4(Ir Spectra)

8/9/2019 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

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CI 6.4(Ir Spectra)