I.R. Spectroscopy

C.I. 6.4

Molecular Spectroscopy

Energy possessed by molecules is quantised.

When a molecule interacts with radiation there can be changes in electronic, vibrational or rotational energy. (C.I. 6.2)

These changes depend on the frequency of the radiation.

Analysis of the energy needed to change from one energy level to another forms basis of molecular spectroscopy.

Infrared Spectroscopy

Substances exposed to radiation from frequency range 1014 Hz to 1013 Hz

(wavelengths 2.5μm -15μm)causing vibrational energy changes in

the molecule these absorb infrared radiation of

specific frequencies.point is to identify functional groups

in the molecule

Remember

c = λv

from this equation we can get the reciprocal of the wavelength (1/λ)

this is a direct measure of the frequency

the reciprocal is described as the wavenumber

it is the wavenumber, measured in cm-

1 that is recorded on an infrared spectrum

wavenumber (1/λ) / cm-1

wavelength (λ) / μm

frequency (v) / Hz

1000200030004000

102.5

2.5 x 10131.0 x 1014

Bond deformation• SIMPLE diatomic molecules can only vibrate one

way, by stretching.

H Br

For these molecules there is only one vibrational infrared absorption.

Bond deformation• More complex molecules have more possible

deformations

O C O

symmetric stretch

Bond deformation

O C O

O C O

asymmetric stretch

Bond deformation

O C O

Bond deformation

O C O

Bond deformation

O C O

Bond deformation

O C O

Bond deformation

O C O

Bond deformation

O C O

Bond deformation

O C O

Bond deformation

O C O

bending

Bond deformation

•Frequencies are different for each molecule

•Energy required for vibration depends on strength of bond

•Weaker bonds requiring less energy.

Simple versionSample placed in ir spectrometerSubjected to ir radiationMolecule absorbs energyMolecule bonds starts to undergo different types of vibration (stretching, bending etc.)

This produces different signals that the detector records as ‘peaks’ on the spectrum.

Important …When an ir spectrum is obtained we do not try to explain the whole thing, simply look for one or two signals that are characteristic of different bonds.

O-H bondstretch

C-H bondstretch

C-O bondstretch

OCC

H

H

H HH

H

O-H bondstretch

3670 cm-1

C-O bondstretch

1050 cm-1

C-H bondstretch

3010 -2850 cm-1

Interpreting the spectra!

• Usually match a particular bond to a particular absorption region.

• The precise position of the peak depends on the bond environment, so only wavenumber regions can be quoted.

absorption intensityThe strongest (more intense)

absorptions occur when a large change in bond polarity associated with the vibration.

e.g. C=O bonds will give more intense absorptions than C=C bonds.

Some typical absorptions

Below 1500cm-1 the ir spectrum can be quite complex

This region is characteristic of a 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 C≡C, C≡N

2000-1500 Double bonds C=C, C=O

Below 1500 various C-O, C-X