1. I.R. Spectroscopy
C.I. 6.4

2. 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.

3. 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

4. Remember
c = λv
from this equation we can get the reciprocal of the wavelength (1/λ)
this is a direct measure of the frequency

5. 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
1000
2000
3000
4000 10
2.5
2.5 x 1013
1.0 x 1014

6. Bond deformation
SIMPLE diatomic molecules can only vibrate one way, by stretching. Br H
For these molecules there is only one vibrational infrared absorption.

7. Bond deformation O C O symmetric stretch
More complex molecules have more possible deformations O C O
symmetric stretch

8. Bond deformation O C O O C O
asymmetric stretch

9. Bond deformation O C O

10. Bond deformation O O C

11. Bond deformation O O C

12. Bond deformation O O C

13. Bond deformation O C O

14. Bond deformation C O O

15. Bond deformation C O O

16. Bond deformation C O O
bending

17. Bond deformation Frequencies are different for each molecule
Energy required for vibration depends on strength of bond
Weaker bonds requiring less energy.

18. Simple version Sample 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.

19. 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.

20. O-H bond
stretch
C-H bond
stretch
C-O bond
stretch

21. H H C C O H H H H C-H bond stretch 3010 -2850 cm-1 C-O bond stretch
O-H bond
stretch
3670 cm-1 O H H H H

22. 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.

23. absorption intensity
The 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.

24. Some typical absorptions
Absorption range / cm-1
Bonds responsible
Examples
Single bonds to H
O-H, C-H, N-H
Triple bonds
C≡C, C≡N
Double bonds
C=C, C=O
Below 1500
various
C-O, C-X
‘can use acid to catalyse’ hyperlink takes to slide 11
‘carboxylate salt’ hyperlink takes to slide 12
Below 1500cm-1 the ir spectrum can be quite complex
This region is characteristic of a particular molecule
Hence known as ‘fingerprint region’