I'R' Spectroscopy

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 ?v
from this equation we can get the reciprocal of
the wavelength (1/?) this is a direct measure of
the frequency
5
the reciprocal is described as the wavenumberit
is the wavenumber, measured in cm-1 that is
recorded on an infrared spectrum
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

• 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
C-O bond stretch 1050 cm-1
C-H bond stretch 3010 -2850 cm-1
H
H
C
C
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. CO bonds will give more intense absorptions
  than CC bonds.

24
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