AS Chemistry (NEW-2008) : Infra-red and Mass Spectroscopy

Notes on the unit 2 topic, Mass and Infra-red Spectroscopy

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Spectroscopy 2814 1
Introduction Different covalent bonds have different strengths due to the masses of different atoms at
either end of the bond. As a result, the bonds vibrate at different frequencies (imagine two
balls on either end of a spring) . The frequency of vibration can be found by detecting
when the molecules absorb electro-magnetic radiation.
Various types of vibration are possible. O O O
Bending and stretching can be found
in water molecules. H H H H H H
Each occurs at a different frequency. STRETCH BEND STRETCH
An equivalent bend at 667cm-1 occurs O C O
in a carbon dioxide molecule.
As molecules vibrate, there can be a change in the dipole moment of the molecule.
The Infra-red M
Spectrophotometer Reference n P
beam o
c o
h o
r m
o e
m t
Incident Transmitted a e
t r
beam beam o
Sample tube r
Source of
Operation The intensity of the incident beam and reference beam is measured (they are the same).
The intensity of the transmitted beam is also measured. The difference in intensity
between the incidence beam and the transmitted beam is a measure of the amount of
radiation absorbed by the sample. The frequency of radiation is examined continuously
by the monochromator. In the photometer the relative intensities of the reference and
transmitted beams are compared; the percentage of the reference beam found in the
transmitted beam can be plotted as a function of frequency, or wavenumber.
Infra-red spectra
Interpretation Infra-red spectra are very complex due to the many types of vibration which occur in each
molecule. Total characterisation of a substance based only on its IR spectrum is almost
impossible unless one has computerised data handling facilities for comparison of the
obtained spectrum with one in memory. However, the technique is useful when used in
conjunction with other analytical methods such as nuclear magnetic resonance (nmr)
spectroscopy and mass spectroscopy.
The position of a peak depends on the · bond strength
· masses of the atoms joined by the bond
· strong bonds and light atoms absorb at lower wavenumbers
· weak bonds and heavy atoms absorb at high wavenumbers

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Axes Vertical Absorbance the stronger the absorbance the larger the peak.
Horizontal Frequency wavenumber (waves per centimetre) / cm-1
or Wavelength microns (µ); 1 micron = 1000 nanometres
A typical IR spectrum
Wavelength (microns)
2.5 3 4 5 6 7 8 9 10 12 15
.00 .00
.10 .10
.20 .20
.40 .40
.80 .…read more

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Spectroscopy 2814 3
Bond Class of compound Range, cm-1 Intensity
C-H Alkane 2965 - 2850 strong
CH3 - 1450 medium
1380 medium
CH2 - 1465 medium
Alkene 3095 - 3010 medium
1000 - 700 strong
Alkyne 3300 (approx) strong
Aldehyde 2900 - 2820 weak
2775 - 2700 weak
C-C Alkane 1200 - 700 weak
C=C Alkene 1680 - 1620 variable
CC Alkyne 2260 - 2100 variable
C=O Ketone 1725 - 1705 strong
Aldehyde 1740 - 1720 strong
Carboxylic…read more


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