Download Spectroscopy

Spectroscopy
Spectroscopy – Getting Ready
 What
happens when an electron absorbs
energy?
 What kind of energy can cause this to
happen?
 Why do different elements emit/absorb
different wavelengths?
 Which electron transitions produce red
light?
Key Terms
 Electromagnetic
spectrum
 Wavelength
 Standard
 Calibration
curve
 Ionisation
 Vibrational
energy level
 Magnetic resonance
The Electromagnetic Spectrum
Using Light

A number of instruments have
been developed that work on
the ability of substances to
absorb or emit certain
wavelengths of light:




Colourimetry
UV-Visible Light Spectroscopy
Atomic Absorption
Spectroscopy
Atomic Emission Spectroscopy
Colourimetry
 Shine
light of a certain colour through a coloured
solution (colour must be complimentary to the
colour of the solution).
 The greater the absorption, the greater the
concentration.
 A set of standards must be used.
UV-Visible Spectroscopy
A


lot like colourimetry except:
Specific wavelengths are selected, not just
colours.
Can detect absorption of UV light.
 Extra

step in the process:
A scan of the pure substance is taken to
see which wavelengths are absorbed the
most.
Atomic Absorption
Spectroscopy
 Developed
by the CSIRO.
 Used for analysing metals.
Atomic Absorption
Spectroscopy
 N2O
(laughing gas) can also be used as an oxidant
in place of air (O2).
 AAS is selective. You don’t need to separate the
components to analyse them.
 It’s also very sensitive, measuring concentrations in
ppm and even ppb.
Atomic Emission Spectroscopy
 Note
– In AAS the flame does not heat the
solution enough to make the metals EMIT
light. The flame only serves to make the
solution a vapour.
 In AES the flame DOES heat the solution to
a point where the atoms begin emitting
their own light.
Mass Spectroscopy
 Measures
the mass of atoms and
molecules.
Mass Spectroscopy
1.
Sample is injected as a gas into the
ionisation chamber.
2.
Sample then bombarded with high
energy electrons, knocking electrons out
of the atoms.
3.
The positive ions formed are accelerated
to high speeds by an electric field.
Mass Spectroscopy
3.
Ions then passes through a magnetic
field, deflecting the ions according to
their mass to charge ratio (m/e).
4.
Number of ions detected.
5.
The data generated can be presented
graphically as a mass spectrum.
Mass Spectroscopy
Infra-Red Spectroscopy
 Identifies
the functional groups attached to
molecules.
 Can




identify:
Methyl groups
Hydroxy groups
Carboxylic acids
Double and triple bonds
Can you remember what the above groups look like?
Infra-Red Spectroscopy
 In
a similar way to electrons
absorbing particular amounts of
energy that cause them to jump
shells, covalent bonds have
certain vibrational energy levels.
 That
means that certain types of
covalent bonds absorb certain
wavelengths of infra-red light.
Infra-Red Spectroscopy
Symmetrical
stretching
Antisymmetrical
stretching
Scissoring
Rocking
Wagging
Twisting
Infra-Red Spectroscopy
 Measures
transmittance, not absorbance,
so graphs look upside down.
 Units are wave number per cm, not
wavelength.
 Readings below 1000cm-1 are ‘whole
molecule’.
 Almost all organic molecules will have a
peak at 2950cm-1 as it is a C-H bond.
Infra-Red Spectroscopy
Bond
Wave
Number
C-Cl
700-800
C-C
750-1100
C-O
1000-1300
C=C
1610-1680
C=O
1670-1750
O-H (as part of an
acid)
2500-3300
C-H
2850-3300
O-H (as part of an
alkanol)
3200-3550
N-H
3350-3500
Nuclear Magnetic Resonance
Spectroscopy (NMR)
 Measures
the ‘spin’ of H or C nuclei.
 Spin is the direction of the magnetic field
in the nucleus.
 Two types of spin – up and down.
 Magnetic energy is absorbed by the
nucleus which changes the direction of its
spin
 The energy is then released and
detected.
NMR Spectra
A
hydrogen attached to a carbon atom
will have a different magnetic resonance
to a hydrogen attached to a nitrogen
atom.
 The
elements that surround an atom are
referred to as its ‘environment’.
NMR Spectra
CNMR Spectra
#
of peaks = # of environments
 Chemical shift = type of environment
(lower electron density, further downfield)
 Peak area = number of similar atoms
CNMR Spectra
 Refer
to the table of C NMR data to interpret the
following spectrum. Empirical formula is C4H10O and
it is an alkanol.
CNMR Spectra
 Molecular
formula of C3H6O2. Does not
react with NaOH.
HNMR Spectra
 In
high resolution proton (H) NMR, the
number of split peaks gives the number of
hydrogens on a neighbouring carbon.
HNMR Spectra
 Molecular
formula of C2H4Br2.
Image Credits

EM Spectrum properties edit, By Inductiveload, NASA
(http://upload.wikimedia.org/wikipedia/commons/c/cf/EM_Sp
ectrum_Properties_edit.svg), via Wikimedia Commons

AASBlock, By K05en01 (Own work) [Public domain]
(http://upload.wikimedia.org/wikipedia/commons/0/08/AASBL
OCK.JPG), via Wikimedia Commons

‘FlammenAAS’, By Talos at de.wikipedia
(http://upload.wikimedia.org/wikipedia/commons/5/51/Flamm
enAAS.jpg), from Wikimedia Commons
Image Credits

Acetone MS, By NIST
(http://webbook.nist.gov/cgi/cbook.cgi?Name=acetone)
[Public domain], via Wikimedia Commons

Ethanol infrared spectrum, By Mfomich (Own work)
(http://upload.wikimedia.org/wikipedia/commons/b/bb/Ethan
ol_IR_Spectrum.png), via Wikimedia Commons