Transcript Spectropscopy master Class - Nottingham Trent University

Spectroscopy
Master Class
Spectroscopy
Master Class
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Spectroscopy
Aims of this session
Mass spectroscopy
NMR spectroscopy
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To understand how chemists
obtain and interpret the
following three types of spectra.
IR spectroscopy
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Spectroscopy
Mass Spectrometry
An analytical technique which uses the differences between
the mass (& charge) of ions as its basis.
Mass spectrometers can be
used as an analytical tool to
measure the relative
molecular
atomic
mass
mass
(RAM)
(RMM)
of an
of
a compound.
element
(and its isotopes).
www.specs.com
35Cl
75.8%
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and
37Cl
24.2%
 35·5
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Spectroscopy
Mass Spectrometer : Basic Structure
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Spectroscopy
Mass Spectrometry
When the vapour is ionized, electrons are lost to form
positive ions.
A mass spectrometer can also create charged fragments
from compounds and so can provide information about
chemical structure.
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Spectroscopy
Mass Spectrometer : Different masses
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Spectroscopy
Mass Spectrometry: Fragments
A mass spectrum contains peaks which correspond to
particular fragments. Stable fragments create larger peaks.
Certain fragments are easily
identifiable:
CH3+= 15
C2H5+= 29
M+ =
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heaviest
peak
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Spectroscopy
Mass Spectrometry: Paired up
A powerful and widely used method is to couple Gas
Chromatography with Mass Spectrometry (GC/MS).
Compound x
Compound y
A mixture of compounds are firstly separated (GC step)
and then analysed (MS step).
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Spectroscopy
Spectroscopy & EMR
1855
Robert Wilhelm Bunsen
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Gustav Kirchhoff
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Spectroscopy
Spectroscopy & EMR
An electron in a lower orbital
receives energy - in this case
by absorbing light. It is then
promoted to a higher energy
orbital.
An excited state electron will
eventually lose energy (emitted
as light) and fall back to the
lower orbital. This is known as
relaxation.
 Hotspot: Play triangle
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Spectroscopy
Spectroscopy & EMR
Emission
Absorption
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Spectroscopy
Hotspots: Beam, Prism, Slider, Autoscan
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Spectroscopy
IR Spectrometry
An analytical technique which uses the differences between
bonds (& electron levels) as its basis.
Bonding electrons
absorbing IR cause the
bonds to deform.
Typical changes to bonds
include…
Stretching
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Bending
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Spectroscopy
Hotspots: Assigned absorption bands
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Spectroscopy
IR Spectrometry
Particular useful technique for helping to identify organic
functional groups.
CH3
deformation
OH
bend
OH
stretch
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CH
stretch
CO
stretch
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Spectroscopy
IR Spectrometry
wavenumbers of IR can be associated with the
Different wavelengths
deformation of particular bonds.
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Spectroscopy
NMR Spectrometry
An analytical technique which uses the differences between
the magnetic property of nuclei as its basis.
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Spectroscopy
NMR Spectrometry: Spin states
Protons possess spin – this makes them behave like tiny
magnets. They will match or oppose an external field.
Energy
Both
Protons
Proton
B
S
Proton A
S
Proton B
N
N
DE
Proton A
Magnetic field
Radio waves are required to ‘flip’ the nuclei. This
technique is important for 1H, 13C, 19F and 31P nuclei.
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Spectroscopy
NMR Spectrometry: Relaxation & detection
A low-energy nuclei (aligned with the applied field) will
jump to a high energy spin state when given a pulse of RF.
Against
field
RF signal coil
With
Induced
signal
field
detector coil
can be the same coil!
When the magnetic field is removed, the nuclei revert back to
their original state.
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Spectroscopy
NMR Spectrometry: Energy levels
The energies of the two spin states relate to the magnetic field.
Proton B
Weak
field
Proton B
Both protons
DE
Both protons
DE
Strong
field
Proton A
Proton A
We can either fix the field strength and vary the radio waves
until the nuclei flip … or… we use one particular radio wave
and vary the magnetic field.
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Spectroscopy
NMR Spectrometry: Shift
DEE
Not all nuclei experience
the same strength of
external magnetic field.
C
DE
H
H
DDEE
C
H
O
The opposing field varies
according to the nearby
bonds and nuclei.
H
In a magnetic field electrons
circulate. This creates an
opposing magnetic field.
DE
This is called the chemical shift phenomenon and causes
a difference in the energy spin states for nuclei.
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Spectroscopy
NMR Spectrometry: Assigning peaks
H
C
C
H
O
For ethanal, its two types of
hydrogen nuclei will produce
different signals during NMR.
H
H
The area under each peak relates to the
number of each type of hydrogen.
Area = 65
CH3
Area = 22
CH3
Si
CH3
TMS
CH3
10
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8
6
4
2
0
Chemical shift
Used to calibrate
The signals
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Spectroscopy
NMR Spectrometry: Spectra
Analysis of many organic compounds has enabled chemists
to create tables of chemical shifts…
The peaks themselves contain additional information that
relates to how neighbouring hydrogens interact in 3D.
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Spectroscopy
Combing Techniques
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Mass spectroscopy
Together these three methods of
spectroscopy form a powerful
tool for the chemist - identifying
functional groups, bonds and the
3D structure of compounds.
NMR spectroscopy
IR spectroscopy
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Spectroscopy
Master Class
Prepared for general school use by
Centre for Effective
Learning in Science
www.ntu.ac.uk/cels
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