1. Spectroscopy Master Class

2. Master Class Spectroscopy Prepared for general school use by
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3. Aims of this session Spectroscopy
Mass spectroscopy
To understand how chemists obtain and interpret the following three types of spectra.
IR spectroscopy
NMR spectroscopy
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4.  35·5 Mass Spectrometry 35Cl and 37Cl Spectroscopy
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 mass (RMM) of a compound.
Mass spectrometers can be
used as an analytical tool to
measure the relative
atomic mass (RAM) of an
element (and its isotopes).
35Cl and 37Cl
 35·5
75.8% %
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5. Mass Spectrometer : Basic Structure
Spectroscopy
Mass Spectrometer : Basic Structure
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6. Mass Spectrometry Spectroscopy
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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7. Mass Spectrometer : Different masses
Spectroscopy
Mass Spectrometer : Different masses
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8. CH3+= 15 C2H5+= 29 M+= Mass Spectrometry: Fragments Spectroscopy
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+=
heaviest
peak
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9. Mass Spectrometry: Paired up
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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10. Spectroscopy & EMR Spectroscopy 1855 Robert Wilhelm Bunsen
Gustav Kirchhoff
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11. Spectroscopy & EMR Spectroscopy
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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12. Spectroscopy
Spectroscopy & EMR
Absorption
Emission
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13. Spectroscopy
Hotspots: Beam, Prism, Slider, Autoscan
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14. IR Spectrometry Spectroscopy
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
Bending
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15. Spectroscopy
Hotspots: Assigned absorption bands
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16. IR Spectrometry CH3 OH CH OH CO Spectroscopy
Particular useful technique for helping to identify organic functional groups.
CH3
deformation OH
bend CH
stretch OH
stretch CO
stretch
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17. IR Spectrometry Spectroscopy
Different of IR can be associated with the deformation of particular bonds.
wavenumbers
wavelengths
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18. NMR Spectrometry Spectroscopy
An analytical technique which uses the differences between the magnetic property of nuclei as its basis.
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19. NMR Spectrometry: Spin states
Spectroscopy
NMR Spectrometry: Spin states
Protons possess spin
– this makes them behave like tiny magnets. They will match or oppose an external field.
– this makes them behave like tiny magnets.
Energy
Both Protons
Proton B N S D E
Proton A N S
Magnetic field
Proton B
Proton A
Radio waves are required to ‘flip’ the nuclei. This technique is important for 1H, 13C, 19F and 31P nuclei.
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20. NMR Spectrometry: Relaxation & detection
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
detector coil
can be the same coil!
RF signal coil
Induced signal
With
field
When the magnetic field is removed, the nuclei revert back to their original state.
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21. NMR Spectrometry: Energy levels
Spectroscopy
NMR Spectrometry: Energy levels
The energies of the two spin states relate to the magnetic field. D E
Proton B
Proton A D E
Proton B
Proton A
Strong
field
Weak
field
Both protons
Both protons
We can either fix the field strength and vary the radio waves until the nuclei flip … or…
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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22. H O C C H H H NMR Spectrometry: Shift D D D D D D D D Spectroscopy
Not all nuclei experience
the same strength of
external magnetic field. H
In a magnetic field electrons
circulate. This creates an
opposing magnetic field. O D E D E C C H H H D E
The opposing field varies
according to the nearby
bonds and nuclei.
This is called the chemical shift phenomenon and causes a difference in the energy spin states for nuclei. D E D E D E
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23. O C H NMR Spectrometry: Assigning peaks Spectroscopy
For ethanal, its two types of
hydrogen nuclei will produce different signals during NMR.
The area under each peak relates to the number of each type of hydrogen.
Area = 65
Area = 22
TMS
Used to calibrate
The signals
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24. NMR Spectrometry: Spectra
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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25. Combing Techniques Spectroscopy
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.
IR spectroscopy
NMR spectroscopy
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26. www.ntu.ac.uk/cels Master Class Spectroscopy
Prepared for general school use by
Centre for Effective
Learning in Science
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