Spectroscopy Master Class

1. SpectroscopyMaster Class

2. Spectroscopy Master Class Prepared for general school use by www.ntu.ac.uk/cels Permission is hereby given to modify the textual content of this file to suit your own classes provided you do not redistribute the work or any derivative version. Please contact CELS if you wish to use either the images or animations in an alternative presentation or related worksheet.

3. Next Back Spectroscopy Aims of this session Mass spectroscopy To understand how chemists obtain and interpret the following three types of spectra. IR spectroscopy NMR spectroscopy

4. Next Back 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 relativemolecular 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). www.specs.com 35Cl and 37Cl  35·5 75.8% 24.2%

5. Next Back Spectroscopy Mass Spectrometer : Basic Structure

6. Next Back 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.

7. Next Back Spectroscopy Mass Spectrometer : Different masses

8. Next Back 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+= heaviest peak

9. Compound x Compound y Next Back Spectroscopy Mass Spectrometry: Paired up A powerful and widely used method is to couple Gas Chromatography with Mass Spectrometry (GC/MS). A mixture of compounds are firstly separated (GC step) and then analysed (MS step).

10. Next Back Spectroscopy Spectroscopy & EMR 1855 Robert Wilhelm Bunsen Gustav Kirchhoff

11. Next Back 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

12. Absorption Emission Next Back Spectroscopy Spectroscopy & EMR

13. Next Back Spectroscopy Hotspots: Beam, Prism, Slider, Autoscan

14. Next Back 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 Bending

15. Next Back Spectroscopy Hotspots: Assigned absorption bands

16. Next Back Spectroscopy IR Spectrometry Particular useful technique for helping to identify organic functional groups. CH3 deformation OH bend CH stretch OH stretch CO stretch

17. Next Back Spectroscopy IR Spectrometry Different of IR can be associated with the deformation of particular bonds. wavenumbers wavelengths

18. Next Back Spectroscopy NMR Spectrometry An analytical technique which uses the differences between the magnetic property of nuclei as its basis.

19. Energy S S D E N N Magnetic field Next Back Spectroscopy NMR Spectrometry: Spin states Protons possess spin – this makes them behave like tiny magnets. – this makes them behave like tiny magnets. They will match or oppose an external field. Both Protons Proton B Proton A Proton B Proton A Radio waves are required to ‘flip’ the nuclei. This technique is important for 1H, 13C, 19F and 31P nuclei.

20. detector coil can be the same coil! RF signal coil Next Back 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 With field Induced signal When the magnetic field is removed, the nuclei revert back to their original state.

21. Proton B Proton B D E D E Proton A Proton A Both protons Both protons Next Back Spectroscopy NMR Spectrometry:Energy levels The energies of the two spin states relate to the magnetic field. Strong field Weak field 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. We can either fix the field strength and vary the radio waves until the nuclei flip … or…

22. D E D D D D D D D Next E E E E E E E Back Spectroscopy NMR Spectrometry: Shift Not all nuclei experience the same strength of external magnetic field. H O In a magnetic field electrons circulate. This creates an opposing magnetic field. C C H H H 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.

23. H O C C H H H Area = 65 Area = 22 Next Back Spectroscopy NMR Spectrometry: Assigning peaks 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. TMS Used to calibrate The signals

24. Next Back 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.

25. Next Back Spectroscopy Combing Techniques 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

26. Back Spectroscopy Master Class Prepared for general school use by Centre for Effective Learning in Science www.ntu.ac.uk/cels