1. Spectroscopy

2. 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?

3. Key Terms  Electromagnetic spectrum  Wavelength  Standard  Calibration curve  Ionisation  Vibrational energy level  Magnetic resonance

4. The Electromagnetic Spectrum

5. 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

6. 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.

7. 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.

8. Atomic Absorption Spectroscopy  Developed by the CSIRO.  Used for analysing metals.

9. Atomic Absorption Spectroscopy  N 2 O (laughing gas) can also be used as an oxidant in place of air (O 2 ).  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.

10. 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.

11. Mass Spectroscopy  Measures the mass of atoms and molecules.

12. 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.

13. 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.

14. Mass Spectroscopy

15. 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?

16. 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.

17. Infra-Red Spectroscopy Symmetrical stretching Antisymmetrical stretching Scissoring RockingWaggingTwisting

18. 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.

19. Infra-Red Spectroscopy BondWave Number C-Cl700-800 C-C750-1100 C-O1000-1300 C=C1610-1680 C=O1670-1750 O-H (as part of an acid) 2500-3300 C-H2850-3300 O-H (as part of an alkanol) 3200-3550 N-H3350-3500

20. 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.

21. 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’.

22. NMR Spectra

23. CNMR Spectra  # of peaks = # of environments  Chemical shift = type of environment (lower electron density, further downfield)  Peak area = number of similar atoms

24. CNMR Spectra  Refer to the table of C NMR data to interpret the following spectrum. Empirical formula is C 4 H 10 O and it is an alkanol.

25. CNMR Spectra  Molecular formula of C 3 H 6 O 2. Does not react with NaOH.

26. HNMR Spectra  In high resolution proton (H) NMR, the number of split peaks gives the number of hydrogens on a neighbouring carbon.

27. HNMR Spectra  Molecular formula of C 2 H 4 Br 2.

28. 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 Commonshttp://upload.wikimedia.org/wikipedia/commons/c/cf/EM_Sp ectrum_Properties_edit.svg  AASBlock, By K05en01 (Own work) [Public domain] (http://upload.wikimedia.org/wikipedia/commons/0/08/AASBL OCK.JPG), via Wikimedia Commonshttp://upload.wikimedia.org/wikipedia/commons/0/08/AASBL OCK.JPG  ‘FlammenAAS’, By Talos at de.wikipedia (http://upload.wikimedia.org/wikipedia/commons/5/51/Flamm enAAS.jpg), from Wikimedia Commonshttp://upload.wikimedia.org/wikipedia/commons/5/51/Flamm enAAS.jpg

29. Image Credits  Acetone MS, By NIST (http://webbook.nist.gov/cgi/cbook.cgi?Name=acetone) [Public domain], via Wikimedia Commonshttp://webbook.nist.gov/cgi/cbook.cgi?Name=acetone  Ethanol infrared spectrum, By Mfomich (Own work) (http://upload.wikimedia.org/wikipedia/commons/b/bb/Ethan ol_IR_Spectrum.png), via Wikimedia Commonshttp://upload.wikimedia.org/wikipedia/commons/b/bb/Ethan ol_IR_Spectrum.png