Spectroscopy Master Class

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

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

Mass Spectrometer : Basic Structure Spectroscopy Mass Spectrometer : Basic Structure Back Next

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. Back Next

Mass Spectrometer : Different masses Spectroscopy Mass Spectrometer : Different masses Back Next

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 Back Next

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). Back Next

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

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 Back Next

Spectroscopy Spectroscopy & EMR Absorption Emission Back Next

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

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 Back Next

Spectroscopy Hotspots: Assigned absorption bands Back Next

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 Back Next

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

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

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. Back Next

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. Back Next

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. Back Next

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 Back Next

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 Back Next

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. Back Next

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 Back Next

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