8303207-61-Low-res-NMR

8303207-61-Low-res-NMR - Chem Factsheet...

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Organic Analysis III - Low Resolution NMR Spectroscopy Number 61 1 C hem F actsheet www.curriculumpress.co.uk Before reading through this Factsheet you should: Understand atomic structure (Factsheet 01); Have a good knowledge of AS/A2 organic chemistry; Understand covalent bonding and molecular structure. After working through this Factsheet you will be able to: Be able to interpret simple low-resonance nuclear magnetic resonance spectra; Recognise NMR spectra as useful tool in organic analysis. Successful A2 Chemistry candidates should be able to interpret simple low-resolution nuclear magnetic resonance (NMR) spectra. The aim of this Factsheet is to provide candidates with the knowledge, understanding and experience to do this. Whilst a brief description of how NMR spectra are produced is given, this is background information which students can research in greater detail from text books or other sources. Exam marks are gained from the interpretation of NMR spectra; hence this is the focus of the Factsheet. Formation of NMR Spectra Any spinning electric charge generates a magnetic field. Atomic nuclei are charged, so if they possess the property of spin, they have an associated magnetic field. If such nuclei are placed in a strong magnetic field, they behave like bar magnets and align themselves with that field. This would be the stable, low energy arrangement. If energy is applied, the nuclear magnets can be forced to ‘flip’, and oppose the strong applied magnetic field. This would be the unstable, higher energy arrangement. The applied energy would be electromagnetic radiation of radio frequencies. Fig. 1 Formation of NMR Spectra E (Energy to cause 'flip') external magnetic field external field nuclear 'magnet' nuclear 'magnet' E If a nucleus aligned with the external magnetic field is supplied with electromagnetic radiation of the correct frequency, it will absorb the radiation and ‘flip’ to the higher energy level. This is nuclear magnetic resonance . The precise frequency of radiation that causes a nucleus to ‘flip’ depends not only on the type of nucleus, but also the environment of that nucleus – that is to say the electrons and other nuclei adjacent to the nucleus in question. By placing an organic sample in a strong magnetic field and measuring the frequencies of radiation it absorbs, information can be gained about the environment of the nuclei, i.e. how certain atoms are arranged within the molecule. Such structural information is invaluable in organic analysis. Proton Magnetic Resonance Some isotopes commonly found in organic molecules, such as 12 C and 16 O, do not have nuclei which possess the property of spin, so are not detected by NMR. The hydrogen nucleus (a single proton), however, does.
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8303207-61-Low-res-NMR - Chem Factsheet...

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