Student%20Notes%20-%20Section%2012 - Section 12 Nuclear...

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Section 12: Nuclear Magnetic Resonance Spectroscopy 152 Section 12: Nuclear Magnetic Resonance Spectroscopy 12.1 Theoretical Considerations Nuclear Magnetic Resonance (NMR) spectroscopy is an extremely powerful method for identifying organic compounds – it provides more information than the previously described methods: UV-visible spectroscopy indicates whether the compound contains and aromatic ring, or conjugated π -systems. Infra-red spectroscopy can identify a wider range of functional groups, and indicate substitution patterns to a certain extent. NMR spectroscopy can identify functional groups, but can also be used to determine the structure of an organic compound, including in many cases the stereochemistry (including absolute configurations of enantiomers). NMR can also be used to calculate equilibrium distributions of different tautomers or conformers, and can be used to measure the thermochemistry of these species. As a final point, NMR techniques may be used to determine the concentrations of hydrogen atoms in various chemical environments in a non-homogeneous sample such as a human body. This is the basis of magnetic resonance imaging (MRI). We recall from earlier discussion that electrons in magnetic fields behave as if they are spinning on their axis – they have a quantized spin angular momentum (or spin). In addition to electrons, some (but not all) nuclei also have spin. Some of these nuclei are abundant in nature ( 1 H, 14 N, 19 F and 31 P), others occur at a considerably lower abundance ( 2 H, 13 C). In the presence of an external magnetic field, nuclear spins may align with or against the magnetic field – the nuclear spin state aligned against the magnetic field has higher energy. The splitting between the two possible spin states increases proportionally with the magnitude of the applied field. In an external magnetic field, we have a quantized energy gap between two spin states: the nucleus may undergo a “spin flip” following absorption of a photon of suitable energy. With the spin flip, the nucleus is said to be in resonance with the applied electromagnetic radiation.
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Section 12: Nuclear Magnetic Resonance Spectroscopy 153 Energy 0 Magnetic Field Strength (Tesla) E spin state aligned against magnetic field spin state aligned with magnetic field To put this energy gap into perspective, it is worth remembering that in order to observe resonances with low energy radiofrequencies, we require magnetic fields in the range 4.0- 21.6 Tesla. The magnetic field of the earth is of the order 10 -4 Tesla. This has important ramifications with respect to instrument design, as we will see later. In organic molecules, nuclei are surrounded by electrons (or electron density), which as we know also have magnetic moments due to their spin. Therefore, each nucleus experiences an effective magnetic field, B eff , which is made up of two components: loc ext eff B B B = The two components are the externally applied magnetic field, B ext , offset by a localized
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This note was uploaded on 10/28/2010 for the course CHEM 235 taught by Professor Dr.poole during the Spring '10 term at Ball State.

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Student%20Notes%20-%20Section%2012 - Section 12 Nuclear...

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