NMR Lecture - 1 Introduction NMR is the most powerful tool...

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Unformatted text preview: 1 Introduction NMR is the most powerful tool available for organic structure determination. It is used to study a wide variety of nuclei: 1 H 13 C 15 N 19 F 31 P => 2 NMR spectroscopy - novel features 1. Involves nuclear transitions rather than electronic transitions. 2. Transitions are very low energy and result from absorbtion of radiowaves. 3. Difference in energy states ONLY exists in the presence of a magnetic field. 4. Great sensitivity of NMR spectroscopy to molecular structure came as a surprise to chemists and physicists. 5. Provides information both about the types and number of atoms in a molecule and about their conformation. 6. The method is still evolving, particularly in the area of the determination of the structure large biological molecules. 3 Nuclear Spin A nucleus with an odd atomic number or an odd mass number has a nuclear spin. The spinning charged nucleus generates a magnetic field. => 4 External Magnetic Field When placed in an external field, spinning protons act like bar magnets. => 5 Two Energy States The magnetic fields of the spinning nuclei will align either with the external field, or against the field. A photon with the right amount of energy can be absorbed and cause the spinning proton to flip. => 6 E and Magnet Strength Energy difference is proportional to the magnetic field strength. E = h = h B 0 2 Gyromagnetic ratio, , is a constant for each nucleus (26,753 s-1 gauss-1 for H). In a 14,092 gauss field, a 60 MHz photon is required to flip a proton. Low energy, radio frequency. => Know this equation! 7 Spin flipping 1. Frequency of radiowave that causes transition from to state depends on strength of total strength of the magnetic field that surrounds the nucleus. 2. This magnetic field depends upon: The field generated by the external magnet. The local field generated by the electrons that surround the nucleus [shielding or deshielding]. The field generated by neighboring nuclei [spin-spin coupling. 8 Magnetic Shielding If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained. But protons are surrounded by electrons that shield them from the external field. Circulating electrons create an induced magnetic field that opposes the external magnetic field. => 9 Shielded Protons Magnetic field strength must be increased for a shielded proton to flip at the same frequency. => 10 Protons in a Molecule Depending on their chemical environment, protons in a molecule are shielded by different amounts....
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NMR Lecture - 1 Introduction NMR is the most powerful tool...

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