Chapter 19 - Chapter 19 Nuclear Magnetic Resonance...

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Chapter 19 Nuclear Magnetic Resonance Spectroscopy Nuclear magnetic resonance (NMR) spectroscopy is based on the measurement of absorption of electromagnetic radiation in the radio-frequency region of roughly 4 to 900 MHz. Nuclei of atoms rather than outer electrons are involved in the absorption process. In order to cause nuclei to develop the energy states required for absorption to occur, it is necessary to place the analyte in an intense magnetic field. Nuclear magnetic resonance spectroscopy is one of the most powerful tools for elucidating the structure of chemical species.
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Quantum Description of NMR The maximum number of spin components or values for a particular nucleus is its spin quantum number I. The nucleus will then have 2I + 1 discrete states. The four nuclei that have been of greatest use are 1 H, 13 C, 19 F, and 31 P. The spin quantum number for these nuclei is ½. Thus, each nucleus has two spin states corresponding to I = + ½ and I = - ½. A spinning, charged nucleus creates a magnetic field.
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Quantum Description of NMR The resulting magnetic moment μ is oriented along the axis of spin and is proportional to the angular momentum P. Thus, μ = γ p where the proportionality constant γ is the magnetogyric ratio, which has a different value for each type of nucleus. Energy Levels in a Magnetic Field: When a nucleus with a spin quantum number of one half is brought into an external magnetic field B 0 , its magnetic moment becomes oriented in one of two directions with respect to the field depending upon its magnetic quantum state.
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Types of NMR Spectra There are several types of NMR spectra, which depend upon the kind of instrument used, the type of nucleus involved, the physical state of the sample, the environment of the analyte nucleus, and the purpose of the data collection. Most NMR spectra can be categorized as either wide line or high resolution . Wide-Line spectra:
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This note was uploaded on 05/06/2010 for the course CHEM 4414 at Arkansas Tech.

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Chapter 19 - Chapter 19 Nuclear Magnetic Resonance...

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