3AL Lecture 9 - Nuclei in different electronic environments...

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Nuclei in different electronic environments resonate at slightly different frequencies. The differences between these are in Hertz not Megahertz! For example, different hydrogens or different carbons within the same molecule or in different molecules will appear as unique signals (peaks).
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Example of a 1 H-NMR Spectrum H 3 C O O CH 3 How many peaks in the 13 C NMR spectrum? 3 Increasing frequency Why are there only two peaks when there are six hydrogens? NMR does not distinguish between equivalent hydrogens.
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Nuclei in different electronic environments resonate at slightly different frequencies. WHY? Electrons are charged particles. As such, they will circulate in the presence of an external magnetic field, creating their own small magnetic field that OPPOSES the external field (Lenz’s Law). Greater electron density around a nuclei: Larger opposing field. Less electron density around a nuclei: Smaller opposing field.
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Larger opposing field. Less electron density around a nuclei: Smaller opposing field. Increasing frequency Which peak is which? The methyl attached to the oxygen will have less electron density around its hydrogens than the other methyl. CH
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This note was uploaded on 08/19/2009 for the course CHEM 3A taught by Professor Fretchet during the Spring '08 term at University of California, Berkeley.

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3AL Lecture 9 - Nuclei in different electronic environments...

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