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Unformatted text preview: Notes from the NMR Review Session 1 21 October, 2010 Section I: What is NMR? NMR stands for Nuclear Magnetic Resonance (Spectroscopy). Focusing on nuclear, which nuclei can we use? Magnetic nuclei have an odd number of protons (p + ), an odd number of neutrons (n ), or both. An even number of p + and an even number of n results in an NMR inactive nucleus. This is because each particle is sensitive to a magnetic eld, and if we have an even number, then their sensitivity is canceled out (if this doesn t make sense, don t worry about it too much). Examples: 1 H: 1p + 12 C: 6p + + 6n (NMR inactive) 13 C: 6p + + 7n 10 B: 5p + + 5n Some important NMR nuclei for organic compounds include: 1 H, 2 H (D), 10 B, 11 B, 13 C, 14 N, 15 N, 17 O, 19 F, 31 P, 35 Cl Note: 31 P is NMR active, you can see that in your spectra from the Wittig reaction. Turning our attention to the word magnetic, we can think of a spinning nucleus as a little bar magnet, in that it has its own magnetic eld. N S Figure 1. Diagram of a bar magnet. The curved lines represent its magnetic eld. 1 This work was produced by Neil Cole-Filipiak, Tristan de Rond, Billy Hart-Cooper, Mike Lipschutz, and Kurt Van Allsburg under the Creative Commons Attribution Non-Commercial Share Alike License on 28 October 2010 for the University of California, Berkeley Chemistry Department. http://creativecommons.org/about/licenses/by-nc-sa/3.0/ In solution, all of these little magnets are randomly oriented. However, if we put them in an external magnetic Feld, i.e ., an NMR spectrometer, they all line up either with or against the Feld (kind of like how iron Flings line up when you put a magnet near them). B o a) b) igure 2. Nuclei spinning in a) free solution and b) a solution in an external magnetic Feld ( B o ). IMPORTANT NOTE: Remember that each nucleus has its own magnetic Feld that that will affect the Feld felt by nearby nuclei. In other words, if nucleus X has a spin up (aligned with B o ), then nucleus Y, its neighbor, will feel the combined magnetic Felds. Also, the electrons around each nucleus will also produce a magnetic Feld which further changes the Feld experienced by nucleus Y. So what about resonance? The spin of a nucleus in a magnetic Feld can be knocked out of alignment by hitting it with electromagnetic radiation (in this case, radio waves). We know when this happens due to a change in the radio waves coming out of the sample as B o pushes the nucleus back into alignment. Because each nucleus feels a different magnetic Feld, it will take a slightly different energy to knock it out of alignment. In other words, it takes different frequencies of light to change the spin of nuclei in different magnetic Felds....
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This note was uploaded on 11/19/2010 for the course CHEMISTRY 110A taught by Professor Tu,t during the Spring '10 term at UCLA.
- Spring '10