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chapter 9 - Chapter 9 NMR(Nuclear Magnetic Resonance and...

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Chapter 9 NMR (Nuclear Magnetic Resonance) and Mass Spectrometry
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§9.2 NMR Spectrometers A sample is placed in a very strong, constant magnetic field produced by a superconducting magnet. The sample is irradiated with a short pulse of radio frequency energy that excites certain nuclei. The nuclei relax, emitting a signal that is converted to a spectrum by a Fourier transformation.
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§9.2 MRI MRI (magnetic resonance imaging) is an NMR where humans are the sample. The names (MRI vs NMR) are different because patients equate “nuclear” with radioactivity. Neither NMR nor MRI use radioactivity.
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§9.2 1 H-NMR 1 H-NMR spectra provide a lot of information: 1. Chemical shift (gives functional group like IR) 2. Integration (number of Hs per signal) 3. Coupling (Hs on adjacent Cs) 4. Number of signals (number of unique Hs) This is a lot of structural information from a single spectrum. This site offers an NMR game that will give you some experience solving NMR problems: http://www. spectralgame .com/
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§9.2 A 1 H-NMR Spectrum 1 H-NMR spectra have a peak for each unique type of hydrogen in a molecule. x -axis units are parts per million (ppm) or δ . Left is “downfield”; right is “upfield”. downfield upfield cyclobutene
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§9.2A Chemical Shift Chemical shift – the x -axis position of an NMR signal. Nuclei of a given functional group usually appear in the same region. Chemical shift charts identify the most likely functional group for a given chemical shift.
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R C H 3 0.9 R C H 3 O 2.1 RR'N C H 3 2.4 R C H 2 X 3.5 R O O 4.5 C H 3 R 2 C C H 2 5.3 R H O 9.7 R O 3.8 C H 3 Chemical shift functional group Memorize
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§9.2B Integration Integration – the area under each signal. For 1 H-NMR, integration is proportional to the number of H atoms producing each signal. Integration is calculated automatically by most NMRs. Not useful for 13 C-NMR.
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How many different types of Hs are there? What is the ratio of red to blue Hs? Which peak belongs to each H? 6 : 4
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§9.2C Coupling Coupling NMR signals are split into 2+ smaller, closely spaced peaks by their neighbors. These Hs couple: vicinal Hs (3 bonds apart) CH 2 next to a chirality center C H C H C C H H Z X Y chirality center
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§9.2C Signals and Peaks There is one NMR signal for each unique nuclei. 13 C-NMR: each nuclei signal has one peak. 1 H-NMR: each nuclei signal can have 1-7 peaks. Number of Peaks Signal Name 1 singlet 2 doublet 3 triplet 4 quartet 5 pentet 6 sextet 7 septet 13 C-NMR 1 H-NMR
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§9.2C Signals vs Peaks How do I know if I’m looking at a triplet or 3 singlets? Doublet peaks have the same height and are close together. Other multiplets (non-singlets) are evenly spaced and pyramid shaped: sextet triplet
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H H H H H H H H adjacent H not adjacent H a b c d d Hydrogen a: ___ adjacent H's --> ___ peaks Hydrogen b: ___ adjacent H's --> ___ peaks Hydrogen c: ___ adjacent H's --> ___ peaks Hydrogen d: ___ adjacent H's --> ___ peaks §9.2C n + 1 Rule A H atom with n adjacent (vicinal, or neighboring) Hs has a signal of n + 1 peaks.
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Singlet No adjacent H Doublet 1 adjacent H Triplet 2 adjacent H Quartet 3 adjacent H
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C O O singlet s q t quartet triplet R C H 3 0.9 R C H 3 O 2.1 R O O 4.5 C H 3
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§9.2C Coupling Equivalent Hs do not couple to each other.
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