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21 Cards in this Set
- Front
- Back
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What does the number of NMR signals represent?
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The number of different types of protons in a compound
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What is resonance (in regard to NMR)?
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A nucleus is in resonance when it absorbs RF radiation and "spin flips" to a higher energy state
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What is a spin flip?
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When a nucleus goes from being aligned with an external magnetic field absorbs enough energy to align itself against the magnetic field
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What is chemical shift?
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the position of the peak where a nucleus absorbs
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What does an NMR spectrum plot?
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The intensity of a peak against its chemical shift, which is measured in parts per million (ppm)
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What is the standard for proton NMR?
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TMS (tetramethylsilane)
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Why is TMS used as a standard in proton NMR?
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TMS gives a single peak upfield from other typical NMR absorptions
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What four different features of a proton NMR spectrum provide information about a compound's structure?
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1. Number of Signals
2. Position of Signals 3. Intensity of Signals 4. Spin-Spin Splitting of Signals |
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Protons in different environments give ______ signals. Equivalent protons give the _____ NMR signal.
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Different
Same |
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When comparing two H atoms on a ring or double bond, two protons are equivalent only if they are ____ or ______ to the same groups
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Cis
Trans |
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Where a particular proton absorbs depends on its electronic ______?
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Environment
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In the vicinity of the nucleus, the magnetic field generated by a circulating electron ______ the external magnetic field that a proton feels
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Decreases
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Why does shielding shift an absorption upfield?
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Shielding implies that an electron is decreasing the external magnetic field that the proton feels. Because the proton experience a lower magnetic field strength, it needs a lower frequency to achieve resonance. And lower frequency is to the right (upfield) in an NMR spectrum
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What can cause deshielding of a proton?
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A proton that is near an electronegative atom (like Cl or F). This is because the electronegative element is more able to hold the electrons
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What is deshielding?
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When the electron density around a nucleus is decreased, causing the nucleus to feel a stronger magnetic field strength; thus requiring greater frequency to achieve resonance
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On an NMR spectrum, more shielded electrons occur where?
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More upfield; to the right
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On an NMR spectrum, more deshielded electrons occur where?
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Downfield; to the left
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The chemical shift of a C-H bond increases with increasing ______
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Alkyl substitution
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Describe the absorption for a proton in a benzene ring
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The six pi electrons in benzene circulate around the ring, creating a ring current. The magnetic field induced by these moving electrons reinforces the applied magnetic field in the vicinity of the protons. The protons thus feel a stronger magnetic field and a higher frequency is needed for resonance.
So the protons are deshielded and the absorption is downfield |
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Describe the absorption for Carbon-Carbon double bonds
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In a magentic field, the loosely held pi electrons create a magnetic field that reinforces the applied field in the vicinity of the protons. Because the protons now feel a stronger magnetic field, they require a higher frequency for resonance
The protons are deshielded and the absorption is downfield |
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Describe the absorption for Carbon-Carbon triple bonds
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In a magnetic field, the pi electrons of the carbon-carbon triple bond are induced to circulate, but in this case the induced magnetic field opposes the applied magnetic field, but in this case the induced magnetic field opposes the applied magnetic field. The proton thus feels a weaker magnetic field, so a lower frequency is needed for resonance.
The nucleus is shielded and the absorption is upfield |
