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The variations in the positions of NMR absorptions, arising from electronic shielding and deshielding, are called __.
Explain it.
chemical shift
the difference (in ppm) between the resonance frequency of the proton being observed and tha tof tetramethylsilane
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A more accurate method for expressing chemical shifts is to do what?
The difference?
determine the value relative to a reference compound added to the sample
in the magnetic field strength between the resonances of hte samlple protons and the reference protons can be measured very accurately
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The most common NMR reference compound is __. Because silicon is __ than carbon, the metyhl groups of TMS are relatively __, and their protons are __. Because they absorb at a higher field strength than nmost hydrogens bonded to carbon or other elements, so most NMR signals appear __ (to the left, __) of the TMS signal.
- tetramethylsilane
- less electronegative
- electron rich
- well shielded
- downfield
- deshielded
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For each type of proton, the distance downfield of TMS is the __ of those protons. Newer spectrometers operate at a constant magnetic field, and they measure the chemical shift as a __.
- the chemical shift
- the difference in the magnetic field between where the protons in the sample absorb and where those in TMS absorb
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For each tpe of proton, th distance downfield of TMS is the __. Newer spectrometers operate at a constant magnetic field, and they measure the chemical shift as a __.
- chemical shift of those protons
- frequency difference between teh resonances of the protons in the sample and those in TMS
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Chemical shifts are measured in __. The difference (the chemical shift) between the NMR signal of a proton and that of TMS is shown on the __ calibrated in __.
A chemical shift in ppm can be calculated by __.
- ppm
- horizontal axis
- frequency units
dividing the shift measured in hertz by the spectrometer frequency measured in millions of hertz spectrum
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chemical shift equation?
shift downfield from TMS/ total spectrometer frequency
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The most common scale of chemical shifts is the __.
delta scale
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The chemical shift of a methyl group in an alkane is about __.
In general, the effect of an electron-withdrawing substituent __, adn the effects are usually negligible on protons that are separated from teh electronegative group by __ bonds.
- delta 0.9
- decreases with increasing distance
- four or more
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Protons that are more distant than the __ are deshieled by a __.
If more than one electron-withdrawing grouop is present, __.
- beta protons
- negligible amount
the deshielding effects are nearly (but not quite) additive
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Double bonds and aromatic rings produce large __ on their __ and __ protons.
These deshielding effects result from the same type of __ that normally shields nuclei from the magnetic field.
In benzene and its derivatives, the aromatic ring of pi bonding electrons acts as a __, and the external magnetic field induces a __. At the center fo the ring the induced field acts to __. These induced field lines curve around, however, and on the edge of the ring the induced field adds to the external field. As a result, the aromatic protons are strongly __, resulting in a large chemical shift.
- deshielding effects
- vinyl
- aromatic protons
- circulation of electrons
- conductor
- ring current
- oppose the external field
- deshielded
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The pi electrons of an alkene do what?
Since the pi bond of an alkene deshields the vinyl protons, we might expect an acetylenic hydrogen to be even __. The opposite is true:
deshield the vinyl protons in the same way that an aromatic ring of electrons deshieds the aromatic protons; but it is not as large
even more deshielded by the two pi bonds of hte triple bond
acetylenic hydrogens absorb around dlta 2.5 compared with 5 or 6 for vinyl protons
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Aldehyde protons absorb at even __ than vinyl and aromatic protons: between __.
The chemical shifts of O-H protons in alcohols and NH protons in amines depends on the __. In concentrated solutions, these protons are deshield by __, and they absorb at a relatively low field: about __for an amine NH and about __for an alcohol OH. When the alcohol or amine is diluted with a non-hydrogen bonding solvent such as CCl4, hydrogen bonding becomes __. In dilute solutions, these signals are observed around __.
- lower fields
- delta 9 and delta 10
- concentration
- hydrogen bonding
- delta 3.5
- delta 4.5
- less important
- delta 2
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Carboxylic acid protons have considerable positive character because??
They are strongly __ and absorb at chemical shifts __.
Carboxylic acids frequently exist as __, with moderate rates of proton exchange that broaden the absorption of the acid proton.
they are bonded to an oxygen next to a carbonyl group
- deshielded
- greater than d10
hydrogen-bonded dimers
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In general, the number of __ corresponds to the number of different kinds of protons present in the molecule.
NMR signals
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Protons in identical chemical environments with the same shielding have the __. Such protons are said to be __. This is what is meant whenever we use teh term __ in discussing NMR spec. In methyl tert-butyl ether, the three __ are chemcialyl equivalent and the nine tert0butly protons are chemically equivalent.
- chemical shift
- chemically equivalent
- equivalent
- methoxy protons
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Protons that are not chemically equivalent but happen to absorb at the same chemical shift are said to be __.
accidentally equivalent
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The area under a peak is proprotional to __.
the number of H contributing to that peak
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