nmr Flashcards
what does nmr stand for?
nuclear magnetic resonance
why are carbon-13 and h-1 nuclei used in nmr?
bc odd numbered nuclei are needed for nmr. they can act as little bar magnets as they have magnetic fields around them
what does a high chemical shift tell you about the position of a nucleus?
it’s near an electronegative atom or its near a benzene group
what causes the peak at zero chemical shift?
TMS - tetramethylsilane
give the structure of tetramethylsilane
Si(CH3)4
why is tms chosen as the standard in nmr?
produces a strong single peak
v far to the right
volatile
non-toxic and inert
why does tms produce a strong single peak?
all carbons are in the same environment. joined to the same Si atom in the same way
why is the tms peak so far to the right
bc the electrons in the C-Si bond are closer to the C than in any other compound so there’s maximum shielding
why is it good that the tms peak is so far to the right?
because it doesn’t interfere with the spectrum
why is it good that tms is inert?
doesn’t react with any of the compounds being measured in the spec
which solvents are used in nmr?
CCl4
CDCl3 - organic compounds
D2O - polar solvents
why is it important that solvents in nmr don’t contain hydrogen?
would interfere with the spectrum
what does a low chemical shift suggest in carbon-13 nmr
more electron shielding so less affected by magnetic field
what does the number of peaks show in carbon-13 nmr
the number of carbon environments
what’s the difference between low and high res nmr?
in high resolution nmr you can see the individual peaks within a cluster (caused by splitting) but in low resolution nmr you cannot
what does the integration in proton nmr show?
the number of hydrogens in that environment
what does the splitting pattern show in proton nmr?
the number of hydrogens in neighbouring environments
explain the n+1 rule and its significance
the peaks in a cluster of peaks follows the n+1 rule.
in a peak there is one more peak than the number of surrounding hydrogens
why are solvents needed in nmr?
because solid samples cannot be analysed by nmr
what is d2o used for?
to detect nh and oh groups
how is d2o used to detect NH and OH groups in nmr?
proton exchange occurs. alcohols are slightly acidic. the h on the oh transfers to the lone pair on the oxygen of d2o and the d takes it place
