NMR

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  • Created by: Rscottqub
  • Created on: 01-12-19 18:24
NMR is an example of a
non destructive technique
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We use NMR mainly to
determine structure and ID molecules
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NMR is the only spec that...
the entire spectra can be interpreted
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Compared to MS the sample size required is
larger <1mg
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Types of spin of nuclei
1. integral 2. Fractional 3. No spin
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Integral
whole numbers eg. 1,2,3,4
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Fractional spin
1/2, 3/2, 5/2
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No spin
it is possible for nuclei to have no spin - NB to be analysed by NMR molecules need a spin
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Who thought of the basis of NMR
Wolfgang pauli 1924
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who won the nobel prize in physics for NMR
Block and Purcel 1952
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The 1st NMR spec to be produced was what compound?
ethanol
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Spin - odd mass nuclei
have an odd number of nucleons - and have FRACTIONAL spin
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nucleon
protons or neutrons
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Spin- even mass nuclei (odd no of protons AND odd no of neutrons)
have Integral spin
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spin - even mass nuclei (even no of neutrons)
have NO SPIN
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Principle of NMR
Spinning charge generates a magnetic field. When a external field is applied 2 spin states exist . +1/2 and -1/2
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+1/2
state where nuclei is alligned with the field
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-1/2
alligned against the field
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Which state is higher
-1/2
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Diff between 2 states
basis for NMR
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Energy change between 2 states is measure in
MHz
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If we apply radio frequency corresponding to diff in energy between states
cause nuclei to allign against external field- and will switch form +1/2 to -1/2
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If apple external field
nuclei tend to allign with the field
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NMR spec must be fine tuned to
a specific nucleus - in most cases this is proton/H
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Process of NMR
sample solution is deuteriated. in uniform 5mm glass tube betwen poles of powerful magnet. Spun to average out mag field variations. Radio freq applied from antenna coil. Receiver coil surrounds sample tube. Emission of absorb freq -> spec
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why do nuclei behave diff in diff compounds?
Electrons move in response to external field, they generate a secondary field. This opposes the applied field. The secondary field shields the nucleus from applied field and therefore the amount of energy req to change spin state is altered.
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continued ....
more energy is required to allign with the field.
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Reference used in NMR
TMS - tetramethylsilane
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Every type of proton envrinoment
produces own peak
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Chem shift units
ppm
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Chemical shift definition
freq of resonance expressed with reference to the standard compound at 0ppm (TMS)
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How to take NMR
dissolve in CCL4 (no protons) place in approp. tub. Place in machine
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Solvent of choice
CCl4, but toxic and poor solvent . CDCl3 better
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Factor affecting chem shift
inductive groups. If more than one present it can have a cumulative effect.
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How to remove OH group from alchol
add heavy water D20.
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Splitting patterns - singlet
no adj H
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doublet
1 adj H
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triplet
2 adj H
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Quartet
3 adj H
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Splitting patterns of alkenes are locate in ..
5-6 ppm region
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singlet in 5-6 ppm
no adj H
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doublet in 5-6pp,
1 adj H
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Doublet of doublets
2 adj H
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doublet of doublet of doublets
3 adj Hs
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Splitting patterns of benzenes are found in the ....
7-8ppm region
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2 Doublets
para sub
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2 doublets and 2 triplets
ortho sub
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singlet,2 doublets , triplet
meta sub
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Other cards in this set

Card 2

Front

We use NMR mainly to

Back

determine structure and ID molecules

Card 3

Front

NMR is the only spec that...

Back

Preview of the front of card 3

Card 4

Front

Compared to MS the sample size required is

Back

Preview of the front of card 4

Card 5

Front

Types of spin of nuclei

Back

Preview of the front of card 5
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