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Chemistry A Level CIE Analytical Techniques
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Chemistry A Level CIE Analytical Techniques
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Analytical TechniquesProton Nuclear Magnetic Resonance (NMR)
-
nucleus of each hydrogen com behoves like a tiny magnet
-
Proton spils and creates magnetic field
-
sample placedIn M f . .
-Protons either line with field or spin
In opposite direction
-
small diff In energy by opposite spinning proton
, corresponds to energy carried by waves in radiowave
range of EM radiation spectrum
-
nuclei 'flip' between two energy levels
-only atoms with mass number is an obd number absorb energy In range of frequencies analysed
NMR
size of the gaps between nuclear energy varies slightly (depending on molecular environment)
-
↳ CH3OH ,
'H atoms in two diff Molecular environments
-
m . .
f
changed because easier than changing - of radiowaves
-
as m f . .
Changed ,
It nuclei In diff environments flip at diff field strengths measured
using reference compound (tetramethylsilane (TMS) =
Si(CH3)4) ,
Value zero
↳ >
gives only one sharp peak
inert ,
volatile liavid which is higher than most
that mixes well >
absorptions measured
All other
with organ ic compounds by shift away iMsline
from
On NMR Spectrum chemical -
Fiesinradiowaves
e In Ppm units
shift measured
n
Energy
-'Against It OH H
I
c -'
(
H - - C- It 3 environments
gre with
it
H
it
O H
i 1 : 1 : b
After I "'
Applied C C C -H 1 environment
H
- - -
it H
"It nuclei In
absorbs energy radiowave range
When they flip from lower energy level , lining cyclohexane 1 environment
C
up with applied m f .
.,
to higher energy level,
lining up against it
. 3 environments
.
2 2 : 1
, NMR measuring
Hydrogen Atoms
·
low resolution NMR
-
single peak for each non-equivalent hydrogen atom
-
heights of peak very
-
area under peak >
-
relative number of equivalentIt atoms responsible for chemical shift
·
high resolution NMR
-
ranges overlap
-
In some molecules , heavy shielding of hydrogen nuclei by lots of e-in surrounding
atoms , peaks shifted beyond usual range
signal NMR made up of cluster of
-
Peaks that appear as one on low grouped peaks , because
m .
f .
generated by spinning nuclei Interfere slightly with neighbouring nuclei (spin-spill couplings
-
exact splitting depends on number of hydrogen atoms on adjacent carbon aroms
-number of signals a peak splitsInto equals n + 1
CH3CH2OH
↳ num of t atoms atom
on adjacent carbon
i H It &
Split (triplet) +(3 + 1) + (3 + 1)
=
↓ .
CH3 TO 3 quadlet H -
C -
c -
C -
i itH
.
2 CH2-split +O 4 (auadruplet) 1 Poly when both sides have hydrogen
lif 1.2. 1.2. Zil
3 . OH not Usually split as 'H atom constantly exchanged with 'H atoms of other molecules
to find presence Of-OH , add D20 13H) -
heavy Water ,
Spectrum for ol will disappear
P T
:
both Odd hence can't be seen in spectrum
4 : I
·
Identifying-OH1-NH-signal
-
-
Oh appears as single peak as it exchanges rapidly with protons of water /acids before interference
CH3CH2OH + H2O CH3CH2OH + HOH
-
if heavy water used ,
-OH/-N-H disappear hence It in them are labile protons
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