2. 11H andH and 1313C NMR compared:C NMR compared:
Both give us information about the number
of chemically nonequivalent nuclei
(nonequivalent hydrogens or nonequivalent
carbons)
both give us information about the
environment of the nuclei (hybridization
state, attached atoms, etc.)
SirAJ/CMR/MScI 2
3. • The 13C nucleus is present in only 1.08% natural abundance. Therefore,
acquisition of a spectrum usually takes much longer than in 1H NMR.
• The magnetogyric ratio of the 13C nucleus is about 1/4 that of the 1H nucleus.
Therefore, the resonance frequency in 13C NMR is much lower than in 1H NMR.
(75 MHz for 13C as opposed to 300 MHz for 1H in a 7.04 Tesla field).
•
• Each unique carbon in a molecule gives rise to a 13C NMR signal. Therefore,
if there are fewer signals in the spectrum than carbon atoms in the compound,
the molecule must possess symmetry.
•
Introduction to C-13 NMR
SirAJ/CMR/MScI 3
4. General
• 13C isotope has a spin I = ½ (is magnetic)
• 1.1% of natural carbon is the 13C isotope
• Due to the low abundance, we do not usually
see 13C-13C coupling
• Chemical shifts are also measured with
respect to tetramethylsilane,(CH3)4Si (i.e. TMS)
• Chemical shift range is normally 0 to 220 ppm
SirAJ/CMR/MScI 4
5. Information by CMRInformation by CMR
• a. The 13C NMR is directly about the carbon skeleton not just the
proton attached to it.
• b. The number of signals tell us how many different carbons or set
of equivalent carbons
• c. The splitting of a signal tells us how many hydrogens are attached
to each carbon. (N+1 rule)
• d. The chemical shift tells us the hybridization (sp3, sp2, sp) of each
carbon.
• e. Integration: Not useful for 13C NMR
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6. Set ofSet of 1313C carbonC carbon
• Like proton we can divide carbon in to
different types of carbons signals
H3C CH3 CH3
CH2
CH3
1 type 2 type 2 type
1 type
2 type 3 type
H3C
O
CH3
O O
3 type
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8. Find total no. of 13C signals
CH3 CH3 CH3
CH3
CH3
CH3
O
H3C
H3C
CH3
O
H3C
H3C
H3C
O
CH3
CH3
CH3
SirAJ/CMR/MScI 8
9. Integration: Not useful forIntegration: Not useful for 1313C NMRC NMR
• Number of signal gives total number of
Carbon present in the molecule.
• Unlike 1H NMR, the area of a peak is not
proportional to the number of carbons giving
rise to the signal. Therefore, integrations are
usually not done.
• If intensity of signal is high then that signal
contain symmetric type of carbon i.e. 2 or
more carbons
SirAJ/CMR/MScI 9
10. SensitivitySensitivity
• The signal for the NMR of a 13 C nucleus is 10-4 times
weaker than the signal for a hydrogen nucleus.
• A signal for a 13C nucleus is only about 1% as intense as
that for 1H because of the magnetic properties of the
nuclei, and at the "natural abundance" level only 1.1% of
all the C atoms in a sample are 13C (most are 12C)
• 13C signals are spread over a much wider range than 1H
signals making it easier to identify and count individual
nuclei
10
11. 1313CC--1313C coupling not observedC coupling not observed
• Coupling between carbons (JCC) is not usually
observed, because two adjacent 13C nuclei
occur in only 1.1% of the carbons.
• It is very less probability to have adjacent
carbon as 13C
• No adjacent carbon as 13C, and sensitivity also
very low so 13C- 13C or Jcc not observed
SirAJ/CMR/MScI 11
12. 1313CC--H coupling may occurH coupling may occur
• When running a spectrum, the protons are usually
coupled from their respective carbons to give a signal
for each carbon atom. This is called a proton-coupled
spectrum.
• The splitting of a signal tells us how many hydrogens
are attached to each carbon. (N+1 rule)
• -CH3 q primary carbon
• -CH2 t Secondary carbon
• -CH- d tertiary carbon
• -C- s quaternary carbon
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13. Type of carbon (N+1)
-CH3 q
-CH2 t
-CH- d
-C- s
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14. Write the Splitting of CMR signal forWrite the Splitting of CMR signal for
each carboneach carbon
O
H3C
H3C
CH3
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18. 1313C Chemical ShiftsC Chemical Shifts
Shift measured inShift measured in ppmppm ((dd) from the carbons of TMS) from the carbons of TMS
SirAJ/CMR/MScI 18
22. Examples (chemical shifts inExamples (chemical shifts in ppmppm from TMS)from TMS)
2323
138138
spsp33 hybridized carbon is more shielded thanhybridized carbon is more shielded than spsp22
SirAJ/CMR/MScI 22
23. Examples (chemical shifts in ppm from TMS)Examples (chemical shifts in ppm from TMS)
OHOH
OO
spsp33 hybridized carbon is more shielded thanhybridized carbon is more shielded than spsp22
6161
202202
SirAJ/CMR/MScI 23
24. Examples (chemical shifts in ppm from TMS)Examples (chemical shifts in ppm from TMS)
OOHH
2323
an electronegative atoman electronegative atom deshieldsdeshields the carbon tothe carbon to
which it is attachedwhich it is attached
6161
SirAJ/CMR/MScI 24
25. Examples (chemical shifts in ppm from TMS)Examples (chemical shifts in ppm from TMS)
OO
138138
an electronegative atom deshields the carbon toan electronegative atom deshields the carbon to
which it is attachedwhich it is attached
202202
SirAJ/CMR/MScI 25
26. Saturated CSaturated C
Type of carbonType of carbon Chemical shift (Chemical shift (dd),),
ppmppm
RCH3 0-35
R2CH2 15-40
R3CH 25-50
R4C 30-40
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27. Type of carbonType of carbon Chemical shift (Chemical shift (dd),),
ppmppm
Type of carbonType of carbon Chemical shift (Chemical shift (dd),),
ppmppm
RCHRCH33 00--3535
CRCR22RR22CC
6565--9090CRCRRCRC
RR22CHCH22 1515--4040
RR33CHCH 2525--5050
RR44CC 3030--4040
100100--150150
110110--175175
SirAJ/CMR/MScI 27
28. C with electronegative atom (O, N, X)C with electronegative atom (O, N, X)
Type of carbonType of carbon Chemical shift (Chemical shift (dd),),
ppmppm
RCHRCH22BrBr 2020--4040
RCHRCH22ClCl 2525--5050
3535--5050RCHRCH22NHNH22
5050--6565RCHRCH22OHOH
RCHRCH22OROR 5050--6565
SirAJ/CMR/MScI 28
29. Type of carbonType of carbon Chemical shift (Chemical shift (dd),),
ppmppm
Type of carbonType of carbon Chemical shift (Chemical shift (dd),),
ppmppm
RCHRCH22BrBr 2020--4040
RCHRCH22ClCl 2525--5050
3535--5050RCHRCH22NHNH22
5050--6565RCHRCH22OHOH
RCHRCH22OROR 5050--6565
RCORRCOR
OO
160160--185185
RCRRCR
OO
190190--220220
Ketones and aldehyde
Conjugated carbonyl (ester, amide etc)
SirAJ/CMR/MScI 29
