K.L.E.S’s College of Pharmacy, BELAGAVI.
PRINCIPLE OF 13
C NMR SPECTROSCOPY
DIFFICULTIES ENCOUNTERED IN 13
C CHEMICAL SHIFT
The first NMR observation regarding 13
were reported in 1957.
The experiment concluded that the direct
observation of carbon nuclei had greater utility over
the equivalent protons studies.
The study of carbon nuclei through magnetic
resonance spectroscopy is important technique for
determining the structure of organic
compounds,using it with proton NMR and IR
spectroscopy organic chemist can often determine
the complete structure of unknown compounds.
Thus, carbon NMR provides direct information
about the carbon skeleton of the molecule.
C NMR ( CMR) Proton NMR ( PMR)
It is study of spin changes of carbon
It is study of spin changes of proton
Chemical shift range is 0-240 ppm. Chemical shift range is 0-14 ppm.
Fourier transform Technique is used. Continuous wave method is used
Very fast process.
Gyromagnetic ratio is 1.4043
Gyromagnetic ratio is 5.5854
Coupling constant range is 125-250Hz. Coupling constant range is 0-15Hz.
Solvent peak is observed. Solvent peak is not observed.
Area under the peak is not considered. Area under the peak is considered
TMS peak is quartet. TMS peak is singlet.
Effect of substitute on adjacent carbon
atom cannot varies chemical shift.
Effect of substituent on adjacent carbon
atom can varies chemical shift.
Any nucleus with odd mass number spins on its own axis
By the application of an external magnetic field (Ho), the
nucleus spins on its own axis and a magnetic moment is created,.
In this ground state the magnetic field caused by a spin of nuclei
is aligned with external magnetic field.
When the energy in the form of radio frequency is applied and
when applied frequency is equal to processional frequency,
absorption of energy occurs and NMR signal is recorded.
Because of absorption of energy, the nucleus moves from
ground state to excited state, which results in spin reversal or
anti-parallel orientation in which magnetic field caused by the spin
of nucleus opposes the external applied magnetic field.
Nuclear SpinNuclear Spin
A spinning charge, such as the nucleus of 1
C, generates a magnetic field. The
magnetic field generated by a nucleus of spin
+1/2 is opposite in direction from that
generated by a nucleus of spin –1/2.
The distribution of
nuclear spins is
random in the
absence of an
An external magnetic field
causes nuclear magnetic
moments to align parallel
and antiparallel to applied
Energy Differences Between Nuclear Spin StatesEnergy Differences Between Nuclear Spin States
no difference in absence of magnetic field
proportional to strength of external magnetic field
∆∆EE ∆∆EE ''
increasing field strengthincreasing field strength
A Modern NMR Instrument
IMPORTANCE OF 13
CMR is nondestructive and noninvasive method.
CMR of biological materials allows for the
assessment of the metabolism of carbon.
CMR, chemical shift range is wider than PMR.
The low natural abundance of 13
C nuclei (1.1%)
can be made use of tagging the specific carbon
position by selective 13
C nucleus is a stable isotope, hence not
subjected to dangers related to radiotracers.
Homo nuclear coupling of 13
C provides novel
DIFFICULTIES ENCOUNTERED IN 13
C nucleus is magnetically active
and which is similar to 1
Recording of CMR nucleus is difficult
due to the following reasons:
1. Natural abundance.
2. Gyro magnetic ratio.
3. Coupling phenomenon.
1. Natural abundance:
The most abundant isotope of carbon 12
C is not detected
by NMR, as it is magnetically inactive (I=0).
The low natural abundant isotope 13
C is magnetically
As a result of the natural abundance of 13
C is 1.1% , the
sensitivity of 13
C nuclei is only 1.6% that of 1
The availability of FT instrumentation enhances the
sensitivity of 13
2. Gyro magnetic ratio:
The gyro magnetic ratio of 13
C is 1.4043 as
compared to 5.5854 of a proton.
C nucleus resonance frequency is only 1/4th
PMR at a given magnetic field.
Thus, CMR is less sensitive than PMR
Sensitivity of CMR can be increased by adopting
3. Coupling phenomenon:
C and 1
H have I =0, so that we expect coupling in the spectrum
C - 13
C and 13
C - 1
The probability of two 13
C nuclei adjacent to each other in the same
molecule is extremely rare due to low natural abundance of 13
So that 13
C coupling will not usually exist. However the 13
C - 1
coupling have observed in CMR spectrum.
As a result of coupling makes the 13
C spectrum extremely complex ,
consequently there is an overlap of multiplets.
C - 1
H coupling can be eliminated by adopting following
a) FT technique
b) Decoupling technique
c) Nuclear overhauser phenomenon for enrichment of the carbon
Earlier, the continuous wave method is used to record
C spectra but it is slow procedure, require large sample and
for assessing takes long time .FT technique increases activity
FT technique permits simultaneous irradiation of all 13
In this method sample is irradiated with a strong pulse of
radio frequency radiation in desired range at once in a fixed
magnetic field .
1)The scanning takes place rapidly
compared to continuous wave NMR.
2)The sensitivity problems are eliminated in NMR,
therefore which helps in
a) Analyses the sample at low conc.
b) NMR studies on nuclei with low
abundance and with
low gyro magnetic ratio.
Generally the probability occurrence of 13
coupling is rare , but the 13
C - 1
H coupling can usually
observed . The problem of 13
C - 1
H coupling can be
eliminated by decoupling the proton from carbon .
Types of decoupling in CMR
1) Proton decoupling or noise decoupling .
2) Coherent and broadband decoupling .
3) Off resonance decoupling .
1) Proton decoupling or noise decoupling:
The proton decoupled CMR spectrum can be
recorded by irradiating the sample at two
The first radio frequency signal is used to affect
carbon magnetic resonance, while simultaneous
exposure to second signal causes all the protons to
be resonance at the same time they spin or flip very
As they flip so fast, there is no coupling and each
carbon appears as a single unsplit peak at
corresponding chemical shift range.
Ex: Proton decoupled spectra of sec-butyl bromide.
2) Broadband decoupling:
In this technique, all the proton resonance can be
reduced and to get sharp CMR spectral peaks,
each directly reflecting a 13
C chemical shift.
The NMR spectrum of nucleus A is split by
nucleus B, because A can see B in different
Off resonance decoupling
1000-2000 Hz above the spectral region
In this primary carbon nuclei (bearing three
protons) yield a quartet of peaks, secondary
carbons give three peaks, tertiary carbon nuclei
appear as doublets, and quaternary carbons
exhibit a single peak.
This division gives a number independent
of the instrument used.
THE CHEMICAL SHIFTTHE CHEMICAL SHIFT
The shifts from TMS in Hz are bigger in higher field
instruments (300 MHz, 500 MHz) than they are in the
lower field instruments (100 MHz, 60 MHz).
We can adjust the shift to a field-independent value,
the “chemical shift” in the following way:
A particular proton in a given molecule will always come
at the same chemical shift (constant value).
= δ =
shift in Hz
spectrometer frequency in MHz
Chemical shift (Chemical shift (δδ, ppm), ppm)
measured relative to TMSmeasured relative to TMS
Increased shieldingIncreased shielding
Decreased shieldingDecreased shielding
(CH(CH33))44Si (TMS)Si (TMS)
Applications of 13
Metabolic studies on human
1. Brain function
2. Glucose metabolism in liver
3. Glucose metabolism in muscle
4. Determination of degree of unsaturation of
fatty acids in adipose tissue
5. Characteristic of body fluids and isolated
6. In diseased state
Industrial applications in solids
A.The differences in the applied magnetic field strength (Angular
Frequency of Precession) at which the various proton configurations
in a molecule Resonate is extremely small.
A.The differences amount to only a few parts per million in the
Magnetic field strength.
A.It is difficult to measure the precise field strength to less than a
part per million.
A.Measuring the difference between absorption positions is much
easier using the difference between the Resonance of the sample and
the Resonance of a standard reference sample.
The Chemical Shift
The Chemical Shift (Con’t)
E. The actual procedure measures the difference between nuclei
resonance energies relative to the universally accepted
standard - Tetramethylsilane (CH3)4Si (TMS).
All protons in TMS have chemically and electronically
They are highly shielded - nothing about them diminishes
the electron density.
They resonate at same field strength, i.e., a single reference
signal is produced.
The proton resonances in Tetramethylsilane (TMS) appear
at a higher magnetic field strength than proton resonances
in most all other molecules.
Chemical Shift (δ) = 0 ppm by Definition
All protons in TMS are in chemically equivalent environments
The protons are in regions of high electron density
Silicon is less electronegative than Carbon
Proton resonances appear at a higher field strength than proton
resonances in most all other molecules
One signal is produced
Small amount produces large signal
Reasons to use TMS as internal Standard
H C H
H C H
Observed Shift from TMS (Hz) Hz
Chemical Shift (δ) = = = PPM60 MHz MHz
The Chemical Shift on (Con’t)
F. Thus, the protons in compounds of interest to the organic
chemist resonate at frequencies greater than the protons in
TMS, i.e., at lower magnetic field strengths.
G. A parameter called the “Chemical Shift (δ)” has been
defined to give the position of the absorption of a proton a
H. The Chemical Shift values are reported in units of
“Parts Per Million” (ppm).
The Chemical Shift (Con’t)
A. A NMR spectrometer increases the Magnetic field
strength as the pen moves from left to right on the chart.
B. The TMS absorption is higher than will be obtained for
just about all organic compounds.
C. Thus, the absorption signal for TMS appears at the far
right side of the chart.
D. The Chemical Shift for TMS is arbitrarily set at “0”
E. By convention, the Chemical Shift values increase from
right to left, with a range of 0 (TMS) to about 13 on the
far left of the chart.
F. In other words: Chemical Shift values decrease with
increasing Magnetic field strength!
OH does not have
C-NMR OH signal