PRINCIPLES of FT-NMR & 13C NMR Fourier Transform FOURIER TRANSFORM NMR SPECTROSCOPY THEORY OF FT-NMR  13C NMR SPECTROSCOPY Principle Why C13-NMR is required though we have H1-NMR? CHARACTERISTIC FEATURES OF 13 C NMR  Chemical Shifts NUCLEAR OVERHAUSER ENHANCEMENT Short-Comings of 13C-NMR Spectra

1. Presented by:
Aditya Sharma
M.S. (Pharm)
Pharmaceutical Analysis
NIPER Guwahati
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PRINCIPLES of FT-NMR & 13C NMR

2. Fourier Transform
• It is the mathematical operation in
which the complex waveform can be
broken-down into simple
mathematical operations.
• It is the mathematical operation
required to convert a time domain
spectrum to frequency domain
spectrum (or vice versa).
• Following an adequate S/N ratio,
digital data must be transformed into
the frequency data.
• A computer is essential to solve the
complex equations. 2

3. FOURIER TRANSFORM NMR SPECTROSCOPY
• In FT-NMR instrument, small energy change takes place in the magnitude,
present in NMR and hence the sensitivity of this instrument is very less.
• The sensitivity in FT-NMR can be increased by adding the square root of recorded
spectra's together.
• Simultaneous irradiation of frequency occurs in a spectrum having a Radio
frequency pulse and then the nuclei returns back to thermal equilibrium on its
normal state.
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4. FOURIER TRANSFORM NMR SPECTROSCOPY
• The Fourier Transformation is the basic mathematical calculation
necessary to convert the data in time domain (interferogram) to
frequency domain(NMR Spectrum)
• i.e., time domain- Intensity v/s Time.
• Frequency domain- Intensity v/s Frequency.
• It was developed by JEAN BAPTISE JOSEPH FOURIER.
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5. THEORY OF FT-NMR
• When magnetic nuclei are placed in a magnetic field and irradiated with a pulse
of radio frequency close to their resonant frequency, the nuclei absorb some of
the energy and precis's like little tops at their resonant frequencies.
• This precession of many nuclei at slightly different frequencies produces a
complex signal that decays as the nuclei loses the energy they had gained from
the pulse. This signal is called as free induction decay(FID) or transient, it contains
all the information needed to calculate a spectrum.
• The free induction decay can be recorded by a radio receiver and a computer in
1-2 seconds and many FIDs can be averaged in few minutes. A computer converts
the averaged transients into a spectrum.
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7. THEORY OF FT-NMR (CONTINUED..)
• A Fourier transform is the mathematical technique used to compute the
spectrum from the free induction decay. This technique of using pulses and
collecting transients is called Fourier transform spectroscopy.
• A Fourier transform spectrometer is usually more expensive than a continuous
wave spectrometer, since it must have fairly sophisticated electronics capable of
generating precise pulses and accurately receiving the complicated transients.
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8. 13C NMR SPECTROSCOPY
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9. Introduction
• In all carbon atoms about 1.1% of all carbon atoms are the C-13 isotope; the rest
(apart from tiny amounts of the radioactive C-14) is C-12.Obtaining C-13 spectra
is more complex than for proton NMR. This is primarily because of the low
isotopic abundance of 13C in nature. C-13 NMR relies on the magnetic properties
of the C-13 nuclei.
• Carbon-13 nuclei fall into a class known as "spin ½" nuclei for reasons. The effect
of this is that a C-13 nucleus can behave as a little magnet. C-12 nuclei don't have
this property.
• Proton Decoupled Techniques:
a. Broad Band hydrogen (or proton) Decoupling
b. Off – resonance Decoupling
c. DEPT – NMR 9

10. Principle
• The theory behind NMR comes from the spin of a nucleus and it generates a
magnetic field. Without an external applied magnetic field, the nuclear spins are
random in directions. But when an external magnetic field(Bo), is present the
nuclei align themselves either with or against the field of the external magnet.
• If an external magnetic field is applied, an energy transfer (ΔE) is possible
between ground state to excited state. when the spin returns to its ground state
level, the absorbed radiofrequency energy is emitted at the same frequency level.
The emitted radio frequency signal that give the NMR spectrum of the concerned
nucleus.
• The emitted radio frequency is directly proportional to the strength of the applied
field.
v = γB0/2П
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11. Why C13-NMR is required though we have H1-
NMR?
• 1H nmr spectroscopy - The powerful and useful tool a tool for structural analysis.
• Useless when portions of a molecule lack C-H bonds, no information is
forthcoming.
• Ex: polychlorinated compounds such as chlordane, polycarbonyl compounds such
as croconic acid, and compounds incorporating triple bonds.
• Even when numerous C-H groups are present, an unambiguous interpretation of
a proton nmr spectrum may not be possible.
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12. CHARACTERISTIC FEATURES OF 13 C NMR
• The chemical shift of the CMR is wider (δ is 0-220ppm relative to TMS) in
comparison to PMR(δ is 0-12ppm relative to TMS).
• 13C - 13C coupling is negligible because of low natural abundance of 13C in the
compound. Thus in one type of CMR.
• Spectrum(proton de coupled) each magnetically non equivalent carbon gives a
single sharp peak that does undergo further splitting.
• The area under the peak in CMR spectrum is not necessary to be proportional to
the number of carbon responsible for the signal. Therefore not necessary to
consider the area under ratio.
• Proton coupled spectra the signal for each carbon or a group of magnetically
equivalent carbon is split by proton bonded directly to that carbon & the n+1 rule
is follwed.
• 13C nucleus is about one-fourth the frequency required to observe proton
resonance.
• The chemical shift is greater for 13C atom than for proton due to direct
attachment of the electronegative atom to 13C 12

13. Chemical Shifts
• 13C chemical shifts follow the same
principles as those of 1H, although the
typical range of chemical shifts is
much larger than for 1H (by a factor of
about 20). The chemical shift
reference standard for 13C is the
carbons in tetramethylsilane (TMS),
chemical shift is considered to be 0.0
ppm.
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14. NUCLEAR OVERHAUSER ENHANCEMENT
• NOE effect for heteronuclear nuclei: when one of two different types of atoms
irradiated & NMR spectrum of other type is determined they show change in
absorption intensities of the observed atom, enhancement occurred, is called
nuclear overhauser effect & the degree of increase in the signal is called nuclear
overhauser enhancement.
• Effect can be either positive or negative.
• In 13 C interacting with 1 H effect is positive.
• NOE is enhancement of signals, it add in to original signal strength.
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15. Short-Comings of 13C-NMR Spectra
• Only 1% of the carbon in the molecule is carbon-13,
• Sensitivity is consequently low, and
• Recording the NMR-spectra is a tedious and time consuming process. However,
with the advent of recent developments in NMR-spectroscopy it is quite possible
to eliminate some of these short comings adequately. They are:
a. Development of powerful magnets (‘supercon’ magnets) has ultimately
resulted in relatively stronger NMR-signals from the same number of atoms,
b. Improved hardware in NMR-spectroscopy has gainfully accomplished higher
sensitivity, and
c. Development of more sensitive strategy
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16. References
• R. M. Silverstein, G. C. Bassler and T. C. Morrill (1991). Spectrometric Identification
of Organic Compounds. Wiley.
• Keeler, James (2010). Understanding NMR Spectroscopy (2nd ed.). John Wiley &
Sons. p. 457. ISBN 978-0-470- 74608-0.
• C.O.Wilson and O. Givold, "Text book of Organic Medicinal and pharmaceutical
Chemistry", 5 Medical Publishing Co. LTD, London copy right. by. J. B. Lippin Cott
Company (1966).
• Ashutosh Kar,”Pharmaceutical Drug Analysis’,2’nd edition, New Age International
Pvt.Ltd.Publishers, Page No:-348
• Eberhard Breitmaier & Wolfgang Voelter Carbon 13 nmr Spectroscopy”,3’rd
revised Edition, VCH Publication ,Page N:-12, 107
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