CARBON-13 NMR
Presented by
Dr.A.Suneetha
Dept of Pharm. Analysis
Hindu College of Pharmacy
Molecular Spectroscopy
Nuclear
Magnetic
Resonance
(NMR)
Spectroscopy: A Spectroscopic Technique that gives us
information about the number and types of atoms in a
molecule.
For ex,About the Number and Types of:
1)Hydrogen Atoms Using 1H-NMR Spectroscopy.
2)Carbon Atoms Using 13C-NMR Spectroscopy.
3)Phosphorus Atoms Using 31P-NMR Spectroscopy.
Nuclear Magnetic Resonance

If the Precessing nucleus is irradiated with
Electromagnetic Radiation of the same frequency
as the rate of Precession,
1. The Two Frequencies Couple.
2. Energy is Absorbed.
3. The Nuclear Spin is Flipped from Spin State +1/2
(with the Applied Field) to -1/2 (against the Applied
Field).
Nuclear Magnetic Resonance/NMR
(a) Precession
(b) Absorption of Electromagnetic Radiation.
Nuclear Magnetic Resonance/NMR

Resonance: In NMR spectroscopy, resonance is the
absorption of energy by a precessing nucleus and the
resulting “flip” of its nuclear spin from a lower energy
state to a higher energy state.
◦ Signal: A recording in an NMR spectrum of a
nuclear magnetic resonance.
Introduction
Carbon-13 NMR was first studied in 1957,but
its widespread use did not begin until the early 1970’s.
Carbon isotopes
12C: 98.9%
13C: 1.1% natural abundance
14C: trace
Low abundance overcome by signal averaging and
Fourier transform NMR
Increases sensitivity and speed of operation
Carbon-13 NMR has several advantages over proton
NMR in terms of its power to elucidate organic and
biochemical structures.
In addition,the chemical shift range for 13c for most
organic compounds is about 200ppm,compared with
10-15ppm for the proton.
Homonuclear spin-spin coupling between carbon atom
is not observed because in natural-abundance samples
the probability of two13c occuring adjacent to each
other is small.
Heteronuclear spin coupling between13c and12c
does not occur because the spin quantum number
of 12c is zero.
So ,the number of excellent methods are there for
decouple the interaction between 13c atoms and
protons.
PROTON DECOUPLING:3 types of decoupling.They are:
1.Broadband decoupling.
2.Off-resonance decoupling.
3.pulsed or gated decoupling.
SPECIFICATIONS OF
• 12C is
13C
NMR
the most abundant natural isotope of carbon, but has
a nuclear spin I = 0, rendering it unobservable by NMR.
•
Limited to the observation of the 13C nucleus which
constitutes only 1.1% of naturally occurring carbon.
Fourier Transform NMR





Radio-frequency pulse given.
Nuclei absorb energy and process (spin) like little tops.
A complex signal is produced, then decays as the nuclei lose
energy.
Free induction decay is converted to spectrum.
Low 13C abundance
13C
 13C
Shielding
Spectra are typically recorded from 0 – 220 ppm; with the
zero being the methyl carbon in TMS (much wider range than 1H
spectra!)
 13C
Nuclei are shielded or deshielded (CHEMICAL SHIFT) due
to the same factors as for 1H NMR.
1. Electron withdrawing ability (by inductance or resonance) of
nearby groups.
2. Hybridization.
3. Electron current effects.
13C
NMR Chemical Shifts
• Magnetic induction in
the p bonds of a carboncarbon triple bond
shields an acetylenic
hydrogen & shifts its
signal lower frequency.
Magnetic induction in
the p bond of a
carbon-carbon double
bond deshields vinylic
hydrogen & shifts
their signal higher
frequency.
13C

NMR Chemical Shifts
The magnetic field induced by circulation of p
electrons in an aromatic ring deshields the
hydrogen on the ring and shifts their signal to
higher frequency.
Effect of Decoupling
Nuclear Overhauser effect
In magnetic resonance spectroscopy, the transfer of spin polarization from
one spin population to another via cross-relaxation is generally called the
Overhauser Effect, after American physicist Albert Overhauser who
hypothesized.
The original Overhauser effect was described in terms of polarization transfer
between electron and nuclear spins, but is now mostly used for transfer
between nuclear spins—the Nuclear Overhauser Effect (NOE or nOe).
A very common application is NOESY (Nuclear Overhauser Effect
Spectroscopy), an NMR technique for structure determination of
macromolecular motifs (see also 2D-FT NMRI and Spectroscopy).
NOE differs from in the respect that NOE is observed through space, not
through bonds. Thus, all atoms that are in close proximity to each other give
a NOE, whereas spin coupling is observed only when the atoms are bonded
to same or neighboring atoms.
Furthermore, the distance can be derived from the observed NOEs, so that
the precise, three-dimensional structure of the molecule can be
reconstructed.
1H
–
13C
Splitting
The splitting follows the simple N+1 rule:
13
C
13
C
H
H
C
H
H
C H
H
13
13
Quaternary
Methine
Methylene
Quaternary
Singlet
Doublet
Triplet
Quartet
The multiplet analysis gives useful information, but there are two
major limitations:
1) If the 13C signal is weak (common) the outer peaks of the
multiplet may be lost in the noise of the spectrum.
2) Due to the large J-constants, the multiplets quickly begin to
overlap and become congested.
Main Difference Between CARBON NMR (C-13 NMR) &
PROTON NMR(H-NMR)
Carbon
NMR shows the number of unique carbon environments in a
compound, the peaks are not quantitative.
H NMR shows the number of protons and how each proton is interacting
(coupling) with other protons.
Carbon
NMR spectra are relatively simple, with one line per carbon
atom.
H NMR spectra are very complex, and each peak can be split into things
like a doublet of doublets, etc. The peak can be integrated to show how
many protons it represents. If a peak integral has a value of 2, it means
that there are 2 protons in identical chemical environments.
Some Of The General Applications Of C13 NMR Are As Follows:
1)
Medicine.
2)
Chemistry.
3)
Non-destructive testing.
4)
Acquisition of dynamic information.
5)
Data acquisition in the petroleum industry.
6)
Flow probes for NMR spectroscopy.
7)
Process control.
8)
Earth's field.
9)
NMR Quantum computing.
10)Magnetometers.
Applications Of C13NMR
1) Solution
structure The only method for atomic-resolution
structure determination of bio-macromolecules in aqueous
solutions under near physiological conditions or membrane
mimeric environments.
2) Molecular
dynamics The most powerful technique for
quantifying motional properties of biomacromolecules.
3) Protein
folding The most powerful tool for determining
the residual structures of unfolded proteins and the
structures of folding intermediates.
4)Ionization state The most powerful tool for determining the
chemical properties of functional groups in biomacromolecules,
such as the ionization states of ionizable groups at the active
sites of enzymes.
5)Weak intermolecular interactions Allowing weak functional
interactions
between
macrobiomolecules
(e.g.,
those
with
dissociation constants in the micro molar to mill molar range) to
be studied, which is not possible with other technologies.
6)Protein hydration A power tool for the detection of interior
water and its interaction with biomacromolecules.
7)Hydrogen bonding A unique technique for the DIRECT detection of
hydrogen bonding interactions.
8)Drug screening and design Particularly useful for identifying drug leads and
determining the conformations of the compounds bound to enzymes, receptors,
and other proteins.
9)Native membrane protein Solid state NMR has the potential for determining
atomic-resolution structures of domains of membrane proteins in their native
membrane environments, including those with bound ligands.
10)Metabolite analysis A very powerful technology for metabolite analysis.
11)Chemical analysis A matured technique for chemical identification and
conformational analysis of chemicals whether synthetic or natural.
12)Material science A powerful tool in the research of polymer chemistry and
physics.
REFERENCES:
 Introduction To SPECTROSCOPY, Donald L.Pavia, Gary M.Lampman &
George S.Krit, Third Edition, Harcourt College Publishers, Pg.No: 167-194.
 Instrumental Methods Of Chemical Analysis, Gurdeep R. Chatwal, Shamk.
Anand, Himalaya publishing house, Pg.No: 2.231-2.234.
 Organic SPECTROSCOPY, Y.R. Sharma, S.Chand & Company Ltd, Pg.No:
232-238.
 Websites:
 www.ionchannels.org
 http://www.springerprotocols.com
 http://www.nrcresearchpress.com