Carbon-13 NMR Spectroscopy& 2D NMR Methods

1. Presented By: Guided By:
Madhavanand Ingalageri Dr. A R Saundane
MSc IV-Sem Professor in Chemistry
Gulbarga University
Kalaburagi

2. Introduction
 Nuclear magnetic resonance concern the magnetic properties of
certain atomic nuclei. It concerns the atoms having spin quantum
number.
 12C nucleus is not magnetically active the spin number I being zero. But
13C Have I = ½
 13C account for only 1.1% of naturally occurring carbon 13C- 13C coupling
is negligible and not observed.
 The gyromagnetic ratio of 13C is one-fourth of that of 1H.
 Each nonequivalent 13C gives a different signal.
 A 13C signal is split by the 1H bonded to it according to the (n + 1) rule.
 The most common mode of operation of a 13C-NMR spectrometer is a
hydrogen-decoupled mode.

3. Principle of NMR
 When energy in the form of radio frequency applied.
 When applied force is equal to precessional frequency.
 Absorption of energy occurs.
 Nucleus is in the resonance.
 NMR signal is recorded.
Why Carbon-13 NMR required?
 Carbon NMR can used to determine the number of non-equivalent
carbons and to identify the types of carbon atoms(methyl, methylene,
aromatic, carbonyl….) which may present in compound.
 13C signals are spread over a much wider range than 1H signals making
it easier to identify & count individual nuclei.

4. Chemical Shift
Carbon-13 chemical shifts are most affected by,
 Hybridiasation state of carbon & Electronegative group attached to carbon.

5. Hydrogen Decoupled mode(Broad Band
Decoupled)
 A sample is irradiated with two different radio frequencies.
 One to excite all 13C nuclei.
 A second broad spectrum of frequencies to cause all
hydrogen's in the molecule to undergo rapid transitions
between their nuclear spin states.
 On the time scale of a 13C-NMR spectrum, each hydrogen is in
an average or effectively constant nuclear spin state, with the
result that 1H-13C spin-spin interactions are not observed; they
are decoupled.
 Thus, each different kind of carbon gives a single, unsplit peak.

6. ETHYL PHENYLACETATE
13C coupled
to the hydrogens
13C decoupled
from the hydrogens
in some cases
the peaks of the
multiplets will
overlap
this is an
easier spectrum
to interpret

7. Off-Resonance Decoupling
 Off-Resonance decoupling simplifies the spectrum by
allowing some of the splitting information to be retained.
 In this technique only the 13C nuclei are split by the protons
directly bounded to them and not by any other protons i.e.,
one observes only one bond coupling 13C -1H
 The coupling between each carbon atom and each
hydrogen attached directly to it, s observed acc to n+1 rule.
 Use of off-resonance decoupled spectra has been replaced
by use of DEPT 13C NMR

8. Example: Propanol

9. DEPT 13C NMR Spectroscopy
 Distortionless Enhancement by Polarization
Transfer (DEPT-NMR) experiment
 Run in three stages

10. Example: 6methylhept-5-en-2-ol
(a) Ordinary broadband-decoupled
spectrum showing signals for all
eight of 6-methylhept-5-en-2-ol
(a) DEPT-90 spectrum showing
signals only for the two C-H
carbons.
(b) DEPT-135 spectrum showing
positive signals for the two CH
carbons and the three CH3
carbons and negative signals for
the two CH2 carbons.

11. COSY Spectrum
 2D NMR spectra have two frequency axes and one intensity
 The common 2D spectra are 1 H -1H shift correlations
known as COSY Spectrum.
 COSY identifies pair of protons which are coupled to each
other.
 The compound is identified using a contour plot
 One dimensional counterpart of a given peak on the
diagonal lies directly below that peak on each axis
 The presence of cross peak normally indicates protons
giving the connected resonance on the diagonal are
geminaly or vicinally coupled.

12. COSY Spectrum of m-dinitrobenzene

13. HECTOR Spectrum
 2D-NMR spectra that displays 13C – 1H-NMR shift correlations are called HETCOR
spectra. It shows coupling between protons and the carbon to which they are
attached. The HETCOR spectrum of 1-chloro-2propanol is shown in the fig.
 The 2-D spectrum is composed only of cross-peaks, each one relating carbon to its
directly bonded proton(s).
 The methyl doublet of 1H-NMR spectrum appears at δ 1.2 when drawn cross-peak
and then dropped down to the 13C spectrum axis indicates that the 13C peak at δ 20
is produced by the methyl carbon of 1-chloro-2-propanol(C-3)
 The 1H -NMR signal at δ 3.9 is due to CH-OH (C-2 proton) tracing out to the
correlation peak and down to the 13C spectrum shows the 13C NMR signal at 67
arises from the C-2 carbon of the compound i.e., the carbon carrying the hydroxyl
group.
 The 1H-NMR peaks at δ 3.4-3.5 for the two protons on the carbon bearing the
chlorine, the interpretation leads us to the cross-peak and down to the 13C peak at δ
51

14. Hector spectrum of 1-chloro-2-propanol

15. Applications of 13C NMR
 CMR is a noninvasive and nondestructive
method,i.e,especially used in repetitive In-vivo analysis of
the sample without harming the tissues .
 CMR, chemical shift range(0-240 ppm) is wider compared
to H-NMR(0-14 ppm), which permits easy separation and
identification of chemically closely related metabolites.
 C-13 enrichment, which the signal intensities and helps in
tracing the cellular metabolism.
 CMR technique is used for quantification of drugs purity to
determination of the composition of high molecular weight
synthetic polymers.

16. THANK YOU…