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. Chemical Shift
Carbon-13 chemical shifts are most affected by
Hybridiasation state of carbon & Electronegative group attached to carbon
4. 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
hydrogens 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. 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Use of off-resonance decoupled spectra has been replaced
by use of DEPT 13C NMR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. DEPT 13C NMR Spectroscopy
Distortionless Enhancement by Polarization
Transfer (DEPT-NMR) experiment
Run in three stages
9. Example: 6methylhept-5-en-2-ol
(a) Ordinary broadband-decoupled
spectrum showing signals for all
eight of 6-methylhept-5-en-2-ol
(b) DEPT-90 spectrum showing
signals only for the two C-H
carbons
(c) DEPT-135 spectrum showing
positive signals for the two CH
carbons and the three CH3
carbons and negative signals for
the two CH2 carbons
10. 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
The off diagonal peaks which are termed as cross peaks provide useful
information. The presence of a cross-peak normally indicates that the
protons giving the connected resonance on the diagonal are geminally
or vicinally coupled. Long range couplings normally do not give
significant cross-peak. However, there may exceptions when long
range coupling are large.
12. 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. 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.
