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## Description

CARBON-13 NUCLEAR
MAGNETIC RESONANCE

SPECTROSCOPY:

CARBON-13 NMR

12C is not NMR-active I = 0

however…. 13C does have spin, I = 1/2 (odd mass)

13C signals are 6000 times weaker than 1H because:

1. Natural abundance of 13C is small (1.08% of all C)

2. Magnetic moment of 13C is small

PULSED FT-NMR IS REQUIRED

The chemical shift range is larger than for protons

0 – 200 ppm

For a given field strength 13C has its resonance at a
different (lower) frequency than 1H.

Divide the hydrogen
1H frequency by 4 (approximately)

for carbon-13
1.41 T 60 MHz

2.35 T 100 MHz

7.05 T 300 MHz 13C
1.41 T 15.1 MHz

2.35 T 25.0 MHz

7.05 T 75.0 MHz

SALIENT FACTS ABOUT 13C NMR (cont)

Because of its low natural abundance (0.0108) there
is a low probability of finding two 13C atoms next to
each other in a single molecule.

not probable
13C – 13C coupling NO!

Spectra are determined by many molecules contributing
to the spectrum, each having only one 13C atom.

However, 13C does couple to hydrogen atoms (I = 1/2)

very common
13C – 1H coupling YES!

COUPLING TO ATTACHED PROTONS

COUPLING TO ATTACHED PROTONS

3 protons 2 protons 1 proton 0 protons

H H
13 13 13 13

C H C H C H C

H
n+1 = 4 n+1 = 3 n+1 = 2 n+1 = 1

Methyl Methylene Methine Quaternary
carbon carbon carbon carbon

The effect of attached protons on 13C resonances

( n+1 rule applies ) (J’s are large ~ 100 – 200 Hz)

FTNMR
 Provides simultaneous and instantaneous

recording of the whole spectrum in the
nuclear magnetic resonance.

 Here the sample is subjected to a high power
short duration pulse of radio frequency
frequency.

All the spin active nuclei resonate all at once at
these larmour frequency

When the pulse is withdrawn all the nuclei relaxed
to lower energy level.

This signal is called FID. It is a time domain spectra
FID signals can be added together by giving

multiple RF pulses and Fourier Transform(FT)
convert this into frequency domain spectra

FT is a computer program which convert all time
domain spectra into frequency domain spectra

.

magnetic
moment

.

Free Induction Decay of an NMR signal in a 1D experiment with a 50 msec
expansion showing the digitization of the signal.

A 90 degree pulse followed by turning on the detector
leads to a Free Induction Decay (FID)

(J) and altered The signal decays
frequency

Different decay rates can be seen

13C NMR FID of Methyl a-D-Arabinofuranoside in CD3CN.
Collected at 11.7 T by Jim Rocca in AMRIS.

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 sec

Fourier Transform of previous FID
Some peaks are bigger
than others (CD3CN)

frequencies. These are called
chemical shifts. This peak has other

stuff going on.
0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 sec

120 110 100 90 80 70 60 50 40 30 20 10 ppm

13C NMR

The principle behind 13C NMR is same as
that of proton NMR.

13C NMR spectra gives direct

13C-NMR SPECTRA IS LESS SENSITIVE

Isotope – 12C,13C

Magnetically active – 13C

Nuclear spin – I = ½

Abundance _ 1.1%

Gamma value _ ¼ of H

Number of signals
The number of signals indicates how many kinds
of carbon are there in a molecule.

Equivalent carbons

In the same chemical environment

Non equivalent carbons
In different chemical environment

Spin-Spin Decoupling

The process of removing the spin-spin
splitting between the spins.

Three types

 Off resonance decoupling
 Gated decoupling

Most commonly used one
All protons are decoupled from the carbons
A single sharp peak is obtained for each

chemically non equivalent carbons
Here a broad band frequency is used to

Proton-Decoupled 13C NMR of
2-Butanol

Off resonance decoupling

Here the coupling due to protons directly
attached to carbon are observed but other
couplings are removed.

13C signals will split into a multiplet of n+1
component peaks, where n is the no: of protons
directly attached to the carbon.

The spectrum will give an idea about how
many protons are attached to each
carbon.

C Singlet

CH
Doublet

CH2
Triplet

Quartet
CH3

This is achieved by irradiating the sample
at a frequency close to but not coinciding
with the resonance frequency of protons.

Thus the coupling due to the directly
attached protons are retained

Proton-Coupled 13C NMR of 2-Butanol

Types of Proton Decoupling in 13C NMR

Decoupling

Off-Resonance
Decoupling

Source: Skoog, Holler, and Nieman, Principles of Instrumental Analysis, 5th edition, Saunders College Publishing.

13C-NMR Spectroscopy

• In a proton-decoupled mode, a sample is
– one to excite all 13C nuclei.
– a second broad spectrum of frequencies to cause all

protons in the molecule to undergo rapid transitions
between their nuclear spin states.

• On the time scale of a 13C-NMR spectrum, each
proton 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.

NUCLEAR OVERHAUSER EFFECT

NOE with reference to 13C

First described by Overhauser in 1953

It involves the saturation of one signal in
the spectrum and observation of the
changes in the intensities of other signals

• Decoupling increases the sensitivity of
NMR

• The intensities of the multiplets lines are
accumulated in one single signal

• The intensity of the 13C signal often
increases much more than expected. This

The irradiation of proton results in
equalisation of the proton spin state
populations.

This in turn causes changes in the normal
population of the 13C states with an
increased fraction in the lower energy state,
thus increasing the intensity of 13C lines

It turns out that the major relaxation route for 13C
involves dipolar transfer of its excitation energy to
the proton directly attached to it.

NOE decreases in the order
CH3 > CH2 > CH

Non proton containing carbon have long T1 so
gives weak signals

NOE is welcome for obtaining qualitative CMR
spectra.

However, the peak areas are no longer
proportional to the concentration of the specific
nuclei.

1H and 13C NMR compared:

Both give us information about the number
of chemically non-equivalent nuclei (non-
equivalent hydrogens or non-equivalent
carbons)

Both give us information about the
environment of the nuclei (hybridization
state, attached atoms, etc.)

1H and 13C NMR compared:

The signal for the NMR of a 13 C nucleus is 10-4

times weaker than the signal for a hydrogen
nucleus.

A signal for a 13C nucleus is only about 1% as
intense as that for 1H because of the magnetic
properties of the nuclei, and

At the “natural abundance” level only 1.1% of all
the C atoms in a sample are 13C (most are 12C)

1H and 13C NMR compared:

13C signals are spread over a much wider range
than 1H signals making it easier to identify and
count individual nuclei

Check the spectra on the next slides:

(a) Shows the 1H NMR spectrum of 1-
chloropentane;

(b) Shows the 13C spectrum. It is much easier to
identify the compound as 1-chloropentane by its
13C spectrum than by its 1H spectrum.

1-Chloropentane 1H

ClCH2 CH3

ClCH2CH2CH2CH2CH3

10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0

Chemical shift (d, ppm)

Note: in these spectra the peak intensities 13
are not exactly proportional to the C

number of carbon atoms.

ClCH2CH2CH2CH2CH3

A separate, distinct
peak appears for
each of the 5
carbons CDCl3

200 180 160 140 120 100 80 60 40 20 0

Chemical shift (d, ppm)

13C Chemical Shifts

These are measured in ppm (d)

from the carbons of TMS

Chemical Shift – 13C-NMR

Type of Chemical Type of Chemical
Carbon Shift (d) Carbon Shift (d)

RCH3 10-40

RCH2R 15-55 C R 110-160

R3CH 20-60
O

RCH2 I 0-40
RCOR 160 – 180

RCH2Br 25-65
O

RCH2Cl 35-80 RCNR2 165 – 180
R3COH 40-80 O
R3COR 40-80 RCCOH 165 – 185

RC CR 65-85 O O

R 100-150 RCH, RCR 180 – 215
2C=CR2

Interpreting NMR Spectra
• Alkanes –

– 1H-NMR signals appear in the range of d 0.8-1.7.
– 13C-NMR signals appear in the considerably wider range

of d 10-60.

• Alkenes –
– 1H-NMR signals appear in the range d 4.6-5.7.
– 1H-NMR coupling constants are generally larger for

trans-vinylic hydrogens (J= 11-18 Hz) compared with cis-
vinylic hydrogens (J= 5-10 Hz).

– 13C-NMR signals for sp2 hybridized carbons appear in the
range d 100-160, which is to higher frequency from the
signals of sp3 hybridized carbons.

Interpreting NMR Spectra
• Alcohols –
• 1H-NMR O-H chemical shift often appears in the range d 3.0-4.0,

but may be as low as d 0.5.
– 1H-NMR chemical shifts of hydrogens on the carbon bearing

the -OH group are deshielded by the electron-withdrawing
inductive effect of the oxygen and appear in the range d 3.0-
4.0.

– 13C-NMR signals appear d 50-65.

• Ethers –
– A distinctive feature in the 1H-NMR spectra of ethers is the

chemical shift, d 3.3-4.0, of hydrogens on the carbons
bonded to the ether oxygen.

– 13C-NMR signals appear d 40-70.

Interpreting NMR Spectra

• Aldehydes and ketones –
– 1H-NMR: aldehyde hydrogens appear at d 9.5-10.1.
– 1H-NMR: a-hydrogens of aldehydes and ketones appear

at d 2.2-2.6.
– 13C-NMR: carbonyl carbons appear at d 180-215.

• Amines –
– 1H-NMR: amine hydrogens appear at d 0.5-5.0

depending on conditions.
– 13C-NMR signals appear d 35-50.

Interpreting NMR Spectra
• Carboxylic acids –

– 1H-NMR: carboxyl hydrogens appear at d 10-13, higher
than most other types of hydrogens.

– 13C-NMR: carboxyl carbons in acids and esters appear at
d 160-180.

13C Chemical shifts are most affected by:

• Hybridization state of carbon.
• Electronegativity of groups attached to carbon.

Examples (Chemical shifts in ppm from TMS)

sp3 d-23 ppm sp2 d-138 ppm

sp3 hybridized carbon is more shielded than sp2

Examples (chemical shifts in ppm from TMS)

OH O

sp3 d -61 ppm sp2 d -202 ppm

sp3 hybridized carbon is more shielded than sp2

Table

Type of carbon Chemical shift (d),
ppm

RCH3 0-35

R2CH2 15-40

R3CH 25-50

R4C 30-40

Table

Type of carbon Chemical shift (d), Type of carbon Chemical shift (d),
ppm ppm

RCH2Br 20-40 O

RCH RCOR 160-185
2Cl 25-50

RCH2NH2 35-50 O

RCH2OH 50-65 RCR 190-220

RCH2OR 40-70

The DEPT Method

• The DEPT methods uses a complex series of pulses
in both the 1H and 13C ranges, with the result that
CH3, CH2, and CH signals exhibit different phases;
– signals for CH3 and CH carbons are recorded as positive

signals.
– signals for CH2 carbons are recorded as negative signals.
– quaternary carbons give no signal in the DEPT method.