Outcomes
• State that NMR spectroscopy involves interaction
of materials with low-energy radio-frequency
radiation.
• State the need for deuterated solvents.
• Describe the use of TMS (tetramethylsilane) as the
standard for chemical shift measurements.
• Analyse carbon-13 NMR spectra to make
predictions about the different types of carbon
atoms present.
• Predict the chemical shifts of carbons within a
given molecule.
Carbon-13 NMR
Spectroscopy
12C
atoms do not have nuclear spin, but 1.1% of carbon atoms
are 13C and these do have nuclear spin and so produce NMR
spectra.
NMR spectra give a lot of valuable information about the
chemical environment of C atoms (e.g. the difference between
C atoms in C=O, C-N, C≡N, C-C, C=C, etc.).
Samples carried out in solution – to prevent the C/H atoms in the
solvent giving a signal Deuterium (isotope of H) is used as it has
an even number of nucleons so will produce no NMR signal.
NMR Spectrometry: Spin states
Neutrons & Protons possess spin – this makes them behave like tiny
magnets. They will match or oppose an external field.
Energy
Both
Protons
Proton
B
S
Proton A
S
Proton B
N
N
Magnetic field
DE
Proton A
Nuclei that
oppose the
magnetic field
have a higher
energy level
than those
aligned with
the field.
Radio waves are required to ‘flip’ the nuclei so that all nuclei are no longer
aligned with the magnetic field.
NMR Spectrometry: Excitation & Relaxation
A low-energy nuclei (aligned with the applied field) will jump to a high energy
spin state (opposing the applied field) when given a pulse of RF. (Excitation)
Against
field
RF signal coil
With
Induced
signal
field
detector coil
can be the same coil!
When the magnetic field is removed, the nuclei revert back to their original
state releasing the energy that was just given to them in the form of
radiation. (Relaxation)
The cycle of excitation and relaxation of the nucleus is called resonance –
hence the name NMR...nuclear magnetic resonance.
Chemical shift
TMS
CH3
CH3
Si
CH3
= 0ppm
CH3
10
8
6
4
2
0
Chemical shift δ (ppm)
• The chemical shift (δ) is measured relative to TMS
• TMS is chemically unreactive and is very volatile so is easily
removed from the sample after running the NMR spectrum.
The carbon in the CH3
group is attached to 3
hydrogens and a carbon.
The carbon in the CH2 group
is attached to 2 hydrogens, a
carbon and an oxygen.
But which is which?
A table of typical chemical shifts in C-13 NMR spectra
carbon
environment
chemical shift
(ppm)
C=O (in ketones)
205 - 220
C=O (in aldehydes)
190 - 200
C=O (in acids and esters)
170 - 185
C in aromatic rings
125 - 150
C=C (in alkenes)
115 - 140
RCH2OH
50 - 65
RCH2Cl
40 - 45
RCH2NH2
37 - 45
R3CH
25 - 35
CH3CO-
20 - 30
R2CH2
16 - 25
RCH3
10 - 15
CH3
CH2
The external magnetic field experienced by the carbon nuclei is affected
by the electronegativity of the atoms attached to them.
The effect of this is that the chemical shift of the carbon increases if you
attach an atom like oxygen to it.
Predict the number of signals
3 - methylpentane
Predict the number of signals
Cyclohexane
Predict the number of signals
Butylamine
Predict the number of signals
2 - chlorobutane
Predict the number of signals
4-pentenoic acid
Predict the number of signals
methyl butanoate
Predict the number of signals
Pentan-3-one