Interpreting NMR Spectra
CHEM 318
Introduction
• You should read the assigned pages in your text (either Pavia
or Solomons) for a detailed description of NMR theory and
instrumentation.
• This presentation will focus on interpreting 1H and 13C NMR
spectra to deduce the structure of organic compounds.
• Here is a reminder of some important points and vocabulary:
Introduction
• Atomic isotopes with either an odd number of protons or
neutrons possess a spin angular momentum (1H, 13C, and 15N
but not 12C and 16O).
• The positively charged nuclei generate a small electric current
and thus also generate an associated magnetic field.
• When the nuclei are placed in an external magnetic field,
their magnetic spins align either with the external field (lower
energy) or against it (higher energy).
Introduction
• When irradiated with electromagnetic radiation (radio
frequency range), the lower energy nuclei absorb the energy
and become aligned against the field in the higher energy
state. The magnetic "spin-flip" transition is in "resonance"
with the applied radiation.
• For nuclei in different bonding environments, the resonance
energy is different.
• The position of the energy absorption signal is known as the
chemical shift, in units of ppm (parts per million) relative to a
standard reference compound.
Introduction
• Most carbons are the 12C6 isotope – 6 protons and 6 neutrons
in the nucleus. ~ 1.1% of carbons are the 13C6 isotope with an
extra neutron. With this odd number of nucleons, the 13C6
atoms can give rise to an NMR signal.
• Most hydrogens are the 1H1 isotope – 1 proton in the nucleus
and no neutrons. The hydrogen atom thus has an odd number
of nucleons and can produce an NMR signal.
13C
NMR Spectroscopy
• Information obtained from a 13C NMR spectrum:
– the number of resonance signals denotes the number of
different carbon atoms present
– the chemical shift reveals the electronic bonding
environment of the carbons
– signal splitting gives the number of hydrogens bonded to
each carbon
13C
NMR Spectroscopy
• The range of chemical shifts is about 0-220 ppm.
• Saturated carbon atoms absorb upfield (lower ppm) relative
to unsaturated carbons (downfield at higher ppm).
• Electron-withdrawing atoms cause a downfield shift
– compare shifts of –CH2– with –CH-O– and C=C with C=O
Hexane
CH3
H3C
Carbon-Hydrogen Spin Splitting
• The spectrum just shown has 1 resonance signal for each
different carbon atom.
• A spectrum can be taken that “couples” the spins of a carbon
and the hydrogens that are bonded directly to it.
• The resulting “splitting” pattern (n+1 peaks) shows the
number of hydrogens (n) bonded to the carbon that is
undergoing resonance.
Quartet (4) (q)
3 H’s -CH3 methyl
Triplet (3) (t)
2 H’s -CH2- methylene
Doublet (2) (d)
1H
>CH- methine
Singlet (1) (s)
0H
Hexane
CH3
H3C
ppm
14.16 q
22.89 t
31.87 t
2,2-Dimethylbutane
H3C
H3C
CH3
CH3
2,2-Dimethylbutane
H3C
H3C
ppm
8.88
28.96
30.42
36.49
q
q
s
t
CH3
CH3
Acetaldehyde
O
H3C
H
Ethyl propionate
H3C
O
H3C
ppm
174.40 s
60.26 t
27.71 t
14.32 q
9.19 q
O
H
ortho-Xylene
H
H3C
H
H
CH3
CH3
1-Butyne
H
ppm
85.96
71.94
13.81
12.27
d
s
q
t
1H
NMR Spectroscopy
• Information obtained from a 1H NMR spectrum:
– the number of resonance signals denotes the number of
different hydrogen atoms present
– the chemical shift reveals the electronic bonding
environment of the hydrogens
– integration of the peaks gives the relative number of
hydrogens causing the resonance
– signal splitting gives the number of hydrogens bonded to
the adjacent atom(s)
1H
NMR Spectroscopy
• The range of chemical shifts is about 0-12 ppm.
• Hydrogens bonded to saturated carbons absorb upfield
relative to hydrogens on unsaturated carbons (downfield).
• Electron-withdrawing atoms cause a downfield shift.
– compare >CH – with >CH-O– and compare C=CH with HC=O
1H
NMR Spectroscopy
Chemical shift: There are two resonance peaks, indicating two different kinds
of H’s. The upfield signal is in the saturated C region; the downfield signal
is in the unsaturated C region.
CH3
1H
NMR Spectroscopy
Integration: The area under the peak is proportional to the number of
hydrogens causing the absorbance.
CH3
After integration
and rounding to
small whole
numbers, the peak
area ratio is 5:3.
1H
NMR Spectroscopy
Spin-spin splitting: A resonance peak is split into n+1 peaks by n H atoms on
adjacent atoms.
The CH3 resonance
(1.8 ppm) is split by
the neighboring
CH2.
I
H3C CH2
There are n=2
neighboring H’s and
so the methyl
resonance is split
into n+1=3 peaks –
a triplet.
Integration: 2:3
1H
NMR Spectroscopy
Spin-spin splitting: A resonance peak is split into n+1 peaks by n H atoms on
adjacent atoms.
The CH2 resonance
(3.2 ppm) is split by
the neighboring
CH3.
I
H3C CH2
There are n=3
neighboring H’s and
so the methylene
resonance is split
into n+1=4 peaks –
a quartet.
Integration: 2:3
1H
NMR Spectroscopy
Spin-spin splitting: A resonance peak is split into n+1 peaks by n H atoms on
adjacent atoms.
These are some simple,
common splitting patterns:
Ethyl group is a quartet-triplet,
integration 2:3
Isopropyl group is a septet
(hard to see) and a doublet,
integration 6:1
Para-disubstituted benzenes
give this symmetrical splitting;
other substituted benzenes are
not well-resolved. Relative
integrations are shown.
Isopropylbenzene (cumene)
CH3
H3C
O
Propanoic acid
H3C
CH2 C
OH
25
O
Acetaldehyde (ethanal)
H3C
H
26
3-Hydroxy-2-butanone (acetoin)
O HO
H3C C
CH
CH3