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B18OA1 H NMR Spectroscopy

Characteristic 1H NMR chemical shifts
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A H NMR (say “proton NMR” or “one H NMR”) spectrum provides 4 key bits of information:
i) Chemical shift — tells you about adjacent atoms (Cl, O, N) or environments (C=C, C=O,
aromatic ring),
ii) Integration — tells you the relative number of protons that share the same environment,
iii) Multiplicity — tells you about the number of adjacent protons, and
iv) Coupling constant — tells you about the angle and distance between coupling protons.
Chemical shifts are measured in δ or ppm (which mean basically the same thing).
An OH/NH/NH2 signal
or this or this
could look like this
CO2H aryl OH amide NH, NH2 alkyl OH alkyl NH, NH2

12 ppm
Aromatic H CH2Br CH2-C-, cycloalkane
-M-substituted +M-substituted

O-CH2-O CH2Cl CH3-C=

O
OCH3, OCH2
CH3
O
H H-C=C NCH3 CH3-C- SiCH3

δ
10 9 8 7 6 5 4 3 2 1 0

Characteristic multiplicities and signal shapes

n =0 1
To a first approximation, protons just couple to protons on the next C. If n =1 1 1
there are n protons on neighbouring carbon(s), coupling splits the n =2 1 2 1
signal into n + 1 lines. The splitting of a signal is called multiplicity. The n =3 1 3 3 1
simplest multiplicities are singlets (n = 0, that is no protons nearby), n =4 1 4 6 4 1
doublets (n = 1 or just 1 proton nearby), triplets (n = 2), quartets (n =
n =5 1 5 10 10 5 1
3), quintets (n = 4), sextets (n = 5) and septets (n = 6). The
n =6 1 6 15 20 15 6 1
theoretical intensity of the individual lines can be derived from Pascal’s
triangle.

Singlet (s) Doublet (d) Triplet (t) Quartet (q) Quintet (qui) Sextet Septet
Note that you will see the symmetrical distribution of intensities predicted by Pascal’s triangle only when the
signals coupling to each other all share the same coupling constant. Pascal’s triangle will no longer be valid
when couplings are different, spin systems move closer together and become higher order, or a molecule
has a (pro)chiral centre.
When a proton signal shows no recognisable symmetry or multiplicity pattern at all, we refer to it as a
multiplet.
Coupling constants (J) are measured in Hz. A typical coupling constant between alkyl protons on adjacent
carbons is about 6 – 8 Hz. The magnitude of the spin-spin coupling between protons in general decreases
as the number of bonds between the coupled nuclei increases. Protons that are more than 3 bonds apart

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B18OA1 H NMR Spectroscopy
tend to show very small or no coupling at all. An exception arises when the protons are rigidly fixed in a W or
zig-zag arrangement. These so-called long-range couplings are seen in aromatic and rigid non-aromatic
rings, as well as (conjugated) alkene systems.

Doublets, triplets, quartets etc. are the simplest multiplicities. However, protons frequently couple to more
than one type of signal groups, with different coupling constants. Such successive splitting will lead to more
complex multiplicities, for example, doublets of doublets, doublets of triplets or triplets of quartets, to name
just a few possibilities.
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Integration: H NMR spectra are routinely integrated.
Simply measuring the integrals with a ruler
The integral of a signal is proportional to the number of
protons contributing to it. Units are irrelevant: use the
14 mm
electronic integral, or simply measure the integral height 9 mm
in mm, then calculate the number of protons 1H, 2H, 3H 4.5 mm
… Integrals have to be taken with a pinch of salt and can
be off by 10%, or more in case of exchangeable OH/NH gives an integral ratio of 1H : 2H : 3H (or 2H : 4H : 6H ...)
signals. It is important that the sum total of your integrals
equals the number of hydrogens in your compound.
OH and NH signals
Signals of OH and NH groups usually don’t show any coupling to other protons at all.
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In the H NMR spectrum you will recognise them as broad singlets.
The OH chemical shift can vary over a wide range depending on whether you are
dealing with an aliphatic alcohol, a phenol, a carboxylic acid or an enol. Similar
variations are seen for the NH chemical shift of aliphatic amines, aromatic amines and
A broad singlet
amides. Their signals are usually somewhat broadened, as well as influenced by
(br s)
sample concentration, water content and the choice of solvent.

δ 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0

Amide NH Amine
NH
Enols RCO 2H ROH

ArOH

An OH or NH signal is D2O exchangeable, and this provides one of the best ways of identification. So, if you
add a drop of D2O to a solution of your sample in CDCl3 and shake it, the signals of exchangeable OH and
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NH protons will no longer be seen when you record the H NMR spectrum again because they are converted
into OD or ND groups.
R OH D2O R OD HDO
Some pecularities of OH and NH signals:
• OH and NH signals are often broad.
• Their chemical shifts depend on solvent, concentration, temperature, water content.
• Their integration often appears too small.
• Exchange with e.g. water (H2O) wipes out all coupling.
• OH and NH signals vanish upon exchange with D2O; this test detects OH and NH signals.
Signal shapes of some common aromatic coupling systems
The coupling system of a monosubstituted benzene is non-first-order, that is, it is best described as multiplet.
However, benzene itself and benzenes with identical substituents in 1,4- or 1,3,5-position will show true
singlets because of their molecular symmetry.

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B18OA1 H NMR Spectroscopy
Unsplit spectra (showing approximate ∼singlets) are sometimes produced by monosubstituted benzene
derivatives if the substituent has no strong electron-withdrawing or electron-donating effect (e.g. toluene).
A single substituent that is either electron-withdrawing (for example, a nitro or carbonyl group) or electron-
donating (e.g. OCH3, OH and NH2), usually causes the ortho protons to move to higher or lower δ values with
respect to the meta or para protons. We then observe a 2-proton complex multiplet that is separated from a
3-proton complex multiplet. Note that the ortho proton signal resembles roughly a doublet (after all, it couples
to only one H on the nearest neighbouring carbon) — unlike meta and para signals which are both ∼triplets.
The smaller splittings, which are almost always seen, are due to long-range (meta) couplings and/or higher
order effects.
meta
ortho meta ortho
O CH3
δH ≈ 7 C OH
due to Chemical shift
–M substituent unaffected by δH < 7
para para
+M substituent due to
∼doublet +M substituent

8.1 8.0 7.9 7.8 7.7 7.6 7.5 7.4 7.3 7.5 7.4 7.3 7.2 7.1 7.0 6.9 6.8 6.7
Chemical Shift (ppm) Chemical Shift (ppm)

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H NMR chemical shifts are quite sensitive towards the electronic environment. While the aromatic protons of
an alkyl-substituted benzene will have almost the same chemical shift as benzene itself (δH 7.36 in CDCl3),
this changes if there is a strongly electron-donating or electron-withdrawing substituent on the benzene ring.
electron-poor δ electron-rich
downfield upfield

O OH
NO2 Cl OH NH2
H H H H H H H H H H H

δ H 8.2 δH 7.8 δ H 7.36 δ H 7.3 δH 6.8 δH 6.6

The ortho protons (and less so the para protons) will be affected the most, and move upfield (= to lower ppm
values) in the case of an electron-donating substituent or downfield (= to higher ppm values) if they are next
to an electron-withdrawing substituent. This can provide useful information on the type of substituent, as well
as help with the assignment of proton signals in substituted benzenes.
An unsymmetrical para-disubstitution leads to a symmetrical pair of signals which look almost like a pair of
doublets. There are, however, a number of tiny extra lines within each “doublet” which you see if you take a
closer look at an expansion. Although you will be allowed to call it a “doublet” at this stage, strictly speaking,
the multiplicity is a higher-order AA’XX’ (pronounced “A A dash X X dash”) or AA’BB’ system, the letters of
the alphabet indicating whether the chemical shifts are far from each other or not.

Typical AA'XX' Typical AA'BB'

Y
X

Mirror plane X
X

Similarly, identical ortho substituents also make the molecule symmetrical but the resulting multiplicity
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becomes more complex due to the three large ortho couplings. The H NMR signal pattern shows a
characteristic symmetry about the mid-point of the 12 − 24 line multiplet (it often looks very much like a pair
of hands). The resulting multiplicity is a non-first-order AA’BB’ spin system, with even more complexity than
seen in the case of para-disubstituted aromatic systems due to there being now three ortho couplings.

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B18OA1 H NMR Spectroscopy
Roofing and AB spectra
When the difference in the chemical shift between two coupled groups of signals becomes smaller, the
intensity distribution in the lines of the two groups of signals can differ considerably from the theoretical
intensities predicted by Pascal’s triangle. The intensity of the lines nearest to the signal of the neighbouring
group becomes larger, while the intensity of the other lines turn out smaller. This is called roofing. In the
extreme case where the difference in chemical shift in Hertz has about the size of the coupling constant, this
results in an AB spectrum.
AB spectrum = two doublets Compare this
Roofing close together with a quartet:
Distorted ... to coupling Distorted ... to coupling
doublet partner to the doublet partner to the
points ... right (upfield) points left (downfield)
∆δ small
J J
or a doublet
of doublets:

Tip: If a signal shows roofing, look for the coupling

partner on the side where the lines are higher. δB δA

NB: These are TWO doublets and belong to TWO protons which both have the same coupling constant,
whereas a “doublet of doublets” has TWO coupling constants because there are TWO coupling partners.
NMR solvents
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Deuterated solvents give rise to residual signals in the H NMR spectrum. You should familiarise yourself
with the chemical shifts and multiplicities of the more common NMR solvents so that you don’t mistake them
for signals belonging to the sample. In addition, “extra signals” often come from traces of water (moisture)
and acetone (from cleaning NMR tubes). By convention, solvent signals due to deuterated solvent are not
quoted when NMR data are tabulated.
CDCl3 D2O CD3OD (CD3)2SO or d6-DMSO
(Deuterochloroform) (Deuterium oxide) (Deuterated methanol) (Deuterated DMSO)
Residual 7.26 4.80 3.30 2.50
solvent
δH

7.3 7.2 7.1 4.9 4.8 4.7 3.4 3.3 3.2 2.6 2.5 2.4

H2O 1.56* 4.80 4.87 3.33

Acetone 2.17 2.22 2.15 2.09

* Exchange, particularly with acidic protons, can shift this signal downfield.
NB: Whereas the residual solvent signal in CDCl3 is due to undeuterated chloroform (CHCl3), the major
impurity in d6-DMSO is d5-DMSO and coupling of the single proton in the CD2H methyl group to the two
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deuteriums (with a nuclear spin of 1) gives 2 n + 1 = 2 × 2 + 1 = 5 lines or a quintet in the H NMR spectrum.
Real-life NMR spectra often show signals due to impurities, such as residual extraction or recrystallisation
solvents. Here is a useful paper: H. E. Gottlieb, V. Kotlyar, A. Nudelman, “NMR Chemical Shifts of Common
Laboratory Solvents as Trace Impurities”, J. Org. Chem. 1997, 62, 7512 – 7515. Access it through VISION.

Online videos
For more on NMR spectroscopy, see online video tutorials on Vision.