Basics of NMR spectroscopy

Basics of NMR spectroscopy
Prof. Klaus Ruhland
N
-
+
p
p
n
+
-
S
1

1. Introduction
Nuclear magnetic resonance spectroscopy (NMR spectroscopy) is one of the most important
characterisation methods in chemistry in particular for soluble low-molecular-weight
compounds.
History
1940‘s first evidence for NMR signals (1945)
1950‘s explanation of the influence on the chemical
environment on the shift of NMR signals
1952 Noble prize for Bloch/Purcell for developing
NMR spectroscopy
1960‘s improvement of sensitivity by multiple
measurements and Fourier-Transformation
evaluation
1970‘s improvement of resolution by application of
superconducting magnets yielding in higher field
strengths
1980‘s development of multidimensional methods
1990‘s development of pulsed field gradient methods
1991 Noble prize for R. Ernst for high-resolution NMR
2000‘s Coupling of NMR with chromatographic methods
development of even higher fields of the magnets
The detectable probe in this case is the magnetic dipole/multipole of the atomic nuclei, about
the cause and existence of it we will talk first.
Atomic nuclei consist as is well known of protons and neutrons, from which one can at least
as a model imagine, that they move around, held together by strong but short-ranging
interactions and especially rotate (quantized) about their own axies. The accelerated
movement of charges results in a magnetic field (Maxwell equations), so it will not come as a
surprise, positively charged protons, when rotating about thier own axis (changes in
orientation are also accelerations!) cause a magnetic field. In fact, both nucleons (protons and
neutrons) contribute to the magnetic field of the atomic nucleus, which we are up to detect.
This, in the case of the neutral neutrons, is not plausible in the first place, since no charge
movement seems to be connected with the rotation about their own axis..
2

N
-
+
p
p
n
+
-
S
To better understand this, we must go a resolution step deeper and have a look, how protons
and in particular neutrons themselves are composed of. Indeed, they consist each of 3 quarks (
protons: up/up/down; neutrons: up/down/down). the up-quarks possess a charge of +⅔ and a
spin of ½. The down-quarks possess a charge of -⅓ and also a spin of ½. If one assumes in a
simplified manner, that the spins of u and d each act in opposite direction, on this basis one
will predict for a proton (uud) the charge +1 and the spin ½. For the neutron (udd) one would
expect a charge of 0 and a spin of ½. Both is in accordance with the experimental findings.
Taking this into account we can easier accept, that also a neutron by rotation about its own
axis contributes to the magnetic field, since the three charge carriers udd can be imagined to
be anisotropically distributed in space, so that a self-rotation of neutrons with spin ½, if not
connected with separated charges, is at least connected with the acceleration of electric
dipoles.
Particle Charge Spin Magnetic Moment
(relative)
Gyromagnetic
constant γ
10 -7 rad T -1 s -1
Proton +1 ½ +1.6 26.8
Neutron 0 ½ -1 -18.3
Electron -1 ½ -1060 -17608
The upper table compares the most decisive properties of the three „elemental“ particles of
matery. We realize, that the magnetic moment of neutrons and protons are approximately of
the same size and opposite in sign, whereas that of an electron is larger by a factor of 1000.
The also listed gyromagnetic constant γ will be introduced and explained later in this text.
3

It is of course not possible, to examine matery only consistent of nuclei. We always have to
carry with us the electrons a neutralising kit. And concerning the magnetic properties, which
we want to use to characterize the material, these obviously show a much more pronounced
influence. In diamagnetic materials however the magnetic contributions of the electrons
cancel out each other, since electrons with opposite spin each are present in the same amount
in the sample, so in such samples we really can exclusively observe the (decisively smaller)
magnetic behaviour of the nuclei. This means at the same time, that we have to expect
complications for measurements of paramagnetic samples, which is indeed true.
For the most important cases we can ignore the electron cloud (in first approximation; it will
become important again later with a secondary effect, but not by its spin) and we can
concentrate completely on the nucleus. In general several protons and neutrons will be sharing
the nucleus. Which consequences will this have on the resulting magnetic moments of the
nucleus?
We will distinguish three different cases:
1. Both the amount of protons and the amount of neutrons is even
In this case the opposite spins of two neutrons and two protons each cancel out (Fermions ⇒
Pauli-Prinziple). The complete spin of the nucleus I is zero, and there does not exist a
resulting magnetic dipole/multipole. Nuclei, e. g. 12 C, for which this constellation is found,
are called magic nuclei. For NMR spectroscopy they are inactive and therefor useless.
2. Either the amount of protons or the amount of neutrons is uneven
In this case the spins of one kind of particle cancel out each other, whereas for the other
particle type with uneven amount at least one spin remains uncompensated. The interaction
with the other existing spins however influences the complete nucleus spin. At least it can be
predicted, that these nuclei will possess a half-numbered resulting nucleus spin I. If exactly
one spin remains uncompensated, the nucleus spin is I=½. Nuclei of this kind are particularly
attractive for NMR spectroscopy, because they do not possess a magnetic quadrupole
moment, only a magnetic dipole moment (please just accept this as a matter of fact without
explanation). Many nuclei important for organic materials, especially 1 H, 13 C, 19 F, 31 P belong
to this group. Not at least because of this fact, NMR spectroscopy has gained its importance
for chemists nowadays.
3. Both the amount of protons and the amount of neutrons are uneven
4

This constellation leads to whole-numbered complete nucleus spins I larger than zero. These
will always possess a magnetic quadrupole moment (necessary and sufficient condition I>½),
which makes the signal detection more difficult.
5

2. Nuclei with spin I=½
We will first concentrate on nuclei with I=½, because they are so important for NMR
spectroscopy. The following periodic system of the elements shows, which nuclei/isotopes we
are talking about (some of the elements possess several isotopes with I=½. In the periodic
system only that with the highest natural abundance is shown).
Group
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
Period
The number below the element symbol represents the natural abundance of the Isotope with I=1/2 in %
1
1
1 H
100
2
He
-
2
3
Li
-
4
Be
-
5
B
-
6
13 C
1.1
7
15 N
0.3
8
O
-
9
19 F
100
10
Ne
-
3
11
Na
-
12
Mg
-
13
Al
-
14
29 Si
4.7
15
31 P
100
16
S
-
17
Cl
-
18
Ar
-
4
19
K
-
20
Ca
-
21
Sc
-
22
Ti
-
23
V
-
24
Cr
-
25
Mn
-
26
57 Fe
2.2
27
Co
-
28
Ni
-
29
Cu
-
30
Zn
-
31
Ga
-
32
Ge
33
As
-
34
Se
-
35
Br
-
36
Kr
-
5
37
Rb
-
38
Sr
-
39
89 Y
100
40
Zr
-
41
Nb
-
42
Mo
-
43
Tc
-
44
Ru
-
45
103 Rh
100
46
Pd
-
47
107 Ag
51.8
48
111 Cd
12.8
49
In
-
50
119 Sn
8.6
51
Sb
-
52
125 Te
7.0
53
I
-
54
129 Xe
26.4
6
55
Cs
-
56
Ba
-
*
71
Lu
-
72
Hf
-
73
Ta
-
74
183 W
14.4
75
Re
-
76
187 Os
1.6
77
Ir
-
78
195 Pt
33.8
79
Au
-
80
Hg
-
81
205 Tl
70.5
82
207 Pb
22.6
83
Bi
-
84
Po
-
85
At
-
86
Rn
-
7
87
Fr
-
88
Ra
-
*
*
103
Lr
-
104
Rf
-
105
Db
-
106
Sg
-
107
Bh
-
108
Hs
-
109
Mt
-
110
Ds
-
111
Rg
-
112
Uub
-
113
Uut
-
114
Uuq
-
115
Uup
-
116
Uuh
-
117
Uus
-
118
Uuo
-
*Lanthanoids
*
57
La
-
58
Ce
-
59
Pr
-
60
Nd
-
61
Pm
-
62
Sm
-
63
Eu
-
64
Gd
-
65
Tb
-
66
Dy
-
67
Ho
-
68
Er
-
69
169 T
m
100
70
173 Yb
14.3
**Actinoids
*
*
89
Ac
-
90
Th
-
91
Pa
-
92
U
-
93
Np
-
94
Pu
-
95
Am
-
96
Cm
-
97
Bk
-
98
Cf
-
99
Es
-
100
Fm
-
101
Md
-
102
No
-
What happens, if an external magnetic field is applied to such kind of nucleus?
For a nucleus with spin I=½ there exist only two allowed quantized states ±½ (in general:
amount of allowed states = 2 I + 1). Without an external magnetic field these two states are
degenerated, but the stronger the external applied magnetic field becomes, the more decisive
6

the degeneration will be lost and an energy difference proportional to the external magnetic
field will results between the two states.
Energy
Magn. field strength
How strong the energy splitting between the two states proportional to the external magnetic
field will be, depends on the proportionality constant γ, which is individual for each isotope:
∆E = γ ħ B 0 = h ν
This proportionality constant is the aforementioned gyromagnetic constant. The larger it is the
more sensitive is the isotope for NMR spectroscopic examinations.
7

The upper table summarizes the most decisive properties concerning NMR spectroscopy of
the most important isotopes.
In NMR spectroscopy will be measured the transition between the allowed states (in case of
I=½ between the two states ±½), resulting from the external magnetic energy irradiation,
which is transformed in an energy-dependent signal. This can be done in two ways. On the
one hand the external magnetic field can be systematically changed on constant irradiation of
an amount of energy h ν and on the other hand the irradiated energy can be systematically
changed at constant external magnetic field strength. In both cases the energy absorption by
the sample results in a signal.
Energy
Energyabsorption
Magn. field strength
Energyabsorption
Magn. field strength
Energyabsorption
Magn. field strength
8

Energyabsorption
Magn. field strength
Energyabsorption
Magn. field strength
Because the external magnetic field should be as constant and homogeneous as possible the
more appropriate method is, to keep the magnetic field unchanged and instead vary the
irradiated energy.
Frequency ν
Energy
Energyabsorption
Magn. field strength
9

Frequency ν
Energy
Energyabsorption
Magn. field strength
Frequency ν
Energy
Energyabsorption
Magn. field strength
Frequency ν
Energy
Energyabsorption
Magn. field strength
10

Frequency ν
Energy
Energyabsorption
Magn. field strength
The measurement is even more simplified nowadays by the fact, that the frequencies of the
irradiated energy (brought about by a AC current device) are not stepwise applied, but all at
once a broad band of frequencies are used. This is done by irradiation of an as short as
possible current signal (the shorter the more frequencies) in an as long as possible periodic
time distance (the longer the time distance the larger the frequency window, that is irradiated
at once). From the temporal response of this at-once-irradiation of a broad frequency band the
intensity of absorption as a function of the irradiating frequency can be received by Fouriertransform
analysis (which we only mention here without further detail). This is then the NMR
spectrum.
1 Frequenz gleichzeitig
5 Frequenzen gleichzeitig
1,5
1
0,5
0
0 1 2 3 4 5 6 7
-0,5
-1
-1,5
10
8
6
4
2
0
0 1 2 3 4 5 6 7
-2
-4
-6
-8
-10
10 Frequenzen gleichzeitig
50 Frequenzen gleichzeitig
25
20
15
10
5
0
0 1 2 3 4 5 6 7
-5
-10
-15
-20
25
20
15
10
5
0
0 1 2 3 4 5 6 7
-5
-10
-15
-25
-20
11

100 Frequenzen gleichzeitig
250 Frequenzen gleichzeitig
80
70
60
50
40
30
20
10
0
0
-10
1 2 3 4 5 6 7
-20
-30
100
80
60
40
20
0
0 1 2 3 4 5 6 7
-20
-40
The pulse length is in the range of µs.
Let us discuss for a short while the energy window of NMR spectroscopy:
Energy differencies in NMR spectroscopy (intensity of signals)
Magnetic Energy
∆E = h ν = ħγ B ˜ 4 10 -25 J
Thermal Energy
E = k T ˜ 4 10 -21 J
∆E
m=½
∆E = h ν
B
n m=½
- ∆E
k T
n =e ≈ 0.9999
m=-½
m=-½
In fact the energy difference between the nucleus spin states is much smaller than thermal
energy at room temperature. A Boltzmann distribution assumed, this means, that the
distribution difference between the two states (I=½), which is the most important parameter
for the sensitivity of the measurement, is very small. This explains the search for as strong as
possible external magnetic fields, because the energy difference between the spin states grows
proportional to this external magnetic field. At the moment the maximum is 900 MHz (21.15
Tesla) and it is determined by the superconducting material of the coil, which provides the
magnetic field, and its critical magnetic field strength, above which the superconducting
properties are lost.
Concerning the range of frequencies (E = h ν) NMR spectroscopy is a fairly „slow“
spectroscopic method.
12

Comparison of energies in spectroscopy
Electromagnetic Wave length Frequency Properties to be examined Spectroscopic method
irradiation
γ rays 100 pm – 1 pm 3 10 18 – 3 10 20 Hz Change of nuclear states γ-Spectroscopy, Mößbauer-
Spectroscopy
x-rays 10 nm – 100 pm 3 10 16 – 3 10 18 Hz Change of state of inner electron x-ray Spectroscopy
core state
UV light, visible light 1 µm – 10 nm 3 10 14 – 3 10 16 Hz Change of state of avlence UV Spectroscopy
electrons
Infrared rays 100 µm – 1 µm 3 10 12 – 3 10 14 Hz Change of vibrational
IR/Raman Spectroscopy
states
Microwaves 1 cm – 100 µm 30 GHz – 3 10 12 Hz Change of rotational
Microwave Spectroscopy
states
Microwaves 1m – 1 cm 300 MHz – 30 GHz Change of electron spin Electron spin resonance (ESR)
states
Radiowaves 100 m – 1 m 3 MHz – 300 MHz Change of nuclear spin
states
Nuclear spin resonance (NMR)
This means, that on doing NMR spectroscopic examinations in several cases only a timeaveraged
picture of the behaviour of matery in comparison with „faster“ methods as for
instance IR spectroscopy with higher time resolution is detected.
IR spectroscopy
13
C-NMR
13

In the upper example for instance only one type of symmetry-equivalent carbons can be
detected by NMR spectroscopy for the compound Co 2 (CO) 9 . The IR spectrum of the very
same compound however shows, that this is only true time-averaged and in fact is based on an
overlay of different isomers/conformers.
Below is shown the schematic composition of an NMR machine.
Let us have a look step by step, what happens during the measurement.
A typical NMR sample tube for solvent NMR spectroscopy is shown in the upper right. This
will be lifted, attached to a probe holder, by release of air pressure along the probe channel
into the strong constant external magnetic field. The probe holder enables (as well by a stream
of air), that the NMR sample tube rotates about its long axis. This contributes decisively to the
fact, that the complete sample time-averaged experiences an as homogeneous magnetic field
as possible. The rotation must be faster than the desired resolution in the spectrum. In general
14

it is spinned at 20 Hz. As the next step one optimizes the emitter coil (which generates a
magnetic field perpendicular to the external strong magnetic field, if a current pulse is
applied) for the measurement. Two actions are implemented to homogenize the emitter
magnetic field concerning time and space. The first action is called „Locking“. For it the
deuterium signal of the NMR solvent is used as leading signal (that is the main reason why
the NMR solvent is deuterated; also of course to hold free the 1 H window from disturbing
signals), to guarantee the time-averaged constancy of the emitter magnetic field. The second
action is called „Shimming“. On doing this the optimal orientation of the emitter coil is
arranged. It is done by maximising the lock signal (again using the deuterium channel).
The measurement can be started now!
Perpendicular to the external strong magnetic field an emitter magnetic field is applied via DC
coil to the sample.
N
B
B'
M
emitter
coil
S
receiver
coil M=0
N
B'
M
emitter
coil
S
rotating angle
is dependent on
the irradiation time
and irradiation power
receiver
coil M>0
N
B'
M
emitter
coil
S
The magnetisations of the sample that are in resonance with the irradiation of the emitter are
rotated the more the longer the emitter coil is applied and the higher its power is. Ideal fort he
sensitivity is a 90°-Puls, because in that case the magnetisation is exactly parallel to the
receiver coil and thus induces the strongest magnetic field as signal.
15

What happens as soon as the emitter pulse stops? In that case the system is no longer at its
equilibrium state, because the magnetisation of the sample is perpendicular to the external
magnetic field, without another magnetic field being operative (the emitter is off). The
equilibrium state a parrallel orientation of the sample magnetisation relative to the external
magnetic field. The equilibrium state however cannot be reached instantaneously. A physical
process, at which an equilibrium state is reached with time delay, is called relaxation. In
NMR spectroscopy there exist two relaxation mechanisms:
A. Longitudinale or Spin-Lattice-Relaxation
Relaxation
A. spin-lattice relaxation T 1 :
receiver
static magnetic field
transmitter
B z
receiver
T 1
B y
transmitter
static magnetic field
B z
receiver
B. spin-spin relaxation T 2 :
transmitter
B z
B z
T 2
B z
∆E ∆t ≥ h
B y
B y
This mechanism describes the loss in sample magnetisation via interaction of the magnetic
mlti(di)poles of the nucleus with other molecular magnetic fields in ist proximity (solvent
molecules, other nclei (↑ towards →)).
B. Transversale or Spin-Spin-Relaxation
This mechanism explains the loss in sample magnetisation by a „Spin-Flipping“-Process (e.
g.: ↑↓ → ↓↑), through which the preferred orientation of the spins along the receiver coil
decreases, until no preferred orientation exists any more.
The next diagram shows the time scale of NMR spectroscopy (spectral resolution) in relation
to chemical and physical processes.
16

Molecular
vibration
Dynamic
Process
Chemical
exchange processes
Molecular
Rotation
Macroscopic
transport processes
slow
s ms µs ns ps fs
fast
Spectroscopic
Finding
slow
exchange
Longitudinal
Magnetization exchange
Line wideperturbation
fast
exchange
The NMR time scale
Relaxation
time scale
Spectral
time scale
Larmortime
scale
The NMR time scale which overlays conveniently with that of chemical exchange processes
is another reason, why this characterization method is so important for chemists.
17

3. The chemical shift
Until now we understand, that, if a particular nucleus (e. g. H or C or P) is present in the
material, we can detect it via NMR spectroscopy (as far as it is NMR-aktive).
In fact the diagnostic information content of NMR spectroscopy is decisively higher, because
the molecular electronic environment of the nucleus influences the resonance frequency as
well. This influence is called chemical shift. Nuclei of one and the same isotope will show a
magnetic transition at a very similar resonance frequency between the spin states, but within
this window the direct electronic environment of the nucleus will lead to small changes in the
resonance frequency. The chemical (electronic) environment acts thus as a perturbation of the
resonance frequency of the bare nucleus. The physical reason is the magnetic Lorentz effect.
Electron density near the nucleus is influenced by the external magnetic field in a way, that it
generates a magnetic field that counteracts the external field. This screens the nucleus against
the external field.
Resonance
frequency
ν Α,Ref.
bare nucleus A
nucleus A
with electron density
e
nucleus A
with more electron density
δ
e
chemical
shift
ν Β,Ref.
bare nucleus B nucleus B
with electron density
e
δ
nucleus B
with electron density
e
A nucleus, surrounded by large electron density, will be exited at a (slightly) higher resonance
frequency excited in comparison to a nucleus of the same isotope, surrounded by a smaller
electron density.
The diagram below shows the different reasons, that contribute to the chemical shift.
Intermolecular influences are (in contrast to solid-state-NMR spectroscopy) in case of
solution NMR spectroscopy of second order, because they cancel out in time via free rotation
of the single molecules (which is faster than the spectral resolution).
18

A quite special influence on the chemical shift is the ring-current effect, which is found in
cyclic conjugated (aromatic) compounds.
The protons in the periphery of the benzene ring are strongly deshielded by the ring-current
effect (resonance frequency decisively smaller than for other protons, e. g. those bound to a
non-cyclic conjugated double bond). The chemical shift is measured in ppm according to the
equation
δ = (ν A, Ref. – ν A ) / ν A,Ref.
The ppm (parts per million) scale stems from the fact, that the difference between ν A and
ν A,Ref . is in the range of Hz, while ν A,Ref. itself has a value of MHz.
19

The upper two diagrams show typical chemical shifts in 1 H (left) and 13 C (right) NMR
spectroscopy for well-established organic functionalities.
4. Scalar Spin-Spin coupling
Let us go one step further, since NMR spectroscopy contains even more information than
mentioned so far. Not only the type of nucleus and its electronic environment, but also the
neighboring nucleus spins influence the appearance of the NMR spectrum, the latter by the
multiplicity of the signal.
In contrast to solid-state-NMR spectroscopy in case of solution NMR spectroscopy only those
nucleus spin interactions contribute, that are mediated by a chemical bond (shared electron
density). Magnetically dipolar couplings through spatial proximity time-average to zero in
solution.
Let us have a look on the situation in formaldehyde H 2 C=O. 16 O and 12 C, the most abundant
isotopes of these two elements, are magic nuclei with spin 0 and thus NMR-inactive. We can
thus concentrate in the following only on the two protons. So let us have a look, how this
constellation will develop in the 1 H-NMR spectrum.
20

Spectrum:
ν 0 ν 1 ν 1
β(β)
H
β
β
β(α)
C
O
H
ν 0 ν 1
ν 1
ν 1
α
α
α(β)
α(α)
bare nucleus
Chemical
shift
by surrounding
electrons
Chemical
shift
additional by
neighboring spins
The protons experience an electronic screening through the neighboring electrons of the COgroup
(chemical shift). Additionally the spins feel each other, which also leads to shielding or
deshielding. Starting from the two energy states for a pare proton now four energy states
result. Only transitions with ∆I=1 are allowed. This selection rule stems from the fact , that on
a transition a photon is absorbed. Photons possess a spin of 1; this means, that on absorption
the moment of inerta of the system changes by –1. Since the moment of inerta must be
retained, the loss must be compensated for an allowed transition; in our case by a change of
the spin nucleus of +1.
It might seem so, that for formaldehyde one should expect two signals in the 1 H NMR
spectrum instead of only one. Butt his is not the case, because the two allowed transitions are
brought about by the same energy h ν and thus appear at the same position in the spectrum.
This is only the case, because the two protons are symmetry-equivalent (remember:
symmetry-equivalent nuclei do not couple with each other!). The situation changes already in
Aceto-chlorimin H 2 C=NCl. Now the symmetry-inequivalent nuclei couple with each other
additionally to the chemical shift. This coupling splits of the signals fort he two protons each
to a doublet. The scalar coupling is transferred via bonding electrons. The size of the coupling
is therefore indicative for the electronic situation of the bond. For example one can deduce the
degree of hybridization of a C-H bond by means of the 1 J 1H/13C -coupling constant.
21

Spectrum:
ν 0
ν 1 ν 2 ν 1 ν 2 ν 1a/b ν 2a/b
β
β(β)
β(α)
β@β
β@α
H
Cl
β
β
β(β)
β(α)
β@β
β@α
C
N
H
ν
ν 2b
ν 2
ν 2 ν 0 ν 1 ν 1b
1 ν 2a
ν 2 ν 1 ν 1a
α
α
α(β)
α(α)
α@β
α@α
bare nucleus
α
chemical
shift
by surrounding
electrons
α(β)
α(α)
chemical
shift
by neighboring spins
α@β
α@α
additional active coupling
between
neighboring sins
If two nuclei interact with each other by magnetic coupling, the visible splitting of the signals
must bet he same for both nuclei (J ab = J ba ). The following diagram shows the continuous
transition starting from an A 2 -system (both nuclei symmetry-equivalent) to an AX-System
(active coupling between the two nuclei).
In general it holds:
3 J > 1 J >> 2 J
The higher number expresses, how many bonds there are between the actively coupling
nuclei.
22

The splitting pattern in organic compounds of a proton CH-X, where X ist he group as shown
below, shows in dependence of the number of bound protons k to the neighboring C-atom a
characteristic behaviour (multiplicity: (k + 1); try to make this plausible to you yourself!):
The splitting follows concerning the multiplicity and intensity of the single multiplett-signals
a Pascal’s triangle.
For the 3 J-coupling the Karplus-equation holds:
3 J = A cos 2 (θ) + b cos(θ) + c
θ is the torsional angle between bond one and three.
23

5. Line broadening
We have by now collected quite a bit of information power, connected with NMR
spectroscopy, but still there is more to come. Additionally to the position of the peak in the
spectrum (electronic environment), the intensity of the peak (amount of the nucleus in a
specific environment) and the multiplicity of the peak (spin environment of the nucleus) the
line-broadening of the signal is another valuable diagnostic criterion, since it tells us
something about the life-time of a state.
Is in particular a chemical exchange between two positions in the spectral time scale of NMR
spectroscopy (1-1000 Hz), this will be detectable by line-broadening, from which (also
quantitatively) the rate of this exchange can be determined. In general at modern machines
there is the possibility, to cool and heat the probe head (from about -110°C, caused by the
melting point of the well-established NMR solvents up to 110°C, caused by the stability limit
of the probe head material (tubing etc.)), so that by temperature-dependent measurement of
the line-broadening very conveniently activation parameters can be determined.
Line broadening (Chemical exchange)
The upper example demonstrates, how the line-broadening of the CH 2 -group with temperature
changes. At low temperatures only two doublets are visible (why?), which at higher
temperatures step by step develop into a singlet as time-averaged signal, because at elevated
temperatures the back- and for-vibration of the CH 2 -groups becomes so fast, That on timeaverage
the appear equivalent.
25

In summary the last diagram shows once again, which conclusions can be drawn from a 1-
dimensional solution NMR spectrum.
Information content of a 1D NMR spectrum
Line width of the signal:
life time of the spin state
in the specific environment
Intensity of the signal:
amount of nuclei with
specific environment
Fine structure of the signal:
neighboring nuclear spins
Position of the signal:
electronic environment of the nucleus
26

Which compound might this be?
27