2. Nuclear spin and the splitting of energy levels in a magnetic field
Subatomic particles (electrons, protons and neutrons) can be imagined as
spinning on their axes. In many atoms (such as 12C) these spins are paired
against each other, such that the nucleus of the atom has no overall spin.
However, in some atoms (such as 1H and 13C) the nucleus does possess an
overall spin. The rules for determining the net spin of a nucleus are as
follows;
If the number of neutrons and the number of protons are both even, then the
nucleus has NO spin.
If the number of neutrons plus the number of protons is odd, then the
nucleus has a half-integer spin (i.e. 1/2, 3/2, 5/2)
If the number of neutrons and the number of protons are both odd, then the
nucleus has an integer spin (i.e. 1, 2, 3)
3. 1. number of signals
2. their intensity (as measured by area
under peak)
3. splitting pattern (multiplicity)
Information contained in an NMR
spectrum includes:
4. Number of Signals
protons that have different chemical shifts
are chemically nonequivalent
exist in different molecular environment
6. are in identical environments
have same chemical shift
replacement test: replacement by some
arbitrary "test group" generates same compound
H3CCH2CH3
chemically equivalent
Chemically equivalent protons
8. replacement by some arbitrary test group
generates diastereomers
diastereotopic protons can have different
chemical shifts
Diastereotopic protons
C C
Br
H3C
H
H
d 5.3 ppm
d 5.5 ppm
9. are in mirror-image environments
replacement by some arbitrary test group
generates enantiomers
enantiotopic protons have the same
chemical shift
Enantiotopic protons
13. Two-bond and three-bond coupling
C C
H
H
C C H
H
protons separated by
two bonds
(geminal relationship)
protons separated by
three bonds
(vicinal relationship)
14. in order to observe splitting, protons cannot
have same chemical shift
coupling constant (2J or 3J) is independent
of field strength
Two-bond and three-bond coupling
C C
H
H
C C H
H
16. Why do the methyl protons of
1,1-dichloroethane appear as a doublet?
C C H
H
Cl
Cl
H
H
signal for methyl
protons is split into
a doublet
To explain the splitting of the protons at C-2,
we first focus on the two possible spin
orientations of the proton at C-1
17. Why do the methyl protons of
1,1-dichloroethane appear as a doublet?
C C H
H
Cl
Cl
H
H
signal for methyl
protons is split into
a doublet
There are two orientations of the nuclear spin
for the proton at C-1. One orientation shields
the protons at C-2; the other deshields the C-
2 protons.
18. Why do the methyl protons of
1,1-dichloroethane appear as a doublet?
C C H
H
Cl
Cl
H
H
signal for methyl
protons is split into
a doublet
The protons at C-2 "feel" the effect of both the
applied magnetic field and the local field
resulting from the spin of the C-1 proton.
19. Why do the methyl protons of
1,1-dichloroethane appear as a doublet?
C C H
H
Cl
Cl
H
H
"true" chemical
shift of methyl
protons (no coupling)
this line corresponds
to molecules in which
the nuclear spin of
the proton at C-1
reinforces
the applied field
this line corresponds
to molecules in which
the nuclear spin of
the proton at C-1
opposes
the applied field
20. Why does the methine proton of
1,1-dichloroethane appear as a quartet?
C C H
H
Cl
Cl
H
H
signal for methine
proton is split into
a quartet
The proton at C-1 "feels" the effect of the
applied magnetic field and the local fields
resulting from the spin states of the three
methyl protons. The possible combinations
are shown on the next slide.
21. C C H
H
Cl
Cl
H
H There are eight combinations of
nuclear spins for the three methyl
protons.
These 8 combinations split the
signal into a 1:3:3:1 quartet.
Why does the methine proton of
1,1-dichloroethane appear as a quartet?
22. High-resolution spectra are run using a higher radio
frequency and the peaks have more detail.
Compare the spectra below for methyl propanoate.
High resolution Spectra
C
C
C
O
O
C
H
H
H H
H
H H
H
Low-resolution NMR
for methyl propanoate
High-resolution NMR for
methyl propanoate.
24. When the spectrum is expanded it can be seen that each peak is made
up of a number of peaks. These are called multiplets.
Quartet: four peaks in
the group.
Other multiplets include
singlets and doublets.
Triplet: three
peaks in the group.
25. n + 1 rule
• The number of peaks in a multiplet can give additional
information about the structure.
• The splitting of peaks is caused by the neighbouring carbon’s
hydrogen atoms.
• Protons in the same environment are said to be equivalent and as
such behave as one proton.
• This follows the n + 1 rule.
– n is the number of hydrogen atoms attached to the next-door
carbon
– n + 1 is how many peaks will be seen in the cluster.
26. Splitting Patterns of Common Multiplets
Number of equivalent Appearance Intensities of lines
protons to which H of multiplet in multiplet
is coupled
0 singlet 1
1 Doublet 1:1
2 Triplet 1:2:1
3 Quartet 1:3:3:1
4 Pentet 1:4:6:4:1
5 Sextet 1:5:10:10:5:1
6 Septet 1:6:15:20:15:6:1
Table
27. Splitting Patterns:
The Ethyl Group
CH3CH2X is characterized by a triplet-quartet
pattern (quartet at lower field than the triplet)
31. Splitting Patterns:
Pairs of Doublets
Splitting patterns are not always symmetrical,
but lean in one direction or the other.
32. Pairs of Doublets
Consider coupling between two vicinal
protons.
If the protons have different chemical shifts,
each will split the signal of the other into a
doublet.
C C
H H
33. Pairs of Doublets
Let Dn be the difference in chemical shift in Hz
between the two hydrogens.
Let J be the coupling constant between them
in Hz.
C C
H H
34. AX
When Dn is much larger than J the signal for
each proton is a doublet, the doublet is
symmetrical, and the spin system is called
AX.
C C
H H
J J
Dn
35. AM
As Dn/J decreases the signal for each proton
remains a doublet, but becomes skewed. The
outer lines decrease while the inner lines increase,
causing the doublets to "lean" toward each other.
C C
H H
J J
Dn
36. AB
When Dn and J are similar, the spin system is
called AB. Skewing is quite pronounced. It is
easy to mistake an AB system of two doublets
for a quartet.
C C
H H
J J
Dn
37. A2
When Dn = 0, the two protons have the same
chemical shift and don't split each other. A
single line is observed. The two doublets
have collapsed to a singlet.
C C
H H
40. m-Nitrostyrene
Consider the proton shown in red.
It is unequally coupled to the protons shown
in blue and white.
Jcis = 12 Hz; Jtrans = 16 Hz
H
H
O2N
H
41. m-Nitrostyrene
16 Hz
12 Hz 12 Hz
The signal for
the proton
shown in red
appears as a
doublet of
doublets.
H
H
O2N
H
44. O—H
The chemical shift for O—H is variable (d 0.5-
5 ppm) and depends on temperature and
concentration.
Splitting of the O—H proton is sometimes
observed, but often is not. It usually appears
as a broad peak.
Adding D2O converts O—H to O—D. Then
O—H peak disappears.
C O
H H
45. 1H NMR—Intensity of Signals
• The area under an NMR signal is proportional to the number of
absorbing protons.
• An NMR spectrometer automatically integrates the area under the
peaks, and prints out a stepped curve (integral) on the spectrum.
• The height of each step is proportional to the area under the peak,
which in turn is proportional to the number of absorbing protons.
• Modern NMR spectrometers automatically calculate and plot the
value of each integral in arbitrary units.
• The ratio of integrals to one another gives the ratio of absorbing
protons in a spectrum. Note that this gives a ratio, and not the
absolute number, of absorbing protons.
46.
47.
48. Excercise: Molecular formula: C4H9BrO
1H-NMR: 4.49 ppm (sextet, integral=1); 3.73 ppm (triplet,
integral = 2); 3.20 ppm (singlet, integral =1); 1.95 ppm (quartet,
integral=2) and 1.70 ppm (doublet, integral = 3)
