Proton NMR Spectroscopy
The NMR Phenomenon
• Most nuclei possess an intrinsic angular
momentum, P.
• Any spinning charged particle generates a
magnetic field.
P = [I(I+1)]1/2 h/2p
where I = spin quantum #
I = 0, 1/2, 1, 3/2, 2, …
Which nuclei have a “spin”?
• If mass # and atomic # are both even, I = 0 and the
nucleus has no spin.
e.g. Carbon-12, Oxygen-16
• For each nucleus with a spin, the # of allowed spin
states can be quantized:
• For a nucleus with I, there are 2I + 1 allowed spin
states.
1H, 13C, 19F, 31P
all have I = 1/2
DE = g(h/2p)Bo
Spin states split in the presence of B0
-1/2 ant iparallel
E
+1/2 parallel
no field
applied field
Bo
When a nucleus aligned with a magnetic
field, B0, absorbs radiation frequency (Rf), it
can change spin orientation to a higher
energy spin state. By relaxing back to the
parallel (+1/2) spin state, the nucleus is said
to be in resonance. Hence,
NMR
Presence of Magnetic Field
NMR instruments typically have a constant
Rf and a variable B0.
A proton should absorb Rf of 60 MHz in a
field of 14,093 Gauss (1.4093 T).
Each unique probe nucleus (1H perhaps) will
come into resonance at a slightly different and a very small percentage of - the Rf.
All protons come into resonance between
0 and 12/1,000,000 (0 – 12 ppm) of the B0.
• Nuclei aligned with the magnetic field are lower
in energy than those aligned against the field
• The nuclei aligned with the magnetic field can
be flipped to align against it if the right amount
of energy is added (DE)
• The amount of energy required depends on the
strength of the external magnetic field
NMR Spectrometer
• Schematic diagram of a nuclear magnetic
resonance spectrometer.
Energy Difference (DE) Between Two Different
Spin States of a Nucleus With I=1/2
-1/2
ant iparallel
E
100 MHz
200 MHz
300 MHz
400 MHz
parallel
+1/2
23,500
47,000
70,500
inc. magnet ic field st rengt h, Gauss
B0
104,000
What Does an NMR Spectrum
Tell You?
• # of chemically unique H’s in the molecule
# of signals
• The types of H’s that are present e.g.
aromatic, vinyl, aldehyde …
chemical shift
• The number of each chemically unique H
integration
• The H’s proximity to eachother
spin-spin splitting
Chemical Equivalence
How many signals in 1H NMR spectrum?
CH3
O
O
O
O
Number of Equivalent Protons
CH3
1
3
5
2
O
O
O
O
6
4
4
Homotopic H’s
– Homotopic Hydrogens
• Hydrogens are chemically equivalent or
homotopic if replacing each one in turn by the
same group would lead to an identical compound
Enantiotopic H’s
• If replacement of each of two
hydrogens by some group leads to
enantiomers, those hydrogens are
enantiotopic
Diastereotopic H’s
• If replacement of each of two hydrogens
by some group leads to diastereomers, the
hydrogens are diastereotopic
– Diastereotopic hydrogens have different
chemical shifts and will give different signals
Vinyl Protons
Typical 1H NMR Scale is
0-10 ppm
The d Scale
Tetramethylsilane (TMS)
Arbitraril y assigne d a ch e m i cal shi
of d 0.00
CH3
CH3SiCH 3
CH3
TMS
Chemical Shift Ranges, ppm
Diamagnetic Anisotropy
Shielding and Deshielding
Deshielding in Alkenes
Shielding in Alkynes
Integration
Br
CH3
CH3
H
H
CH3
Methyl t-butyl ether (MTBE)
Toluene at Higher Field
Splitting patterns in aromatic groups can be
confusing
A monosubstituted aromatic ring can appear as an
apparent singlet or a complex pattern of
peaks
Integral Trace
Signal Splitting; the (n + 1) Rule
• Peak: The units into which an NMR signal is split;
doublet, triplet, quartet, multiplet, etc.
• Signal splitting: Splitting of an NMR signal into a
set of peaks by the influence of neighboring
nonequivalent hydrogens.
• (n + 1) rule: If a hydrogen has n hydrogens
nonequivalent to it but equivalent among
themselves on the same or adjacent atom(s), its
1H-NMR signal is split into (n + 1) peaks.
Spin-Spin Splitting
The Doublet in
1H
NMR
H a Hb
C
B0
C
Ha i s cou ple d to bH
Hb i s parall e l or an ti -parall e l 0to B
Ha spli ts i n to a 1:1
doublet pe ak
Hb in 1,1,2-Tribromoethane
The Triplet in
1H
NMR
H a Hb
C
C
Hb
Ha i s cou ple d to bHan d Hb
B0
Hb can both be parall e l, anti-parall e
or on e paral le l an d on e an ti -paral l
Ha spli ts i n to a 1:2:1
triplet pe ak
Ha in 1,1,2-Tribromoethane
1,1,2-Tribromoethane
The Quartet in 1HMR
C
H
H
C H
H
proton spl its i n to n +1
qu arte t 1:3:3:1
n = # adjacent H's
B0
deshielded
shielded
Chemical Shift
Summary of Signal Splitting
• The origins of signal splitting patterns. Each arrow
represents an Hb nuclear spin orientation.
1,1-Dichloroethane
Ethyl benzene
CH3CH2OCH3
Equivalent Protons do not
Couple
Pascal’s Triangle
Signal Splitting (n + 1)
Problem: Predict the number of 1H-NMR signals
and the splitting pattern of each.
O
(a) CH3 CCH2 CH3
O
(b) CH3 CH2 CCH2 CH3
O
(c) CH3 CCH( CH3 ) 2
Differentiate using 1H NMR
Methyl Isopropyl Ketone
1-Nitropropane
Para Nitrotoluene
Coupling Constants (J values)
Bromoethane
Coupling Constants
– An important factor in vicinal coupling is the angle a
between the C-H sigma bonds and whether or not it is
fixed.
– Coupling is a maximum when a is 0° and 180°; it is a
minimum when a is 90°.
Physical Basis for (n + 1) Rule
• Coupling of nuclear spins is mediated through
intervening bonds.
– H atoms with more than three bonds between them
generally do not exhibit coupling.
– For H atoms three bonds apart, the coupling is called
vicinal coupling.
Physical Basis for (n + 1) Rule
• Coupling that arises when Hb is split by two
different nonequivalent H atoms, Ha and Hc.
More Complex Splitting Patterns
– Complex coupling that arises when Hb is split
by Ha and two equivalent atoms Hc.
More Complex Splitting Patterns
– Since the angle between C-H bond determines the extent of
coupling, bond rotation is a key parameter.
– In molecules with free rotation about C-C sigma bonds, H
atoms bonded to the same carbon in CH3 and CH2 groups are
equivalent.
– If there is restricted rotation, as in alkenes and cyclic structures,
H atoms bonded to the same carbon may not be equivalent.
– Nonequivalent H on the same carbon will couple and cause
signal splitting.
– This type of coupling is called geminal coupling.
More Complex Splitting Patterns
– In ethyl propenoate, an unsymmetrical terminal
alkene, the three vinylic hydrogens are
nonequivalent.
More Complex Splitting Patterns
– Tree diagram for the complex coupling seen for the
three alkenyl H atoms in ethyl propenoate.
More Complex Splitting Patterns
– Cyclic structures often have restricted rotation about
their C-C bonds and have constrained conformations.
– As a result, two H atoms on a CH2 group can be
nonequivalent, leading to complex splitting.
More Complex Splitting Patterns
– A tree diagram for the complex coupling seen for the
vinyl group and the oxirane ring H atoms of 2methyl-2-vinyloxirane.
More Complex Splitting Patterns
• Complex coupling in flexible molecules.
– Coupling in molecules with unrestricted bond rotation
often gives only m + n + I peaks.
– That is, the number of peaks for a signal is the number
of adjacent hydrogens + 1, no matter how many
different sets of equivalent H atoms that represents.
– The explanation is that bond rotation averages the
coupling constants throughout molecules with freely
rotation bonds and tends to make them similar; for
example in the 6- to 8-Hz range for H atoms on freely
rotating sp3 hybridized C atoms.
More Complex Splitting Patterns
– Simplification of signal splitting occurs when
coupling constants are the same.
More Complex Splitting Patterns
– Peak overlap occurs in the spectrum of 1-chloro-3iodopropane.
– Hc should show 9 peaks, but because Jab and Jbc are so
similar, only 4 + 1 = 5 peaks are distinguishable.
Styrene
Ha splitting in Styrene
“Tree” Diagram
Hc
Ha
C
C
Hb
In the system below, Hb is split by two
different sets of hydrogens : Ha and Hc
– Theortically Hb could be split into a triplet of quartets
(12 peaks) but this complexity is rarely seen in
aliphatic systems
Why go to a higher field strength?
d0.50
d0.75
60 MHz
300
240
180
120
60
0 Hz
100 MHz
300
240
180
120
60
0 Hz
300 MHz
300
240
180
120
60
0 Hz
d0.50 (t, 2H, J=10)
d0.75 (t, 2H, J=10)
60 MHz
300
240
180
120
60
0 Hz
100 MHz
300
240
180
120
60
0 Hz
300 MHz
300
240
180
120
60
0 Hz
Homonuclear Decoupling
Allyl Bromide
C H2=C HC H2Br
C H2=C HC H2Br
irradiate
1-Phenyl-1,2-dihydroxyethane
H a Hb
C C Hc
OH OH
1-Phenyl-1,2-dihydroxyethane
Irradiate at d 4.8