Organic
Chemistry
William H. Brown
Christopher S. Foote
Brent L. Iverson
13-1
Nuclear Magnetic
Resonance
Chapter 13
13-2
Molecular Spectroscopy
 Nuclear
magnetic resonance (NMR)
spectroscopy: a spectroscopic technique that
gives us information about the number and types
of atoms in a molecule, for example, about the
number and types of
• hydrogen atoms using 1H-NMR spectroscopy
• carbon atoms using 13C-NMR spectroscopy
• phosphorus atoms using 31P-NMR spectroscopy
13-3
Nuclear Spin States
 An
electron has a spin quantum number of 1/2
with allowed values of +1/2 and -1/2
• this spinning charge creates an associated magnetic
field
• in effect, an electron behaves as if it is a tiny bar
magnet and has what is called a magnetic moment
 The
same effect holds for certain atomic nuclei
• any atomic nucleus that has an odd mass number, an
odd atomic number, or both also has a spin and a
resulting nuclear magnetic moment
• the allowed nuclear spin states are determined by the
spin quantum number, I, of the nucleus
13-4
Nuclear Spin States
• a nucleus with spin quantum number I has 2I + 1 spin
states; if I = 1/2, there are two allowed spin states
• Table 13.1 gives the spin quantum numbers and
allowed nuclear spin states for selected isotopes of
elements common to organic compounds
Element
Nuclear spin
quantum
number (I )
Number of
spin states
1
H
2
H
12
C
13
C
14
N
16
O
31
P
32
S
1/2
1
0
1/2
1
0
1/2
0
2
3
1
2
3
1
2
1
13-5
Nuclear Spins in B0
• within a collection of 1H and 13C atoms, nuclear spins
are completely random in orientation
• when placed in a strong external magnetic field of
strength B0, however, interaction between nuclear
spins and the applied magnetic field is quantized, with
the result that only certain orientations of nuclear
magnetic moments are allowed
13-6
Nuclear Spins in B0
• for 1H and 13C, only two orientations are allowed
13-7
Nuclear Spins in B0
 In
an applied field strength of 7.05T, which is
readily available with present-day
superconducting electromagnets, the difference
in energy between nuclear spin states for
• 1H is approximately 0.120 J (0.0286 cal)/mol, which
corresponds to electromagnetic radiation of 300 MHz
(300,000,000 Hz)
• 13C is approximately 0.030 J (0.00715 cal)/mol, which
corresponds to electromagnetic radiation of 75MHz
(75,000,000 Hz)
13-8
Nuclear Spin in B0
• the energy difference between allowed spin states
increases linearly with applied field strength
• values shown here are for 1H nuclei
13-9
Nuclear Magnetic Resonance
• when nuclei with a spin quantum number of 1/2 are
placed in an applied field, a small majority of nuclear
spins are aligned with the applied field in the lower
energy state
• the nucleus begins to precess and traces out a coneshaped surface, in much the same way a spinning top
or gyroscope traces out a cone-shaped surface as it
precesses in the earth’s gravitational field
• we express the rate of precession as a frequency in
hertz
13-10
Nuclear Magnetic Resonance
 If
the precessing nucleus is irradiated with
electromagnetic radiation of the same frequency
as the rate of precession,
• the two frequencies couple,
• energy is absorbed, and
• the nuclear spin is flipped from spin state +1/2 (with
the applied field) to -1/2 (against the applied field)
13-11
Nuclear Magnetic Resonance
• Figure 13.3 the origin of nuclear magnetic “resonance
13-12
Nuclear Magnetic Resonance
 Resonance:
in NMR spectroscopy, resonance is
the absorption of electromagnetic radiation by a
precessing nucleus and the resulting “flip” of its
nuclear spin from a lower energy state to a
higher energy state
 The instrument used to detect this coupling of
precession frequency and electromagnetic
radiation records it as a signal
• signal: a recording in an NMR spectrum of a nuclear
magnetic resonance
13-13
Nuclear Magnetic Resonance
• if we were dealing with 1H nuclei isolated from all other
atoms and electrons, any combination of applied field
and radiation that produces a signal for one 1H would
produce a signal for all 1H. The same is true of 13C
nuclei
• but hydrogens in organic molecules are not isolated
from all other atoms; they are surrounded by
electrons, which are caused to circulate by the
presence of the applied field
• the circulation of electrons around a nucleus in an
applied field is called diamagnetic current and the
nuclear shielding resulting from it is called
diamagnetic shielding
13-14
Nuclear Magnetic Resonance
• the difference in resonance frequencies among the
various hydrogen nuclei within a molecule due to
shielding/deshielding is generally very small
• the difference in resonance frequencies for hydrogens
in CH3Cl compared to CH3F under an applied field of
7.05T is only 360 Hz, which is 1.2 parts per million
(ppm) compared with the irradiating frequency
360 Hz
6
300 x 10 Hz
=
1.2
6
10
= 1.2 ppm
13-15
Nuclear Magnetic Resonance
• signals are measured relative to the signal of the
reference compound tetramethylsilane (TMS)
CH3
CH3
Si CH3
CH3
Tetramethylsilane (TMS)
• for a 1H-NMR spectrum, signals are reported by their
shift from the 12 H signal in TMS
• for a 13C-NMR spectrum, signals are reported by their
shift from the 4 C signal in TMS
• Chemical shift (): the shift in ppm of an NMR signal
from the signal of TMS
13-16
NMR Spectrometer
13-17
NMR Spectrometer
 Essentials
of an NMR spectrometer are a
powerful magnet, a radio-frequency generator,
and a radio-frequency detector
 The sample is dissolved in a solvent, most
commonly CDCl3 or D2O, and placed in a sample
tube which is then suspended in the magnetic
field and set spinning
 Using a Fourier transform NMR (FT-NMR)
spectrometer, a spectrum can be recorded in
about 2 seconds
13-18
NMR Spectrum
 1H-NMR
spectrum of methyl acetate
• Downfield: the shift of an NMR signal to the left on the
chart paper
• Upfield: the shift of an NMR signal to the right on the
chart paper
13-19
Equivalent Hydrogens
 Equivalent
hydrogens: have the same chemical
environment
• a molecule with 1 set of equivalent hydrogens gives 1
NMR signal
O
CH3 CCH3
ClCH 2 CH2 Cl
C
H3 C
Propanone
(Acetone)
CH3
H3 C
1,2-Dichloroethane
Cyclopentane
C
CH3
2,3-Dimethyl2-butene
13-20
Equivalent Hydrogens
• a molecule with 2 or more sets of equivalent
hydrogens gives a different NMR signal for each set
Cl
CH3 CHCl
1,1-Dichloroethane
(2 signals)
Cl
O
Cyclopentanone
(2 signals)
CH3
C C
H
H
(Z)-1-Chloropropene
(3 signals)
Cyclohexene
(3 signals)
13-21
Signal Areas
 Relative
areas of signals are proportional to the
number of H giving rise to each signal
 Modern NMR spectrometers electronically
integrate and record the relative area of each
signal
13-22
Chemical
Shifts
1H-NMR
Type of
Hydrogen
(CH3 ) 4 Si
RCH3
RCH2 R
R3 CH
R2 C=CRCHR2
RC CH
ArCH3
ArCH2 R
ROH
RCH2 OH
RCH2 OR
R2 NH
O
RCCH3
O
RCCH2 R
Chemical
Shift ()
0 (by definition)
0.8-1.0
1.2-1.4
1.4-1.7
1.6-2.6
2.0-3.0
2.2-2.5
2.3-2.8
0.5-6.0
3.4-4.0
3.3-4.0
0.5-5.0
2.1-2.3
2.2-2.6
Type of
Hydrogen
O
RCOCH3
O
RCOCH2 R
RCH2 I
RCH2 Br
RCH2 Cl
RCH2 F
ArOH
R2 C=CH2
R2 C=CHR
ArH
O
RCH
O
RCOH
Chemical
Shift ()
3.7-3.9
4.1-4.7
3.1-3.3
3.4-3.6
3.6-3.8
4.4-4.5
4.5-4.7
4.6-5.0
5.0-5.7
6.5-8.5
9.5-10.1
10-13
13-23
Chemical Shift - 1H-NMR
13-24
Chemical Shift


Depends on (1) electronegativity of nearby atoms, (2) the
hybridization of adjacent atoms, and (3) diamagnetic
effects from adjacent pi bonds
Electronegativity
Electronegativity of X
Chemical
Shift ()
CH3 OH
4.0
3.5
4.26
3.47
CH3 Cl
3.1
3.05
CH3 Br
CH3 I
2.8
2.5
2.68
(CH3 ) 4 C
(CH3 ) 4 Si
2.1
0.86
1.8
0.00
CH3 -X
CH3 F
2.16
13-25
Chemical Shift
 Hybridization
of adjacent atoms
Type of Hydrogen
(R = alkyl)
Name of
Hydrogen
Chemical
Shift ()
RCH3 , R2 CH2 , R3 CH
Alkyl
0.8 - 1.7
R2 C=C(R)CHR2
Allylic
1.6 - 2.6
RC CH
Acetylenic
2.0 - 3.0
R2 C=CHR, R2 C=CH2
Vinylic
4.6 - 5.7
RCHO
Aldehydic
9.5-10.1
13-26
Chemical Shift
 Diamagnetic
effects of pi bonds
• a carbon-carbon triple bond shields an acetylenic
hydrogen and shifts its signal upfield (to the right) to a
smaller  value
• a carbon-carbon double bond deshields vinylic
hydrogens and shifts their signal downfield (to the left)
to a larger  value
Type of H
RCH3
Name
Alkyl
RC CH
R2 C=CH2
Acetylenic
Vinylic
Chemical
Shift ()
0.8- 1.0
2.0 - 3.0
4.6 - 5.7
13-27
Chemical Shift
• magnetic induction in the pi bonds of a carbon-carbon
triple bond (Fig 13.9)
13-28
Chemical Shift
• magnetic induction in the pi bond of a carbon-carbon
double bond (Fig 13.10)
13-29
Chemical Shift
• magnetic induction of the pi electrons in an aromatic
ring (Fig. 13.11)
13-30
Signal Splitting; the (n + 1) Rule
 Peak:
the units into which an NMR signal is split;
doublet, triplet, quartet, 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
13-31
Signal Splitting (n + 1)
• 1H-NMR spectrum of 1,1-dichloroethane
For these hydrogens, n = 1;
their signal is split into
(1 + 1) = 2 peaks; a doublet
CH3 -CH-Cl
Cl
For this hydrogen, n = 3;
its signal is split into
(3 + 1) = 4 peaks; a quartet
13-32
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
13-33
Origins of Signal Splitting
 Signal
coupling: an interaction in which the
nuclear spins of adjacent atoms influence each
other and lead to the splitting of NMR signals
 Coupling constant (J): the separation on an NMR
spectrum (in hertz) between adjacent peaks in a
multiplet;
• a quantitative measure of the influence of the spin-spin
coupling with adjacent nuclei
13-34
Origins of Signal Splitting
13-35
Origins of Signal Splitting
• because splitting patterns from spectra taken at 300
MHz and higher are often difficult to see, it is common
to retrace certain signals in expanded form
• 1H-NMR spectrum of 3-pentanone; scale expansion
shows the triplet quartet pattern more clearly
13-36
Coupling Constants

Coupling constant (J): the distance between peaks in a
split signal, expressed in hertz
• the value is a quantitative measure of the magnetic
interaction of nuclei whose spins are coupled
Ha
Ha Hb
C C
6-8 Hz
C
C
C
Hb
11-18 Hz
0-5 Hz
Hb
Ha
C
H
Hb a
Hb
Hb
8-14 Hz
Ha
Ha
5-10 Hz
C
0-5 Hz
Ha
Ha
Hb
Hb
C
0-5 Hz
8-11 Hz
13-37
Origins of Signal Splitting
13-38
Signal Splitting
 Pascal’s
Triangle
• as illustrated by the
highlighted entries,
each entry is the sum
of the values
immediately above it to
the left and the right
13-39
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 noticeable coupling
• for H atoms three bonds apart, the coupling is referred
to as vicinal coupling
13-40
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°
13-41
More Complex Splitting Patterns
• thus far, we have concentrated on spin-spin coupling
with only one other nonequivalent set of H atoms
• more complex splittings arise when a set of H atoms
couples to more than one set H atoms
• a tree diagram shows that when Hb is adjacent to
nonequivalent Ha on one side and Hc on the other, the
resulting coupling gives rise to a doublet of doublets
13-42
More Complex Splitting Patterns
• if Hc is a set of two equivalent H, then the observed
splitting is a doublet of triplets
13-43
More Complex Splitting Patterns
• because the angle between C-H bond determines the
extent of coupling, bond rotation is a key parameter
• in molecules with relatively free rotation about C-C
sigma bonds, H atoms bonded to the same carbon in
CH3 and CH2 groups generally 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
13-44
More Complex Splitting Patterns
• in ethyl propenoate, an unsymmetrical terminal alkene,
the three vinylic hydrogens are nonequivalent
13-45
More Complex Splitting Patterns
• a tree diagram for the complex coupling of the three
vinylic hydrogens in ethyl propenoate
13-46
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
13-47
More Complex Splitting Patterns
• a tree diagram for the complex coupling in 2-methyl-2vinyloxirane
13-48
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
13-49
More Complex Splitting Patterns
• simplification of signal splitting occurs when coupling
constants are the same
13-50
More Complex Splitting Patterns
• an example of peak overlap occurs in the spectrum of
1-chloro-3-iodopropane
• the central CH2 has the possibility for 9 peaks (a triplet
of triplets) but because Jab and Jbc are so similar, only
4 + 1 = 5 peaks are distinguishable
13-51
Stereochemistry & Topicity
 Depending
on the symmetry of a molecule,
otherwise equivalent hydrogens may be
• homotopic
• enantiotopic
• diastereotopic
 The
simplest way to visualize topicity is to
substitute an atom or group by an isotope; is the
resulting compound
• the same as its mirror image
• different from its mirror image
• are diastereomers possible
13-52
Stereochemistry & Topicity
 Homotopic
H
C
H
Cl
Cl
Dichloromethane
(achiral)
atoms or groups
Substitute
one H by D
H
Substitution does not
produce a stereocenter;
C
Cl therefore hydrogens
D
are homotopic.
Achiral
Cl
• homotopic atoms or groups have identical chemical
shifts under all conditions
13-53
Stereochemistry & Topicity
 Enantiotopic
H
Cl
C
H
F
Chlorofluoromethane
(achiral)
groups
Substitute
one H by D
Substitution produces a
H
stereocenter;
Cl therefore, hydrogens are
C
F enantiotopic. Both
hydrogens are prochiral;
D
one is pro-R-chiral, the
Chiral
other is pro-S-chiral.
• enantiotopic atoms or groups have identical chemical
shifts in achiral environments
• they have different chemical shifts in chiral
environments
13-54
Stereochemistry & Topicity
 Diastereotopic
groups
• H atoms on C-3 of 2-butanol are diastereotopic
• substitution by deuterium creates a chiral center
• because there is already a chiral center in the
molecule, diastereomers are now possible
H
OH
H
H
2-Butanol
(chiral)
Substitute one
H on CH2 by D
H
OH
H
D
Chiral
• diastereotopic hydrogens have different chemical
shifts under all conditions
13-55
Stereochemistry & Topicity
 The
methyl groups on carbon 3 of 3-methyl-2butanol are diastereotopic
• if a methyl hydrogen of carbon 4 is substituted by
deuterium, a new chiral center is created
• because there is already one chiral center,
diastereomers are now possible
OH
3-Methyl-2-butanol
• protons of the methyl groups on carbon 3 have
different chemical shifts
13-56
Stereochemistry and Topicity
 1H-NMR
spectrum of 3-methyl-2-butanol
• the methyl groups on carbon 3 are diastereotopic and
appear as two doublets
13-57
13C-NMR
 Each
Spectroscopy
nonequivalent
13C
gives a different signal
• a 13C signal is split by the 1H bonded to it according to
the (n + 1) rule
• coupling constants of 100-250 Hz are common, which
means that there is often significant overlap between
signals, and splitting patterns can be very difficult to
determine
most common mode of operation of a 13CNMR spectrometer is a hydrogen-decoupled
mode
 The
13-58
13C-NMR
Spectroscopy
 In
a hydrogen-decoupled mode, a sample is
irradiated with two different radio frequencies
• one to excite all 13C nuclei
• a second broad spectrum of frequencies to cause all
hydrogens in the molecule to undergo rapid
transitions between their nuclear spin states
the time scale of a 13C-NMR spectrum, each
hydrogen is in an average or effectively constant
nuclear spin state, with the result that 1H-13C
spin-spin interactions are not observed; they are
decoupled
 On
13-59
13C-NMR
Spectroscopy
• hydrogen-decoupled 13C-NMR spectrum of 1bromobutane
13-60
Chemical Shift - 13C-NMR
Type of
Carbon
Chemical
Shift ()
RCH3
RCH2 R
10-40
15-55
20-60
R3 CH
0-40
RCH2 I
RCH2 Br
25-65
RCH2 Cl
35-80
R3 COH
40-80
R3 COR
40-80
65-85
RC CR
R2 C=CR2
100-150
Type of
Carbon
C R
Chemical
Shift ()
110-160
O
RCOR
160 - 180
O
RCNR2
165 - 180
O
RCCOH
165 - 185
O
O
RCH, RCR
180 - 215
13-61
Chemical Shift - 13C-NMR
13-62
The DEPT Method
 In
the hydrogen-decoupled mode, information on
spin-spin coupling between 13C and hydrogens
bonded to it is lost
 The DEPT method is an instrumental mode that
provides a way to acquire this information
• Distortionless Enhancement by Polarization Transfer
(DEPT): an NMR technique for distinguishing among
13C signals for CH , CH , CH, and quaternary carbons
3
2
13-63
The DEPT Method
 The
DEPT methods uses a complex series of
pulses in both the 1H and 13C ranges, with the
result that CH3, CH2, and CH signals exhibit
different phases;
• signals for CH3 and CH carbons are recorded as
positive signals
• signals for CH2 carbons are recorded as negative
signals
• quaternary carbons give no signal in the DEPT method
13-64
Isopentyl acetate
• 13C-NMR: (a) proton decoupled and (b) DEPT
13-65
Interpreting NMR Spectra
 Alkanes
• 1H-NMR signals appear in the range of  0.8-1.7
• 13C-NMR signals appear in the considerably wider
range of  10-60
 Alkenes
• 1H-NMR signals appear in the range  4.6-5.7
• 1H-NMR coupling constants are generally larger for
trans vinylic hydrogens (J= 11-18 Hz) compared with
cis vinylic hydrogens (J= 5-10 Hz)
• 13C-NMR signals for sp2 hybridized carbons appear in
the range  100-160, which is downfield from the
signals of sp3 hybridized carbons
13-66
Interpreting NMR Spectra
• 1H-NMR spectrum of vinyl acetate (Fig 13.33)
13-67
Interpreting NMR Spectra
 Alcohols
 1H-NMR
O-H chemical shifts often appears in the
range  3.0-4.0, but may be as low as  0.5.
• 1H-NMR chemical shifts of hydrogens on the carbon
bearing the -OH group are deshielded by the electronwithdrawing inductive effect of the oxygen and appear
in the range  3.0-4.0
 Ethers
• a distinctive feature in the 1H-MNR spectra of ethers is
the chemical shift,  3.3-4.0, of hydrogens on carbon
attached to the ether oxygen
13-68
Interpreting NMR Spectra
• 1H-NMR spectrum of 1-propanol (Fig. 13.34)
13-69
Interpreting NMR Spectra
 Aldehydes
and ketones
• 1H-NMR: aldehyde hydrogens appear at  9.5-10.1
• 1H-NMR: a-hydrogens of aldehydes and ketones
appear at  2.2-2.6
• 13C-NMR: carbonyl carbons appear at  180-215
 Amines
• 1H-NMR: amine hydrogens appear at  0.5-5.0
depending on conditions
13-70
Interpreting NMR Spectra
 Carboxylic
acids
• 1H-NMR: carboxyl hydrogens appear at  10-13, lower
than most any other hydrogens
• 13C-NMR: carboxyl carbons in acids and esters appear
at  160-180
13-71
Interpreting NMR Spectra
 Spectral
Problem 1; molecular formula C5H10O
13-72
Interpreting NMR Spectra
 Spectral
Problem 2; molecular formula C7H14O
13-73
Nuclear
Magnetic
Resonance
End Chapter 13
13-74