Origins of Signal Splitting
1H-NMR
signals are split into multiple peaks when
molecules containing nonequivalent hydrogen atoms
that are separated by no more than three bonds
‹ Signal
Si
l
coupling:
coupling
li : an interaction
i t
ti
i which
in
hi h the
th
nuclear spins of adjacent atoms influence each
other and lead to the splitting of NMR signals
‹ Coupling constant (J)
(J):: the separation on an
NMR spectrum (in hertz) between adjacent
peaks in a multiplet;
• a quantitative measure of the influence of the spinspin coupling with adjacent nuclei
16--1
16
Origins of Signal Splitting
16--2
16
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
16--3
16
Coupling Constants
‹
Coupling constant (J): the distance between peaks in a
split signal, expressed in hertz
• the
th value
l is
i a quantitative
tit ti measure off the
th magnetic
ti
interaction of nuclei whose spins are coupled
Ha
Ha Hb
C C
6-8 Hz
C
C
C
Hb
11 18 H
11-18
Hz
0-5 Hz
Hb
Ha
C
H
Hb a
Hb
Hb
8-14 Hz
Ha
Ha
5 10 H
5-10
Hz
C
0-5 Hz
Ha
Ha
Hb
Hb
C
0 5 Hz
0-5
8 11 H
8-11
Hz
16--4
16
Origins of Signal Splitting
16--5
16
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
Careful with electronic noise
that prevents distinguishing
the smaller side peaks for
a particular H with a signal
of low intensity
16--6
16
Physical Basis for (n
(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
apart, the coupling is referred
to as vicinal coupling
16--7
16
Coupling Constants
• an important factor in vicinal coupling is the angle α
between the C-H sigma bonds and whether or not it is
fixed
• coupling is a maximum when α is 0° and 180°; it is a
minimum when α is 90°
Bonds that rotate rapidly at RT do not have a fixed angle between adjacent
C-H bonds: average angle and average coupling are observed
16--8
16
More Complex Splitting Patterns
• thus far, we have concentrated on spin-spin coupling
with only one other nonequivalent set of H atoms
• more complex
l splittings
litti
arise
i when
h a sett off H atoms
t
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
16--9
16
More Complex Splitting Patterns
• if Hc is a set of two equivalent H, then the observed splitting is a
doublet of triplets
In the g
general case,, a signal
g
will be split
p into (n
( + 1)) x ((m + 1)) p
peaks for
an H atom coupled to a set of n H atoms with one coupling constant,
and to a set of m H atoms with another coupling constant
16--10
16
More Complex Splitting Patterns
• because the angle between C-H bond determines the
extent of coupling, bond rotation is a key parameter
• in
i molecules
l
l with
ith relatively
l ti l free
f
rotation
t ti about
b t C-C
CC
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
g
splitting
p
g
• this type of coupling is called geminal coupling (J = 00-5
Hz)
16--11
16
More Complex Splitting Patterns
• in ethyl propenoate, an unsymmetrical terminal alkene,
the three vinylic hydrogens are nonequivalent
16--12
16
More Complex Splitting Patterns
• a tree diagram for the complex coupling of the three
vinylic hydrogens in ethyl propenoate
16--13
16
More Complex Splitting Patterns
• cyclic structures often have restricted rotation about
their C-C bonds and have constrained conformations
• as a result,
lt two
t
H atoms
t
on a CH2 group can be
b
nonequivalent, leading to complex splitting
16--14
16
More Complex Splitting Patterns
• a tree diagram for the complex coupling in 2-methyl-2vinyloxirane
16--15
16
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
b off adjacent
dj
h d
hydrogens
+ 1,
1 no matter how
h
many different sets of equivalent H atoms that
p
represents
• the explanation is that bond rotation averages the
coupling constants throughout molecules with freely
rotation bonds and tends to make them similar;
similar for
example in the 6- to 8-Hz range for H atoms on freely
rotating sp3 hybridized C atoms
16--16
16
More Complex Splitting Patterns
• simplification of signal splitting occurs when coupling
constants are the same
16--17
16
More Complex Splitting Patterns
• an example of peak overlap occurs in the spectrum of
1-chloro-3-iodopropane
• the
th central
t l CH2 has
h the
th possibility
ibilit for
f 9 peaks
k (a
( triplet
ti l t
of triplets) but because Jab and Jbc are so similar, only
4 + 1 = 5 peaks are distinguishable
16--18
16
More Complex Splitting Patterns
‹
Fast exchange:
- Hydrogen atoms bonded to oxygen or nitrogen atoms
can exchange with each other faster than the time it takes
to acquire a 1H
H-NMR
NMR spectrum
- Process facilitated by traces of acid or base
- Consequences:
a) Broad signals
b) Signals will disappear when D2O or deuterated
alcohol
l h l is
i added
dd d
Important affected functional groups: carboxylic acids, alcohols,
amines, and amides
16--19
16
Stereochemistry & Topicity
‹ Depending
on the symmetry of a molecule,
otherwise equivalent hydrogens may be
• homotopic
• enantiotopic
• diastereotopic
di t
t i
‹ 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
possible
• are diastereomers p
16--20
16
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
16--21
16
Stereochemistry & Topicity
‹ Enantiotopic
H
Cl
C
H
F
Chlorofluoromethane
(achiral)
groups
Substitute
one H by D
Substitution produces a
H
t
t
stereocenter;
Cl therefore, hydrogens are
C
F enantiotopic. Both
hydrogens
y
g
are prochiral;
p
;
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 (e.g. chiral solvents)
16--22
16
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
molecule,
H
OH
H
H
2 Butanol
2-Butanol
(chiral)
Substitute one
H on CH 2 by D
H
OH
H
D
Chiral
• diastereotopic hydrogens have different chemical
shifts
hift under
d all
ll conditions
diti
(can
(
lead
l d to
t unexpected
t d
complexity in spectra of simple compounds)
16--23
16
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
center,
diastereomers are now possible
OH
3 Methyl 2 butanol
3-Methyl-2-butanol
• protons of the methyl groups on carbon 3 have
different chemical shifts
16--24
16
Stereochemistry and Topicity
‹ 1H-NMR
spectrum of 3-methyl-2-butanol
• the methyl groups on carbon 3 are diastereotopic and
appear as two
t
doublets
d bl t
16--25
16
13C-NMR
Spectroscopy
abundance of 13C (1.1%) results in weak
signals
‹ Magnetic
M
ti momentt off 13C is
i much
h smaller
ll than
th
that of 1H
‹ Each nonequivalent 13C gives a different signal
‹ Low
• 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,
g
, and splitting
p
gp
patterns can be very
y difficult to
determine
most common mode of operation of a 13CNMR spectrometer is a hydrogen-decoupled
hydrogen decoupled
mode
‹ The
16--26
16
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
C-NMR
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
16--27
16
13C-NMR
Spectroscopy
• hydrogen-decoupled 13C-NMR spectrum of 1bromobutane
Caution: integration of signals is not reliable in 13C NMR
spectroscopy (long relaxation times)
16--28
16
Chemical Shift - 13C-NMR
Typ e of
Carbon
Chemical
S hift (δ)
RCH3
RCH2 R
10-40
15-55
20-60
20
60
R3 CH
Type of
Carb on
C R
Chemical
Sh ift ((δ))
110-160
O
RCOR
160 - 180
RCH2 Cl
25-65
35 80
35-80
O
RCNR2
165 - 180
R3 COH
40-80
R3 COR
40-80
65-85
O
RCCOH
165 - 185
O
O
RCH, RCR
180 - 215
RCH2 I
RCH2 Br
RC CR
R2 C=CR2
0-40
100-150
16--29
16
Chemical Shift - 13C-NMR
16--30
16
The DEPT Method
‹ In
the hydrogen-decoupled mode, information on
spin-spin
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):
DEPT): an NMR technique for distinguishing among
13C signals for CH , CH , CH, and quaternary carbons
3
2
16--31
16
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
i
l for
f
CH3 and
d CH carbons
b
are recorded
d d as
positive signals
• s
signals
g a s for
o C
CH2 ca
carbons
bo s a
are
e recorded
eco ded as negative
egat e
signals
• quaternary carbons give no signal in the DEPT
method
th d
16--32
16
Isopentyl acetate
• 13C-NMR: (a) proton decoupled and (b) DEPT
16--33
16
Interpreting NMR Spectra
‹ Alkanes
• 1H-NMR signals appear in the range of δ 0.8-1.7
• 13C-NMR signals appear in the considerably wider
range of δ 10-60
‹ Alkenes
Alk
• 1H-NMR signals appear in the range δ 4.6-5.7
• 1H-NMR
H 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 δ 100-160, which is downfield from the
signals of sp3 hybridized carbons
16--34
16
Interpreting NMR Spectra
• 1H-NMR spectrum of vinyl acetate (Fig 13.31)
16--35
16
Interpreting NMR Spectra
‹ Alcohols
‹ 1H-NMR
O-H chemical shifts often appears
pp
in the
range δ 3.0-4.0, but may be as low as δ 0.5.
• 1H-NMR chemical shifts of hydrogens on the carbon
b i the
bearing
th -OH
OH group are deshielded
d hi ld d by
b the
th electronl t
withdrawing inductive effect of the oxygen and appear
in the range
g δ 3.0-4.0
‹ Ethers
• a distinctive feature in the 1H-MNR spectra of ethers is
the chemical shift, δ 3.3-4.0, of hydrogens on carbon
attached to the ether oxygen
16--36
16
Interpreting NMR Spectra
• 1H-NMR spectrum of 1-propanol (Fig. 13.32)
16--37
16
Interpreting NMR Spectra
‹ Aldehydes
and ketones
• 1H-NMR: aldehyde hydrogens appear at δ 9.5-10.1
• 1H-NMR: α-hydrogens of aldehydes and ketones
appear at δ 2.2-2.6
• 13C-NMR:
C NMR: carbonyl carbons appear at δ 180-215
180 215
‹ Amines
• 1H
H-NMR:
NMR: amine hydrogens appear at δ 0.5-5.0
0550
depending on conditions
16--38
16
Interpreting NMR Spectra
‹ Carboxylic
C b
li
acids
id
• 1H-NMR: carboxyl hydrogens appear at δ 10-13, lower
than most any other hydrogens
• 13C-NMR: carboxyl carbons in acids and esters appear
at δ 160-180
16--39
16
Interpreting NMR Spectra
‹ Spectral
Problem 1; molecular formula C5H10O
16--40
16
Interpreting NMR Spectra
‹ Spectral
Problem 2; molecular formula C7H14O
16--41
16