Transcript Chemical Shift

Chapter 13
Spectroscopy
Introduction
• Spectroscopy is a technique used to
determine the structure of a compound.
• Most techniques are nondestructive
(destroys little or no sample).
• Absorption spectroscopy measures the
amount of light absorbed by the sample
as a function of wavelength.
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Chapter 12
2
Types of Spectroscopy
• Infrared (IR) spectroscopy measures the bond
vibration frequencies in a molecule and is used to
determine the functional group.
• Mass spectrometry (MS) fragments the molecule and
measures the mass. MS can give the molecular weight
of the compound and functional groups.
• Nuclear magnetic resonance (NMR) spectroscopy
analyzes the environment of the hydrogens in a
compound. This gives useful clues as to the alkyl and
other functional groups present.
• Ultraviolet (UV) spectroscopy uses electronic
transitions to determine bonding patterns.
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Chapter 12
3
Electromagnetic Radiation
Is propagated at the speed of light
Has properties of particles and waves
The energy of a photon is proportional
to its frequency
E = h 
Electromagnetic radiation is absorbed when the
energy of photon corresponds to difference in
energy between two states.
The Electromagnetic Spectrum
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Chapter 12
6
Introduction to
1H NMR Spectroscopy
The Nuclei that are Most Useful to
Organic Chemists are:
1H
and 13C
both have spin = ±1/2
1H
is 99% at natural abundance
13C
is 1.1% at natural abundance
Nuclear Spin
+
+
A spinning charge, such as the nucleus of 1H
or 13C, generates a magnetic field. The
magnetic field generated by a nucleus of spin
+1/2 is opposite in direction from that
generated by a nucleus of spin –1/2.
The distribution of
nuclear spins is
random in the
absence of an
external magnetic
field.
+
+
+
+
+
An external magnetic
field causes nuclear
magnetic moments to
align parallel and
antiparallel to applied
field.
+
+
+
B0
+
+
There is a slight
excess of nuclear
magnetic moments
aligned parallel to
the applied field.
+
+
+
B0
+
+
Energy Differences Between Nuclear Spin States
+
E
E '
+
increasing field strength
No difference in absence of magnetic field
Proportional to strength of external magnetic field
Some Important Relationships in NMR
Units
The frequency of absorbed
electromagnetic radiation
is proportional to
the energy difference between
two nuclear spin states
which is proportional to
the applied magnetic field.
Hz
kJ/mol
(kcal/mol)
tesla (T)
Some Important Relationships in NMR
The frequency of absorbed electromagnetic
radiation is different for different elements,
and for different isotopes of the same element.
For a field strength of 4.7 T:
1H absorbs radiation having a frequency
of 200 MHz (200 x 106 s-1)
13C absorbs radiation having a frequency
of 50.4 MHz (50.4 x 106 s-1)
Some Important Relationships in NMR
The frequency of absorbed electromagnetic
radiation for a particular nucleus (such as 1H)
depends on its molecular environment.
This is why NMR is such a useful tool
for structure determination.
Nuclear Shielding
and
1H Chemical Shifts
What do we mean by "shielding"?
What do we mean by "chemical shift"?
Shielding
An external magnetic field
affects the motion of the
electrons in a molecule,
inducing a magnetic field
within the molecule.
The direction of the
induced magnetic field is
opposite to that of the
applied field.
C
H
B0
Shielding
The induced field shields
the nuclei (in this case, C
and H) from the applied
field.
A stronger external field is
needed in order for energy
difference between spin
states to match energy of
rf radiation.
C
H
B0
Chemical Shift
Chemical shift is a
measure of the degree to
which a nucleus in a
molecule is shielded.
Protons in different
environments are shielded
to greater or lesser
degrees; they have
different chemical shifts.
C
H
B0
Chemical Shift
Chemical shifts (d) are
measured relative to the
protons in
tetramethylsilane (TMS)
as a standard.
d =
CH3
H3C
Si
CH3
position of signal - position of TMS peak
spectrometer frequency
CH3
x 106
Downfield
Decreased shielding
Upfield
Increased shielding
(CH3)4Si (TMS)
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
2.0
Chemical shift (d, ppm)
measured relative to TMS
1.0
0
Chemical Shift
Example: The signal for the proton in chloroform
(HCCl3) appears 1456 Hz downfield from TMS at
a spectrometer frequency of 200 MHz.
d =
d =
position of signal - position of TMS peak
spectrometer frequency
1456 Hz - 0 Hz
200 x 106 Hx
d = 7.28
x 106
x 106
Cl
d 7.28 ppm
H
C
Cl
Cl
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
Chemical shift (d, ppm)
2.0
1.0
0
Effects of Molecular Structure
on
1H Chemical Shifts
Protons in different environments experience
different degrees of shielding and have
different chemical shifts.
Electronegative Substituents Decrease
the Shielding of Methyl Groups
least shielded H
CH3F
CH3OCH3
d 4.3
d 3.2
most shielded H
(CH3)3N
d 2.2
CH3CH3
(CH3)4Si
d 0.9
d 0.0
Electronegative Substituents Decrease Shielding
d 0.9
d 1.3
d 0.9
H3C—CH2—CH3
d 4.3
d 2.0
d 1.0
O2N—CH2—CH2—CH3
Effect is Cumulative
CHCl3
CH2Cl2
CH3Cl
d 7.3
d 5.3
d 3.1
Methyl, Methylene, and Methine
CH3 more shielded than CH2 ;
CH2 more shielded than CH
d 0.9
CH3
H3C
C
CH3
H d 1.6
d 0.9
CH3
H3 C
C
CH3
d 1.2
CH2
d 0.8
CH3
Protons Attached to sp2 Hybridized Carbon
are Less Shielded than those Attached
to sp3 Hybridized Carbon
H
H
H
H
H
C
H
H
CH3CH3
C
H
H
H
d 7.3
d 5.3
d 0.9
But Protons Attached to sp Hybridized Carbon
are More Shielded than those Attached
to sp2 Hybridized Carbon
d 5.3
H
H
C
H
C
H
d 2.4
H
C
C
CH2OCH3
Protons Attached to Benzylic and Allylic
Carbons are Somewhat Less Shielded than Usual
H3C
CH3
d 0.8
d 1.5
d 0.9
d 1.3
d 0.9
H3C—CH2—CH3
d 1.2
H3C
d 2.6
CH2
Proton Attached to C=O of Aldehyde
is Most Deshielded C—H
d 2.4
H
H3C
C
O
C
CH3
d 1.1
H d 9.7
Table 13.1
Type of proton
H
H
C
C
R
C
Chemical shift (d), Type of proton
ppm
0.9-1.8
C
C
H
C
C
H
C
Ar
N
2.1-2.3
C 1.5-2.6
O
H
Chemical shift (d),
ppm
2.0-2.5
2.3-2.8
Table 13.1
Type of proton
H
C
NR
Chemical shift (d), Type of proton
ppm
2.2-2.9
H
C
H
H
C
C
Cl
Br
Chemical shift (d),
ppm
C
4.5-6.5
3.1-4.1
2.7-4.1
H
Ar
6.5-8.5
O
H
C
O
3.3-3.7
H
C
9-10
Table 13.1
Type of proton
Chemical shift (d),
ppm
H
NR
1-3
H
OR
0.5-5
H
OAr
6-8
O
HO
C
10-13
Interpreting 1H NMR
Spectra
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Information Contained in an NMR
Spectrum Includes:
1. Number of signals
2. Their intensity (as measured by area
under peak)
3. Splitting pattern (multiplicity)
Number of Signals
Protons that have different chemical shifts
are chemically nonequivalent.
They exist in different molecular environments.
N
Figure 13.12
CCH2OCH3
OCH3
NCCH2O
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
Chemical shift (d, ppm)
2.0
1.0
0
Chemically Equivalent Protons
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
Replacing protons at C-1 and C-3 gives same
compound (1-chloropropane).
C-1 and C-3 protons are chemically
equivalent and have the same chemical shift.
ClCH2CH2CH3
CH3CH2CH2Cl
H3CCH2CH3
chemically equivalent
Diastereotopic Protons
Replacement by some arbitrary test group
generates diastereomers.
Diastereotopic protons can have different
chemical shifts.
Br
C
H3C
H
d 5.3 ppm
H
d 5.5 ppm
C
Enantiotopic Protons
Are in mirror-image environments.
Replacement by some arbitrary test group
generates enantiomers.
Enantiotopic protons have the same
chemical shift.
Enantiotopic
Protons
H
H
C
CH2OH
H3C
H
Cl
Cl
H
C
H3C
CH2OH
R
C
H3C
CH2OH
S
13.7
Spin-Spin Splitting
in
1H NMR Spectroscopy
Not all peaks are singlets.
Signals can be split by coupling of
nuclear spins.
Cl2CHCH3
Figure 13.13
4 lines;
quartet
2 lines;
doublet
CH3
CH
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
Chemical shift (d, ppm)
2.0
1.0
0
Two-bond and Three-bond Coupling
H
C
H
C
C
C
H
H
protons separated by
two bonds
(geminal relationship)
protons separated by
three bonds
(vicinal relationship)
Two-bond and Three-bond Coupling
H
C
H
C
C
C
H
H
In order to observe splitting, protons cannot
have same chemical shift.
Coupling constant (2J or 3J) is independent
of field strength.
Figure 13.13
4 lines;
quartet
Cl2CHCH3
2 lines;
doublet
CH
CH3
coupled protons are vicinal (three-bond coupling)
10.0 9.0
8.0
7.0
6.0
5.0
4.0
3.0 2.0
1.0
CH splits CH3 into a doublet
CH3 splits CH into aChemical
quartet shift (d, ppm)
0
Why Do the Methyl Protons of
1,1-Dichloroethane Appear as a Doublet?
Cl
H
H
C
C
Cl
H
signal for methyl
H 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.
Why Do the Methyl Protons of
1,1-Dichloroethane Appear as a Doublet?
Cl
H
H
C
C
Cl
H
signal for methyl
H 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 C2 protons.
Why Do the Methyl Protons of
1,1-Dichloroethane Appear as a Doublet?
Cl
H
H
C
C
Cl
H
signal for methyl
H 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.
Why Do the Methyl Protons of
1,1-Dichloroethane Appear as a Doublet?
H
Cl
H
C
C
Cl
H
H
"true" chemical
shift of methyl
protons (no coupling)
This line corresponds
to molecules in which
This line corresponds
to molecules in which
the nuclear spin of
the proton at C-1
the nuclear spin of
the proton at C-1
reinforces
the applied field.
opposes
the applied field.
Why Does the Methine Proton of
1,1-Dichloroethane Appear as a Quartet?
signal for methine
proton is split into
a quartet
H
Cl
H
C
C
Cl
H
H
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.
Why Does the Methine Proton of
1,1-Dichloroethane Appear as a Quartet?
H
Cl
H
C
C
Cl
H
There are eight combinations of
nuclear spins for the three methyl
H protons.
These 8 combinations split the
signal into a 1:3:3:1 quartet.
The Splitting Rule for 1H NMR
For simple cases, the multiplicity of a signal
for a particular proton is equal to the number
of equivalent vicinal protons + 1.
Splitting Patterns:
The Ethyl Group
CH3CH2X is characterized by a triplet-quartet
pattern (quartet at lower field than the triplet).
BrCH2CH3
Figure 13.16
4 lines;
quartet
3 lines;
triplet
CH3
CH2
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
Chemical shift (d, ppm)
2.0
1.0
0
Table 13.2
Splitting Patterns of Common Multiplets
Number of equivalent
protons to which H
is coupled
Appearance Intensities of lines
of multiplet in multiplet
1
Doublet
1:1
2
3
4
5
6
Triplet
Quartet
Pentet
Sextet
Septet
1:2:1
1:3:3:1
1:4:6:4:1
1:5:10:10:5:1
1:6:15:20:15:6:1
Splitting Patterns:
The Isopropyl Group
(CH3)2CHX is characterized by a doubletseptet pattern (septet at lower field than the
doublet).
ClCH(CH3)2
Figure 13.18
2 lines;
doublet
7 lines;
septet
CH3
CH
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
Chemical shift (d, ppm)
2.0
1.0
0
Splitting Patterns:
Pairs of Doublets
Splitting patterns are not always symmetrical,
but lean in one direction or the other.
Pairs of Doublets
H
C
C
H
Consider coupling between two vicinal
protons.
If the protons have different chemical shifts,
each will split the signal of the other into a
doublet.
Pairs of Doublets
H
C
C
H
Let  be the difference in chemical shift in Hz
between the two hydrogens.
Let J be the coupling constant between them
in Hz.
AX
H
C
C
J
H
J

When  is much larger than J the signal for
each proton is a doublet, the doublet is
symmetrical, and the spin system is called
AX.
AM
H
C
C
J
H
J

As /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.
AB
H
C
H
C
J
J

When  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.
A2
H
C
C
H
When  = 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.
H
Figure 13.20
skewed doublets
H
OCH3
Cl
H
H
OCH3
10.0
9.0
8.0
7.0
6.0
5.0
4.0
3.0
Chemical shift (d, ppm)
2.0
1.0
0
Complex Splitting Patterns
Multiplets of multiplets
m-Nitrostyrene
H
H
O2N
H
Consider the proton shown in red.
It is unequally coupled to the protons shown
in blue and green.
Jcis = 12 Hz; Jtrans = 16 Hz
H
m-Nitrostyrene
H
O2N
The signal for
the proton
shown in red
appears as a
doublet of
doublets.
16 Hz
12 Hz
H
12 Hz
Figure 13.21
H
doublet doublet
H
H
O2N
doublet of doublets
1H
NMR Spectra of Alcohols
What about H bonded to O?
O—H
H
C
O
H
The chemical shift for O—H is variable (d 0.55 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. The
O—H peak disappears.
NMR and Conformations
NMR is “Slow”
Most conformational changes occur faster
than NMR can detect them.
An NMR spectrum shows the weighted
average of the conformations.
For example: Cyclohexane gives a single
peak for its H atoms in NMR. Half of the time
a single proton is axial and half of the time it is
equatorial. The observed chemical shift is
halfway between the axial chemical shift and
the equatorial chemical shift.
13C
NMR Spectroscopy
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
1H
and 13C NMR Compared:
Both give us information about the number of
chemically nonequivalent nuclei
(nonequivalent hydrogens or nonequivalent
carbons).
Both give us information about the
environment of the nuclei (hybridization state,
attached atoms, etc.).
It is convenient to use FT-NMR techniques for
1H; it is standard practice for 13C NMR.
1H
and 13C NMR Compared:
13C
requires FT-NMR because the signal for a
carbon atom is 10-4 times weaker than the
signal for a hydrogen atom.
A signal for a 13C nucleus is only about 1% as
intense as that for 1H because of the
magnetic properties of the nuclei, and
at the "natural abundance" level only 1.1% of
all the C atoms in a sample are 13C (most are
12C).
1H
and 13C NMR Compared:
13C
signals are spread over a much wider
range than 1H signals, making it easier to
identify and count individual nuclei.
Figure 13.26 (a) shows the 1H NMR spectrum
of 1-chloropentane; Figure 13.26 (b) shows
the 13C spectrum. It is much easier to identify
the compound as 1-chloropentane by its 13C
spectrum than by its 1H spectrum.
1H
Figure 13.26(a)
ClCH2CH2CH2CH2CH3
10.0
9.0
8.0
7.0
6.0
CH3
ClCH2
5.0
4.0
3.0
Chemical shift (d, ppm)
2.0
1.0
0
13C
Figure 13.26(b)
ClCH2CH2CH2CH2CH3
A separate,
distinct peak
appears for each
of the 5 carbons.
200
180
160
140
CDCl3
120
100
80
60
Chemical shift (d, ppm)
40
20
0
13C
Chemical Shifts
are measured in ppm (d)
from the carbons of TMS
13C
Chemical Shifts are Most Affected By:
• Electronegativity of groups attached to carbon
• Hybridization state of carbon
Electronegativity Effects
Electronegativity has an even greater effect
on 13C chemical shifts than it does on 1H
chemical shifts.
Types of Carbons
Classification
CH4
Chemical shift, d
1H
13C
0.2
-2
CH3CH3
primary
0.9
8
CH3CH2CH3
secondary
1.3
16
(CH3)3CH
tertiary
1.7
25
(CH3)4C
quaternary
28
Replacing H with C (more electronegative) deshields
C to which it is attached.
Electronegativity Effects on CH3
Chemical shift, d
1H
13C
CH4
0.2
-2
CH3NH2
2.5
27
CH3OH
3.4
50
CH3F
4.3
75
Electronegativity Effects and Chain Length
Cl
Chemical
shift, d
CH2
CH2
CH2
CH2
CH3
45
33
29
22
14
Deshielding effect of Cl decreases as
number of bonds between Cl and C increases.
13C
Chemical Shifts are Most Affected By:
• Electronegativity of groups attached to carbon
• Hybridization state of carbon
Hybridization Effects
sp3 hybridized
carbon is more
shielded than sp2.
sp hybridized
carbon is
more
shielded than
sp2, but less
shielded than
sp3.
H
36
114
138 36 126-142
C
C
68
84
CH2
22
CH2
20
CH3
13
Carbonyl Carbons are Especially Deshielded
O
127-134
CH2
C
41
171
O
CH2
CH3
61
14
Table 13.3
Type of carbon Chemical shift (d), Type of carbon
ppm
Chemical shift (d),
ppm
RCH3
0-35
RC
CR
65-90
R2CH2
15-40
R2C
CR2
100-150
R3CH
25-50
110-175
R4C
30-40
Table 13.3
Type of carbon Chemical shift (d), Type of carbon
ppm
RCH2Br
RCH2Cl
20-40
25-50
RC
Chemical shift (d),
ppm
N
110-125
RCOR
160-185
O
RCH2NH2
35-50
RCH2OH
50-65
O
RCH2OR
50-65
RCR
190-220
13C
NMR and Peak Intensities
Pulse-FT NMR distorts intensities of signals.
Therefore, peak heights and areas can be
deceptive.
Figure 13.27
CH3
7 carbons give 7
signals, but
intensities are not
equal.
OH
200
180
160
140
120
100
80
60
Chemical shift (d, ppm)
40
20
0
13C—H
Coupling
Peaks in a 13C NMR Spectrum are Typically
Singlets
13C—13C
splitting is not seen because the
probability of two 13C nuclei being in the same
molecule is very small.
13C—1H
splitting is not seen because spectrum
is measured under conditions that suppress
this splitting (broadband decoupling).