Chapter 13
Nuclear Magnetic Resonance
Spectroscopy
Assignment for Chapter 13
• 13.1 through 13.14
13.16 and 13.18
• SKIP 13.15 and 13.17
Problem Assignment
• In text problems
2 - 16 22 a, b, d, g
24, 25
• End of chapter problems
27, 28, 29, 30, 31, 35, 39a, b
40 a, c, d, f
41 a, b, c, d
13.1 Spin Quantum Numbers of Some Nuclei
The most abundant isotopes of C and O do not have spin.
Element
Nuclear Spin
Quantum No
1H
2H
12C
13C
14N
16O
17O
19F
1/2
1
0
1/2
1
0
5/2
1/2
2
3
0
2
3
0
6
2
(I)
No. of Spin
States
Elements with either odd mass or odd atomic number
have the property of nuclear “spin”.
The number of spin states is 2I + 1,
where I is the spin quantum number.
13.2
Nuclear Resonance
absorption of energy by the
spinning nucleus
Nuclear Spin Energy Levels
N
-1/2
unaligned
In a strong magnetic
field (Bo) the two
spin states differ in
energy.
+1/2
Bo
S
aligned
The Energy Separation Depends on Bo
- 1/2
DE
= kBo = hn
degenerate
at Bo = 0
+ 1/2
Bo
increasing magnetic field strength
The Larmor Equation!!!
DE = kBo = hn
can be transformed into
gyromagnetic
frequency of
the incoming
radiation that
will cause a
transition
nn ==
gBg0
2p
2p
ratio g
Bo
strength of the
magnetic field
g is a constant which is different for
each atomic nucleus (H, C, N, etc)
Resonance Frequencies of Selected Nuclei
Isotope Abundance Bo (Tesla)
1H
13C
Frequency(MHz)
g(radians/Tesla)
99.98%
1.00
1.41
2.35
7.05
42.6
60.0
100.0
300.0
267.53
1.108%
1.00
2.35
7.05
10.7
25.0
75.0
67.28
13.3 Classical Instrumentation:
The Continuous-Wave NMR
typical before 1965;
field is scanned
A Simplified 60 MHz
NMR Spectrometer
RF (60 MHz)
Oscillator
hn
Transmitter
absorption
signal
RF
Detector
Recorder
Receiver
MAGNET
MAGNET
N
S
Probe
~ 1.41 Tesla
(+/-) a few ppm
NMR Spectrum of Phenylacetone
O
CH2 C CH3
EACH DIFFERENT TYPE OF PROTON COMES AT A DIFFERENT
PLACE - YOU CAN TELL HOW MANY DIFFERENT TYPES OF
PROTONS THERE ARE BY INTEGRATION.
13.4 Modern Instrumentation:
the Fourier-Transform NMR
FT-NMR
requires a computer
PULSED EXCITATION
N
n1
BROADBAND
RF PULSE
contains a range
of frequencies
(n1 ..... nn)
n2
O
CH2 C CH3
n3
S
All types of hydrogen are excited
simultaneously with the single RF pulse.
13.5 Proton NMR Spectrum
NMR Spectrum of Phenylacetone
O
CH2 C CH3
EACH DIFFERENT TYPE OF PROTON COMES AT A DIFFERENT
PLACE - YOU CAN TELL HOW MANY DIFFERENT TYPES OF
PROTONS THERE ARE BY INTEGRATION.
Some Generalizations
•
•
•
•
NMR solvents contain deuterium
Tetramethylsilane (TMS) is the reference
Spectrum of 1,2,2-trichloropropane
Chemical shift in Hz from TMS vary
according to frequency of spectrometer!
• Delta values (d) are independent of
frequency of spectrometer (ppm)
PEAKS ARE MEASURED RELATIVE TO TMS
Rather than measure the exact resonance position of a
peak, we measure how far downfield it is shifted from TMS.
reference compound
tetramethylsilane
“TMS”
CH3
CH3 Si CH3
CH3
Highly shielded
protons appear
way upfield.
TMS
shift in Hz
downfield
n
0
Chemists originally
thought no other
compound would
come at a higher
field than TMS.
HIGHER FREQUENCIES GIVE LARGER SHIFTS
The shift observed for a given proton
in Hz also depends on the frequency
of the instrument used.
Higher frequencies
= larger shifts in Hz.
TMS
shift in Hz
downfield
n
0
THE CHEMICAL SHIFT
The shifts from TMS in Hz are bigger in higher field
instruments (300 MHz, 500 MHz) than they are in the
lower field instruments (100 MHz, 60 MHz).
We can adjust the shift to a field-independent value,
the “chemical shift” in the following way:
parts per
million
chemical
=
shift
d
shift in Hz
= ppm
=
spectrometer frequency in MHz
This division gives a number independent
of the instrument used.
A particular proton in a given molecule will always come
at the same chemical shift (constant value).
13.6 and 13.7
The Chemical Shift
Generalizations
• electrons shield nucleus
• electronegativity: withdraws electrons to
deshield nucleus
• downfield (deshielding) = left side of
spectrum
• upfield (shielding) = right side of
spectrum
• delta values increase from right to left!
PROTONS DIFFER IN THEIR SHIELDING
All different types of protons in a molecule
have a different amounts of shielding.
They all respond differently to the applied magnetic
field and appear at different places in the spectrum.
This is why an NMR spectrum contains useful information
(different types of protons appear in predictable places).
DOWNFIELD
Less shielded protons
appear here.
SPECTRUM
UPFIELD
Highly shielded
protons appear here.
It takes a higher field
to cause resonance.
Overview of where protons
appear in an NMR spectrum
C-H where C is
CH on C
attached
to
an
aliphatic
acid
aldehyde benzene alkene
next to
C-H
COOH
CHO
CH
=C-H electronega- pi bonds
tive atom
X=C-C-H
X-C-H
12
10
9
7
6
4
3
2
0
NMR Correlation Chart
-OH -NH
DOWNFIELD
DESHIELDED
UPFIELD
SHIELDED
CHCl3 , H
TMS
12
11
10
9
8
7
6
H
RCOOH
RCHO
C=C
5
4
CH2F
CH2Cl
CH2Br
CH2I
CH2O
CH2NO2
3
2
1
0
CH2Ar
C-CH-C
CH2NR2
C
CH2S
C-CH2-C
C C-H
C=C-CH2 C-CH3
CH2-CO
d (ppm)
Some approximate NMR
values (part 1)
H
R
C
O
7-8 ppm
11-12 ppm
H-O
carboxylic acid
benzene ring protons
R
C
H
aldehyde
O
9-10 ppm
C
5-7 ppm
C
H
vinyl protons (alkene)
Some approximate NMR
values (part 2)
-CH2-O-
about 4ppm
-CH2-X
about 3.5 ppm
-CH2-CH2-O-
about 1.2 ppm
for boldfaced CH2
X = Cl, Br, I
If two chlorine atoms are attached,
shifts to 5.3 ppm
CH3
CH2
CH
about 1 to 1.5 ppm
O
and
CH 2
long way from electronegative atoms
R
about 2 ppm
CH 2-
DESHIELDING AND ANISOTROPY
Three major factors account for the resonance
positions (on the ppm scale) of most protons.
1. Deshielding by electronegative elements.
2. Anisotropic fields usually due to pi-bonded
electrons in the molecule.
3. Deshielding due to hydrogen bonding.
DESHIELDING BY
ELECTRONEGATIVE ELEMENTS
DESHIELDING BY AN ELECTRONEGATIVE ELEMENT
d-
Cl
d+
C
d-
electronegative
element
H
d+
Chlorine “deshields” the proton,
that is, it takes valence electron
density away from carbon, which
in turn takes more density from
hydrogen deshielding the proton.
NMR CHART
“deshielded“
protons appear
at low field
highly shielded
protons appear
at high field
deshielding moves proton
resonance to lower field
Electronegativity Dependence
of Chemical Shift
Dependence of the Chemical Shift of CH3X on the Element X
Compound CH3X
Element X
Electronegativity of X
Chemical shift
d
most
deshielded
CH3F
CH3OH
CH3Cl
CH3Br
CH3I
CH4
(CH3)4Si
F
O
Cl
Br
I
H
Si
4.0
3.5
3.1
2.8
2.5
2.1
1.8
4.26
3.40
3.05
2.68
2.16
0.23
0
TMS
deshielding increases with the
electronegativity of atom X
Substitution Effects on
Chemical Shift
most
deshielded
most
deshielded
CHCl3 CH2Cl2 CH3Cl
7.27 5.30
3.05 ppm
-CH2-Br
3.30
-CH2-CH2Br
1.69
The effect
increases with
greater numbers
of electronegative
atoms.
-CH2-CH2CH2Br
1.25
ppm
The effect decreases
with incresing distance.
ANISOTROPIC FIELDS
DUE TO THE PRESENCE OF PI BONDS
The presence of a nearby p bond
greatly affects the chemical shift.
Benzene rings have the greatest effect.
Ring Current in Benzene
Circulating p electrons
H
Bo
H
Deshielded
fields add together
Secondary magnetic field
generated by circulating p
electrons deshields aromatic
protons
ANISOTROPIC FIELD IN AN ALKENE
protons are
deshielded
Deshielded
fields add
H
shifted
downfield
C=C
H
Bo
H
H
secondary
magnetic
(anisotropic)
field lines
ANISOTROPIC FIELD FOR AN ALKYNE
H
C
C
H
Bo
Shielded
fields subtract
hydrogens
are shielded
secondary
magnetic
(anisotropic)
field
HYDROGEN BONDING
HYDROGEN BONDING DESHIELDS PROTONS
R
O
H
H
O
H
O R
The chemical shift depends
on how much hydrogen bonding
is taking place.
Alcohols vary in chemical shift
from 0.5 ppm (free OH) to about
5.0 ppm (lots of H bonding).
R
Hydrogen bonding lengthens the
O-H bond and reduces the valence
electron density around the proton
- it is deshielded and shifted
downfield in the NMR spectrum.
SOME MORE EXTREME EXAMPLES
O
H
O
C R
R C
O
H
O
Carboxylic acids have strong
hydrogen bonding - they
form dimers.
With carboxylic acids the O-H
absorptions are found between
10 and 12 ppm very far downfield.
H3C O
O
H
O
In methyl salicylate, which has strong
internal hydrogen bonding, the NMR
absortion for O-H is at about 14 ppm,
way, way downfield.
Notice that a 6-membered ring is formed.
13.8 Integration
INTEGRATION OF A PEAK
Not only does each different type of hydrogen give a
distinct peak in the NMR spectrum, but we can also tell
the relative numbers of each type of hydrogen by a
process called integration.
Integration = determination of the area
under a peak
The area under a peak is proportional
to the number of protons that
generate the peak.
Benzyl Acetate
The integral line rises an amount proportional to the number of H in each peak
METHOD 1
integral line
integral
line
55 : 22 : 33
=
5:2:3
simplest ratio
of the heights
Benzyl Acetate (FT-NMR)
Actually :
5
58.117 / 11.3
= 5.14
2
21.215 / 11.3
= 1.90
3
33.929 / 11.3
= 3.00
O
CH2 O C CH3
METHOD 2
digital
integration
assume CH3
33.929 / 3 = 11.3 per H
Integrals are
good to about
10% accuracy.
Modern instruments report the integral as a number.