Chapter 13 NMR Spectroscopy

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Chapter 13 NMR Spectroscopy

• NMR - Nuclear Magnetic Resonance
• NMR is a form of spectroscopy that uses an
  instrument with a powerful magnet to analyze
  organic compounds.
• Invented by physicists (1950s), then used by
  chemists (1960s).

MRI Magnetic Resonance Imaging (1980s) A
special form of NMR used in medicine.
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What is NMR?

• NMR a tool to determine the structure of an
  organic compound.

NMRSpectrometer
magnet
1H NMRSpectrum
This instrument gives you information about an
organic compounds structure.
computer
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What is NMR?

• NMR Instruments

computer
Small, 60 mHz instrument for undergraduate
student use.
magnet
student
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What is NMR?

• NMR Instruments

Research grade instrument, 300 mHz magnet, that
we use at Western.
magnet
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What is NMR?

• NMR Instruments

Research grade instrument, 300 mHz magnet, that
we use at Western.
computer
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What is NMR?

• NMR Instruments

researcher
State-of-the-art instrument, 950 mHz
magnet. Rather large and expensive!
magnet
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Why is it called NMR?

• Nuclear Magnetic Resonance
• Nuclear because it looks at the nucleus of an
  atom, most commonly a hydrogen atom.
• A hydrogen atom nucleus consists of one proton
  with a 1 charge and spin of ½. It acts like a
  tiny bar magnet.

proton
generatesmagneticfield
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NMR Effect of Magnetic Field

• No External Magnetic FieldNuclear spins are
  pointedin random directions

Sample in Magnetic FieldSpins align with or
against the external magnetic field
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NMR Effect of Magnetic Field
Sample placed in an external magnetic field H0
No external magnetic field applied to
sample Random orientation of nuclear spins
Spins align with or against field (most align
with field)
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NMR Absorption of Energy
radio waves
nucleus absorbs energy
Scan with RF field nucleus absorbs energy,
giving a signal in the NMR spectrum
Initial State nucleus at low energy level
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NMR Information Obtained from a Spectrum

• An NMR Spectrum will generally provide three
  types of information
• Chemical Shift indicates the electronic
  environment of the nucleus (shielded or
  deshielded)

• Integration gives the relative number of nuclei
  producing a given signal
• Spin-Spin Coupling describes the connectivity

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1H NMR Spectrum H2O
scanning
A sample of water is placed in an NMR instrument,
and a proton spectrum is recorded (scanned from
left to right).
An NMR signal appears. This proves thatwater
contains hydrogen atoms!
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NMR Effect of Magnetic Field
Magnetic Fields 1. from magnet 2. from
protons
Sample placed in an external magnetic field
Spins align with or against field (most align
with field)
No external magnetic field applied to
sample Random orientation of nuclear spins
H0 strong external magnetic field from NMR
instrument
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When does nucleus absorb energy?
Not all protons are the same!
Magnetic Fields 1. from spinning proton 2.
from magnet 3. from electrons
3.
2, External Field (Ho)from magnet
Absorption depends on shielding by electron cloud
around the nucleus. More electron density
more shielding signal shifted to the right.
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NMR Scanning for All Nuclei
13C area is wider
1H area is small
To see both proton and C-13 nuclei, a very wide
region would have to be scanned.
An instrument can only examine one area at a
time.
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NMR Simple 1H NMR Spectrum Showing Chemical
Shift
Chemical Shift location of the signalon the
spectrum.
Right Sidehigh electrondensity
Left Sidelow electrondensity
Two types of protons (a CH2 and a CH3) give two
separate signals at two different chemical shifts.
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NMR Chemical Shift Practice
-OCH3
-CCH3
Cl3C-H
EN
Group
3 electronegative atoms
-O-CH3 -Si-CH3 -C-CH3 Cl3C-H
3.5 1.8 2.5 3.0
Left Sidelow electrondensity(high EN)
-SiCH3
Assign the four groups shown to the four NMR
singals, based on each elements
electronegativity.
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NMR Chemical Shift Reference
Chemical shift zero is set to TMS
(tetramethylsilane).
Chemical shift measured in ppm. For 1H roughly
0 to 10 ppm.
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NMR Chemical Shift Regions
-CH2-CH3
Alkane region (high electron density) is from
about .8 2.5 ppm.
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NMR Chemical Shift Regions
-O-CH3
Heteroatom region (low electron density) is from
about 2.5 to 5.
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NMR Chemical Shift Regions
Double bond region is on the left, from about 5
10 ppm.
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NMR Chemical Equivalence and Number
of Signals
How many signals will the following compounds
show in their 1H NMR Spectrum?
(Hint check for symmetry)
2 4 5 2 4 7
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NMR Chemical Equivalence and Number
of Signals
How many signals should appear in the proton NMR
spectrum for these compounds?
In theory 9 4
Signals actually resolved 3-4
2
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NMR Overlapping Proton Signals
octane
The -CH2- groups allappear in the same spot (not
resolved)
Protons b, c, and d are in roughly the same
environment, and their chemical shifts are
also about the same.
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Review How Many NMR Signals?
How many signals will the following compounds
show in their 1H NMR Spectrum?
(Hint check for symmetry)
1 2 5
Fast chair flips at RT
Fast rotation about C-C single bond
No rotation about double bonds
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NMR Chloroethane
Fast rotation around single bonds gives an
averaged spectrum for the three methyl
hydrogens.
An NMR spectrometer is like a camera with a slow
shutter speed.
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NMR Chair Cyclohexane
Rapid chair flipping makes all Hs equivalent.
Cylcohexane gives one peak in the 1H NMR
spectrum.
An NMR spectrometer is like a camera with a slow
shutter speed.
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NMR A Second Proton Spectrum
Note the signal for the nine methyl Hs (red) is
larger than the signal for the CH2 group (blue)
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NMR Information Obtained from a Spectrum

• An NMR Spectrum will generally provide three
  types of information
• Chemical Shift indicates the electronic
  environment of the nucleus (shielded or
  deshielded)

• Integration gives the relative number of nuclei
  that produces a given
  signal. The integral (area under the curve) is
  drawn on the spectrum by the instrument.
• Spin-Spin Coupling describes the connectivity

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NMR Integration Indicates Relative
Number of Nuclei
The height of the integration line (integral)
gives you the relative number of nuclei
producing each signal.
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NMR Information Obtained from a Spectrum

• An NMR Spectrum will generally provide three
  types of information
• Chemical Shift indicates the electronic
  environment of the nucleus (shielded or
  deshielded)

• Integration gives the relative number of nuclei
  producing a given signal
• Spin-Spin Coupling - describes the carbon
  connectivity - follows the n1rule

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NMR Splitting into a Doublet
doublet
Note that the red signal at 1.6 ppm for the
methyl group is split into two peaks.
Remember that this is one signal, composed of two
separate peaks.
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NMR Signal Splitting, n1 Rule

• A signal is often split into multiple peaks due
  to interactions with protons on carbons next
  door. Called spin-spin splitting
• The splitting is into one more peak than the
  number of Hs on adjacent carbons (n1 rule)
• Splitting of a signal can give doublets (two
  peaks), triplets (three peaks), quartets (4
  peaks), ect.
• The relative intensities given by Pascals
  Triangle doublet 1 1 triplet 1
  2 1 quartet 1 3 3 1 pentet
  1 4 6 4 1

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NMR Signal Splitting, n1 Rule
n1 Rule A signal in the proton NMR spectrum
will be split into n1 peaks, where n is the
number of protons on adjacent carbons.
Example CH3-CH2-Br
For the Methyl Group There are two protons
next door (n2), so the methyl signal will be
split into three peaks (21), which is called a
triplet. Chemical shift will be about 1.5
(alkane region), integration 3.
For the -CH2- Group Three protons next door
means the CH2 signal will be split into 4 (31)
peaks, called a quartet. Chemical shift 3.3
(heteroatom region), integration 2.
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1H NMR Spectrum for Bromoethane
integration 2 H 3 H

Note the expansionsprinted above the spectrum
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NMR Signal Splitting, n1 Rule

Peak Heights - Pascals Triangle singlet
1doublet 1 1triplet 1 2
1quartet 1 3 3 1pentet 1 4 6
4 1
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NMR Signal Splitting, n1 Rule

many lines mulitplet
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NMR Signal Splitting, n1 Rule
H
n 1 7
n 6
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NMR Origin of Spin-Spin Splitting
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NMR Origin of Spin-Spin Splitting
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NMR Doublets and Triplets
Triplet for the two protons next door,there
are four combinations possible a a a
ß ß ß ß a
Doublet the one proton next doorcan be either
up or down (a or ß)
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NMR Signal Splitting, n1 Rule
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NMR Using the n1 Rule
Using the n1 rule, predict the 1H NMR spectrum
of 2-iodopropane.Give splitting pattern,
integration, and approximate chemical shift.
six neighbors
one neighbor
Note that the methyl groups are equivalent, so
they will give one signal in the NMR spectrum.
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NMR Spectrum of 2-iodopropane
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NMR Rules for Spin-Spin Splitting

• The signal of a proton with n equivalent
  neighboring Hs is split into n 1 peaks

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Common 1H NMR Patterns
1. triplet (3H) quartet (2H)
-CH2CH3 2. doublet (1H) doublet (1H)
-CH-CH- 3. large singlet (9H) t-butyl
group 4. singlet 3.5 ppm (3H) -OCH3
group 5. large double (6H) muliplet (1H)
isopropyl 6. singlet 2.1 ppm (3H)
methyl ketone
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Common 1H NMR Patterns
7. multiplet 7.2 ppm (5H) aromatic
ring, monosubstituted 8. multiplet
7.2 ppm (4H) aromatic ring,
disubstituted 9. broad singlet, variable
-OH or NH chemical shift (H on
heteratom)
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Solving NMR Problems
1. Check the molecular formula and degree of
unsaturation. How many rings/double bonds?
2. Make sure that the integration adds up to the
total number of Hs in the formula. 3.
Are there signals in the double bond region? 4.
Check each signal and write down a possible
sub-structure for each one. 5. Try to put the
sub-structures together to find the
structure of the compound.
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Proton NMR Spectrum C9H12
Degree of Unsat 4
aromatic, disubst.
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1H NMR Spectrum C4H7O2Br
Degree of Unsat 1
s 3H
t 2H
t 2H
5.0 4.0 3.0 2.0 1.0
0
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Electronegative Substituents Shift Left
Propane
smalleffect
noeffect
heteroatomregion
d 0.9
d 0.9
d 1.3
d 1.0
d 4.3
d 2.0
H3CCH2CH3
O2NCH2CH2CH3
Effect is cumulative

• CH3Cl 3.1 (one Cl)
• CH2Cl2 5.3 (two Cls)
• CHCl3 7.3 (three Cls)

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Hydrogens on Heteroatoms
Chemical shifts for protons on heteroatoms are
variable, and signals are often broad (not
generally useful).
Type of proton
Chemical shift (ppm)
1-3
0.5-5
6-8
farleft
may beuseful
10-13
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13C NMR Spectroscopy

• Carbon-13 only carbon isotope with a nuclear
  spin natural abundance of 13C is only 1.1
  (99 of carbon atoms are 12C, with no NMR signal)
• All signals are obtained simultaneously using a
  broad pulse of energy. The resulting mass
  signal changed into an NMR spectrum
  mathematically using the operation of Fourier
  transform (FT-NMR)
• Frequent repeated pulses give many data sets that
  are averaged to eliminate noise

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13C NMR Spectroscopy

• 13C signals go from 0 to 240 ppm. 13C
  signals always sharp singlets.
• (wider range than in 1H NMR) (1H signals
  broad multiplets)
• These two facts mean that in carbon-13 NMR, each
  separate signal is usually visible, and you can
  accurately count the number of different carbons
  in the molecule.
• Chemical shift affected by electronegativity of
  nearby atoms alkane-like range
  0 40 ppm (R-CH2-R)
  heteroatom range 50 100 ppm
  (O-CH2-R) double bond range 100
  220 ppm (sp2 carbons)

No signal overlap!
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NMR Scanning for All Nuclei
13C area is much wider
1H area is small
An instrument can only examine one area at a
time.
To see both proton and C-13 nuclei, a very wide
region would have to be scanned.
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Why does 13C NMR give singlets?
13C is only 1.1 natural abundant, so most
carbons are 12C, and give no NMR signal. No
splitting seen with carbon, because carbons next
to the 13C are likely to be carbon-12
Sample of 1-Propanol 12CH3-12CH2-12CH2-OH
12CH3-12CH2-12CH2-OH
12CH3-13CH2-12CH2-OH 13CH3-12CH2-12CH2-OH 12C
H3-12CH2-12CH2-OH
12CH3-12CH2-12CH2-OH 12CH3-13CH2-13CH2-OH
12CH3-12CH2-12CH2-OH
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NMR Number of Signals for 13C NMR
How many signals should appear in the carbon-13
NMR spectrum for these compounds?
In theory 10 4 Signals
actually resolved 10 4
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13C NMR Example
Note the wide spectral width and the sharp
singlets in the spectrum below. Also note that
there is no integration with 13C NMR.
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13C NMR smaller signal to noise ratio
Noise
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13C NMR smaller signal to noise ratio
Signal
Noise
more scans (noisesmaller)
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13C NMR Spectrum C5H11Cl
D. of Unsat 0
five 13Csignals
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13C NMR Spectrum C4H7O2Br
D. of Unsat 1
CDCl3
200 150 100 50 0
double bond region