NMR – Effect of Magnetic Field

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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 (1950’s), then used
by chemists (1960’s).
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.
generates
magnetic
field
NMR – Effect of Magnetic Field
No external magnetic
field applied to sample
Random orientation of
nuclear spins
Sample placed in an external
magnetic field
NMR: Absorption of Energy
Initial State – nucleus
at low energy level
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
1H
NMR Spectrum – H2O
A sample of water is placed in an NMR instrument, and a
proton spectrum is recorded (scanned from left to right).
When does nucleus absorb energy?
3.
2, External Field (Ho)
from magnet
NMR: Simple 1H NMR Spectrum
Showing Chemical Shift
Two types of protons (a CH2 and a CH3) give two
separate signals at two different chemical shifts.
NMR: Chemical Shift Practice
Group
EN
-O-CH3
-Si-CH3
-C-CH3
Cl3C-H
Assign the four groups shown to the four NMR
singals, based on each element’s electronegativity.
NMR: Chemical Shift Regions
Chemical shift zero is set to
TMS (tetramethylsilane),
a reference compound
Chemical shift measured in ppm.
NMR: Chemical Shift Regions
Alkane Region (high electron density):
Heteroatom Region:
Double Bond Region:
NMR: Chemical Equivalence and
Number of Signals
How many signals will the following compounds show in their
1H NMR Spectrum?
(Hint: check for symmetry)
O
OMe
Br
H
Cl
H
NH2
Cl
Cl
H
H
H
Cl
H
NMR: Chemical Equivalence and
Number of Signals
How many signals should appear in the proton NMR
spectrum for these compounds?
O
octane
In theory:
Signals actually
resolved:
NMR: Overlapping Proton Signals
Protons b, c, and d are all nearly the same, and
their signals are not resolved in this spectrum.
Review: How Many NMR Signals?
How many signals will the following compounds show in their
1H NMR Spectrum?
(Hint: check for symmetry)
CH3CH2Cl
H
H
H
C
H
H
H
Cl
C
H
H
H H
CH3
H
H
No rotation about
double bonds
NMR: Chloroethane
HH
C
H
C
HH
Cl
Fast rotation around single bonds
gives an “averaged” spectrum for
the three methyl hydrogens.
NMR: A Second Proton Spectrum
Note: the signal for the nine methyl H’s is larger than the CH2 signal
NMR: Integration Indicates
Relative Number of Nuclei
The height of the integration line (“integral”) gives you
the relative number of nuclei producing each signal.
NMR: Splitting into a Doublet
doublet
Note that the 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.
NMR: Signal Splitting, n+1 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 H’s on adjacent carbons (“n+1 rule”)
• Splitting of a signal can give doublets (two peaks),
triplets (three peaks), quartets (4 peaks), ect.
• The relative intensities given by Pascal’s Triangle:
doublet
1:1
triplet
1:2:1
quartet
1:3:3:1
pentet:
1:4:6:4:1
NMR: Signal Splitting, n+1 Rule
n+1 Rule:
A signal in the proton NMR spectrum will be split
into n+1 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’ (n=2), so the methyl signal will
be split into three peaks (2+1), which is called a triplet.
For the -CH2- Group:
Three protons next door means the CH2 signal will be split into
4 (3+1) peaks, called a quartet.
1H
NMR Spectrum for Bromoethane
integration:
2H
3H
Note the expansions printed above
NMR: Signal Splitting, n+1 Rule
NMR: Signal Splitting, n+1 Rule
H
NMR: Origin of Spin-Spin Splitting
Net result:
NMR: Doublets and Triplets
Doublet: the one proton next door
can be either up or down (α or β)
Triplet: for the two protons next door,
there are four combinations possible:
α α
α β
β α
β β
NMR: Signal Splitting, n+1 Rule
NMR: Using the n+1 Rule
Using the n+1 rule, predict the 1H NMR spectrum
of 2-iodopropane.
Give splitting pattern, integration, and approximate
chemical shift.
I
H
I
C
H3C
CH3
Note that the methyl groups are equivalent, so
they will give one signal in the NMR spectrum.
NMR: Spectrum of 2-iodopropane
doublet
Seven line
pattern
NMR: Rules for Spin-Spin Splitting
• The signal of a proton with n equivalent neighboring H’s is split
into n + 1 peaks
• Protons farther than two carbon atoms apart do not split
each other
• Equivalent protons do not split each other
Common 1H NMR Patterns
1. triplet (3H) + quartet (2H)
2. doublet (1H) + doublet (1H)
3. large singlet (9H)
4. singlet 3.5 ppm (3H)
5. large double (6H) + muliplet (1H)
6. singlet 2.1 ppm (3H)
Common 1H NMR Patterns
7. multiplet ~7.2 ppm (5H)
8. multiplet ~7.2 ppm (4H)
9. broad singlet, variable
chemical shift
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