Training

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13.9 Spin-Spin Splitting
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SPIN-SPIN SPLITTING
Often a group of hydrogens will appear as a
multiplet rather than as a single peak.
Multiplets are named as follows
Singlet Quintet Doublet Septet Triplet Octet Quart
et Nonet
This happens because of interaction with
neighboring hydrogens and is called
SPIN-SPIN SPLITTING.
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1,1,2-Trichloroethane
integral 2
integral 1
triplet
doublet
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n 1 RULE
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MULTIPLETS
this hydrogens peak is split by its two neighbors
these hydrogens are split by their single neighbor
singlet doublet triplet quartet quintet sextet sep
tet
two neighbors n1 3 triplet
one neighbor n1 2 doublet
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Some Common Patterns
7
SOME COMMON SPLITTING PATTERNS
CH-CH3
( x y )
CH-CH2
-CH2-CH3
CH3
( x y )
CH
CH3
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tert-butyl group
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EXCEPTIONS TO THE N1 RULE
IMPORTANT !
Protons that are equivalent by symmetry usually
do not split one another
1)
no splitting if xy
no splitting if xy
Protons in the same group usually do not split
one another
2)
or
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SOME EXAMPLE SPECTRA WITH SPLITTING
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NMR Spectrum of Bromoethane
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NMR Spectrum of 2-Nitropropane
13
NMR Spectrum of Acetaldehyde
offset 2.0 ppm
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The propyl group
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INTENSITIES OF MULTIPLET PEAKS
PASCALS TRIANGLE
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PASCALS TRIANGLE
Intensities of multiplet peaks
1
singlet
1 1
doublet
1 2 1
triplet
1 3 3 1
quartet
1 4 6 4 1
quintet
1 5 10 10 5 1
sextet
1 6 15 20 15 6 1
septet
1 7 21 35 35 21 7 1
octet
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THE ORIGIN OF SPIN-SPIN SPLITTING
HOW IT HAPPENS
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THE CHEMICAL SHIFT OF PROTON HA IS AFFECTED
BY THE SPIN OF ITS NEIGHBORS
aligned with Bo
opposed to Bo
1/2
-1/2
50 of molecules
50 of molecules
H
H
H
H
A
A
C
C
C
C
Bo
upfield
downfield
neighbor aligned
neighbor opposed
At any given time about half of the molecules in
solution will have spin 1/2 and the other half
will have spin -1/2.
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SPIN ARRANGEMENTS
one neighbor n1 2 doublet
one neighbor n1 2 doublet
H
H
H
H
C
C
C
C
yellow spins
blue spins
The resonance positions (splitting) of a given
hydrogen is affected by the possible spins of
its neighbor.
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SPIN ARRANGEMENTS
two neighbors n1 3 triplet
one neighbor n1 2 doublet
methine spins
methylene spins
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SPIN ARRANGEMENTS
three neighbors n1 4 quartet
two neighbors n1 3 triplet
methylene spins
methyl spins
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13.10 The Coupling Constant
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THE COUPLING CONSTANT
J
J
J
J
J
J
The coupling constant is the distance J (measured
in Hz) between the peaks in a multiplet.
J is a measure of the amount of interaction
between the two sets of hydrogens creating the
multiplet.
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FIELD COMPARISON
100 MHz
200 Hz
100 Hz
Coupling constants are constant - they do not
change at different field strengths
7.5 Hz
J 7.5 Hz
1
2
3
4
5
6
200 MHz
400 Hz
200 Hz
7.5 Hz
The shift is dependant on the field
J 7.5 Hz
ppm
1
2
3
25
50 MHz
J 7.5 Hz
Why buy a higher field instrument?
1
2
3
Spectra are simplified!
100 MHz
J 7.5 Hz
Overlapping multiplets are separated.
1
2
3
200 MHz
J 7.5 Hz
Second-order effects are minimized.
1
2
3
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NOTATION FOR COUPLING CONSTANTS
The most commonly encountered type of coupling is
between hydrogens on adjacent carbon atoms.
This is sometimes called vicinal coupling. It
is designated 3J since three bonds intervene
between the two hydrogens.
3J
Another type of coupling that can also occur in
special cases is
2J or geminal coupling
( most often 2J 0 )
Geminal coupling does not occur when the two
hydrogens are equivalent due to rotations around
the other two bonds.
2J
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LONG RANGE COUPLINGS
Couplings larger than 2J or 3J also exist, but
operate only in special situations, especially
in unsaturated systems.
Couplings larger than 3J (e.g., 4J, 5J, etc)
are usually called long-range coupling.
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SOME REPRESENTATIVE COUPLING CONSTANTS
6 to 8 Hz
three bond
3J
vicinal
11 to 18 Hz
three bond
3J
trans
6 to 15 Hz
three bond
3J
cis
0 to 5 Hz
two bond
2J
geminal
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cis
6 to 12 Hz
three bond
3J
trans
4 to 8 Hz
4 to 10 Hz
three bond
3J
0 to 3 Hz
four bond
4J
0 to 3 Hz
four bond
4J
Couplings that occur at distances greater than
three bonds are called long-range couplings
and they are usually small (lt3 Hz)
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13.11 NMR Spectra of Carbonyl Compounds

• Anisotropy in carbonyl compounds
• Anisotropy deshields C-H on aldehydes
• 9-10 ppm
• Anisotropy also deshields methylene and methyl
  groups next to CO 2.0 - 2.5 ppm
• Methylene groups directly attached to oxygen
  appear near 4.0 ppm

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2-Butanone (Methyl Ethyl Ketone) 60 MHz Spectrum
1
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2-butanone, 300 MHz spectrum
WWU Chemistry
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Ethyl Acetate
2
Compare the methylene shift to that of Methyl
Ethyl Ketone (previous slide).
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t-Butyl Methyl Ketone
3
(3,3-dimethyl-2-butanone)
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4
Phenylethyl Acetate
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5
Ethyl Succinate
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a-Chloropropionic Acid
6
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13.12 and 13.13 Alkenes, Alkynes and
Aromatic Compounds
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CHEMICAL SHIFTS
Alkenes and alkynes

• vinyl protons appear between
• 5 to 6.5 ppm (anisotropy)
• methylene and methyl groups next to a double bond
  appear at about 1.5 to 2.0 ppm
• for terminal alkynes, proton appears near 2 ppm

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BENZENE RING HYDROGENS
Ring current causes protons attached to the
ring to appear in the range of 7 to 8 ppm.
Protons in a methyl or methylene group attached
to the ring appear in the range of 2 to 2.5 ppm.
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NMR Spectrum of Toluene
5
3
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THE EFFECT OF CARBONYL SUBSTITUENTS
When a carbonyl group is attached to the ring
the o- and p- protons are deshielded by the
anisotropic field of CO
Only the o- protons are in range for this effect.
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Acetophenone (90 MHz)
3
2
3
deshielded
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NMR Spectrum of 1-iodo-4-methoxybenzene
3
CHCl3 impurity
2
2
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NMR Spectrum of 1-bromo-4-ethoxybenzene
3
4
2
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THE p-DISUBSTITUTED PATTERN CHANGES AS THE TWO
GROUPS BECOME MORE AND MORE SIMILAR
All peaks move closer. Outer peaks get smaller
.. and finally disappear. Inner peaks get
taller. and finally merge.
all H equivalent
X X
X Y
X X
same groups
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NMR Spectrum of 1-amino-4-ethoxybenzene
3
4
2
2
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NMR Spectrum of p-Xylene (1,4-dimethylbenzene)
6
4
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13.14 Hydroxyl and Amino Protons
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Hydroxyl and Amino Protons
Hydroxyl and amino protons can appear almost
anywhere in the spectrum (H-bonding).
These absorptions are usually broader than other
proton peaks and can often be identified because
of this fact.

• Carboxylic acid protons generally appear far
• downfield near 11 to 12 ppm.

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SPIN-SPIN DECOUPLING BY EXCHANGE
In alcohols coupling between the O-H hydrogen
and those on adjacent carbon atoms is usually not
seen.
This is due to rapid exchange of OH protons
between the various alcohol molecules in the
solution. The OH peak is usually broad.
C
O
H
H
In ultrapure alcohols, however, coupling will
sometimes be seen.
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NMR Spectrum of Ethanol
3
2
1
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1-propanol
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13.16 Unequal Couplings Tree Diagrams
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WHERE DOES THE N1 RULE WORK ?
The n1 rule works only for protons in aliphatic
chains and rings, and then under special
conditions.
There are two requirements for the n1 rule to
work
1) All 3J values must be the same all along the
chain.
2) There must be free rotation or inversion
(rings) to make all of the hydrogens on a
single carbon be nearly equivalent.
Hydrogens can interchange their positions
by rotations about the C-C bonds.
The typical situation where the n1 rule applies.
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WHAT HAPPENS WHEN THE J VALUES ARE NOT EQUAL ?
H
H
H
3Ja 3Jb
C
C
C
H
H
H
3Ja 3Jb
In this situation each coupling must be
considered independently of the other.
A splitting tree is constructed as shown on
the next slide.
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CONSTRUCTING A TREE DIAGRAM
( SUPPOSE 3Ja 7 Hz and 3Jb 3 Hz )
The largest J value is usually used first.
H
H
H
-CH2-CH2-CH2-
C
C
C
H
H
H
3Ja 7
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WHEN BOTH 3J VALUES ARE THE SAME
The n1 rule is followed
-CH2-CH2-CH2-
n1 (4 1) 5
.. because of overlapping legs you get the
quintet predicted by the n1 rule.
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2-PHENYLPROPANAL
A case where there are unequal J values.
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Spectrum of 2-Phenylpropanal
a
b
d
TMS
c
a
c
d
b
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7 Hz
2 Hz
Rather than the expected quintet ..
the methine hydrogen is split by two
different 3J values.
quartet by -CH3
3J1 7 Hz
3J2 2 Hz
doublet by -CHO
ANALYSIS OF METHINE HYDROGENS SPLITTING
quartet of doublets
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2-PHENYLPROPANAL

• Adjacent protons are three bonds away from
• each other 3J, often 7 Hz

• The aldehyde proton d has a 3J 2 Hz
• coupling to the single proton b

• the methyl protons a have a 3J 7 Hz
• coupling to proton b

• proton b is a quartet of doublets

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VINYL ACETATE
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COUPLING CONSTANTS
PROTONS ON CC DOUBLE BONDS
PROTONS ON CC DOUBLE BONDS

• In alkenes, 3J-cis 8 Hz
• In alkenes, 3J-trans 16 Hz
• In alkenes, when protons
• are on the same carbon,
• 2J-geminal 0-2 Hz

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NMR Spectrum of Vinyl Acetate
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Analysis of Vinyl Acetate
3J-trans gt 3J-cis gt 2J-gem
HC
HB
HA
3JAC
3JBC
3JBC
cis
trans
trans
3JAC
2JAB
2JAB
cis
gem
gem
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OVERVIEW
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TYPES OF INFORMATION FROM THE NMR SPECTRUM
1. Each different type of hydrogen gives a peak
or group of peaks (multiplet).
2. The chemical shift (d, in ppm) gives a clue
as to the type of hydrogen generating the
peak (alkane, alkene, benzene, aldehyde,
etc.)
3. The integral gives the relative numbers of
each type of hydrogen.
4. Spin-spin splitting gives the number of
hydrogens on adjacent carbons.
5. The coupling constant J also gives
information about the arrangement of the
atoms involved.
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SPECTROSCOPY IS A POWERFUL TOOL
Generally, with only three pieces of data
1) empirical formula (or composition) 2)
infrared spectrum 3) NMR spectrum
a chemist can often figure out the
complete structure of an unknown molecule.
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EACH TECHNIQUE YIELDS VALUABLE DATA
FORMULA
Gives the relative numbers of C and H and other
atoms
INFRARED SPECTRUM
Reveals the types of bonds that are present.
NMR SPECTRUM
Reveals the environment of each hydrogen and the
relative numbers of each type.