13.9 spin-spin splitting 13.9 spin-spin splitting
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13.9 Spin-Spin Splitting13.9 Spin-Spin Splitting
Often a group of hydrogens will appear as a multipletrather than as a single peak.
SPIN-SPIN SPLITTINGSPIN-SPIN SPLITTING
Multiplets are named as follows:
Singlet QuintetDoublet SeptetTriplet OctetQuartet Nonet
This happens because of interaction with neighboring hydrogens and is called SPIN-SPIN SPLITTING.
C CH
Cl
Cl H
H
Cl
integral = 2
integral = 1
triplet doublet
1,1,2-Trichloroethane1,1,2-Trichloroethane
nn + 1 RULE + 1 RULE
C C
H H
H
C C
H H
H
two neighborsn+1 = 3triplet
one neighborn+1 = 2doublet
singletdoublettripletquartetquintetsextetseptet
MULTIPLETSthis hydrogen’s peakis split by its two neighbors
these hydrogens aresplit by their singleneighbor
Some Common PatternsSome Common Patterns
SOME COMMON SPLITTING PATTERNSSOME COMMON SPLITTING PATTERNS
CH2 CH2X Y
CH CHX Y( x = y )
( x = y )CH3
CH
CH3
-CH2-CH3
CH-CH3
CH-CH2
tert-butyl group
CH3
C Cl
CH3
H3C 9 equivalent protons = singlet
EXCEPTIONS TO THE N+1 RULEEXCEPTIONS TO THE N+1 RULEIMPORTANT !
Protons that are equivalent by symmetryusually do not split one another
CH CHX Y CH2 CH2X Y
no splitting if x=y no splitting if x=y
1)
2) Protons in the same group usually do not split one another
C
H
H
H or C
H
H
SOME EXAMPLE SPECTRASOME EXAMPLE SPECTRA WITH SPLITTINGWITH SPLITTING
NMR Spectrum of Bromoethane
CH2CH3Br
NMR Spectrum of 2-Nitropropane
CCH3 CH3
N
H
O O+
-
1:6:15:20:16:6:1 in higher multiplets the outer peaksare often nearly lost in the baseline
NMR Spectrum of Acetaldehyde
offset = 2.0 ppm
CCH3
O
H
The propyl group
CH3-CH2-CH2-X
Can you predict the splitting patterns for this compound?
INTENSITIES OF INTENSITIES OF MULTIPLET PEAKSMULTIPLET PEAKS
PASCAL’S TRIANGLE
1 2 1
PASCAL’S TRIANGLEPASCAL’S TRIANGLE
11 1
1 3 3 11 4 6 4 1
1 5 10 10 5 11 6 15 20 15 6 1
1 7 21 35 35 21 7 1
singlet
doublet
triplet
quartet
quintet
sextet
septet
octet
The interiorentries arethe sums ofthe two numbersimmediatelyabove.
Intensities ofmultiplet peaks
THE ORIGIN OF THE ORIGIN OF SPIN-SPIN SPLITTINGSPIN-SPIN SPLITTING
HOW IT HAPPENS
C C
H H
C C
H HA A
upfielddownfield
Bo
THE CHEMICAL SHIFT OF PROTON HTHE CHEMICAL SHIFT OF PROTON HAA IS IS
AFFECTED BY THE SPIN OF ITS NEIGHBORSAFFECTED BY THE SPIN OF ITS NEIGHBORS
50 % ofmolecules
50 % ofmolecules
At any given time about half of the molecules in solution willhave spin +1/2 and the other half will have spin -1/2.
aligned with Bo opposed to Bo
neighbor aligned neighbor opposed
+1/2 -1/2
C C
H H
C C
H H
one neighborn+1 = 2doublet
one neighborn+1 = 2doublet
SPIN ARRANGEMENTSSPIN ARRANGEMENTS
yellow spins
blue spins
The resonance positions (splitting) of a given hydrogen is affected by the possible spins of its neighbor.
C C
H H
H
C C
H H
H
two neighborsn+1 = 3triplet
one neighborn+1 = 2doublet
SPIN ARRANGEMENTSSPIN ARRANGEMENTS
methylene spins
methine spins
three neighborsn+1 = 4quartet
two neighborsn+1 = 3triplet
SPIN ARRANGEMENTSSPIN ARRANGEMENTS
C C
H H
H
H
H
C C
H H
H
H
H
methyl spins
methylene spins
13.10 The Coupling Constant13.10 The Coupling Constant
J J
J
J J
THE COUPLING CONSTANTTHE COUPLING CONSTANT
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.
C
H
H
C H
H
H
J
100 MHz
200 MHz
123456
123
100 Hz
200 Hz
200 Hz
400 Hz
J = 7.5 Hz
J = 7.5 Hz
7.5 Hz
7.5 Hz
Coupling constants areconstant - they do not change at differentfield strengths
The shift isdependanton the field
ppm
FIELD COMPARISON
Separationis larger
123
123
100 MHz
200 MHz
Why buy a higherfield instrument?
Spectra aresimplified!
Overlapping multiplets areseparated.
Second-ordereffects are minimized.
123
50 MHz
J = 7.5 Hz
J = 7.5 Hz
J = 7.5 Hz
NOTATION FOR COUPLING CONSTANTSNOTATION FOR COUPLING CONSTANTSThe most commonly encountered type of coupling is between hydrogens on adjacent carbon atoms.
C C
HH This is sometimes called vicinal coupling.It is designated 3J since three bondsintervene between the two hydrogens.
Another type of coupling that can also occur in special cases is
C H
H2J or geminal coupling
Geminal coupling does not occur whenthe two hydrogens are equivalent due torotations around the other two bonds.
( most often 2J = 0 )
3J
2J
Couplings larger than 2J or 3J also exist, but operate only in special situations, especially in unsaturatedsystems.
Couplings larger than 3J (e.g., 4J, 5J, etc) are usually called “long-range coupling.”
LONG RANGE COUPLINGSLONG RANGE COUPLINGS
C C
H H
C CH
H
C CHH
CH
H
6 to 8 Hz
11 to 18 Hz
6 to 15 Hz
0 to 5 Hz
three bond 3J
two bond 2J
three bond 3J
three bond 3J
SOME REPRESENTATIVE COUPLING CONSTANTSSOME REPRESENTATIVE COUPLING CONSTANTS
trans
cis
geminal
vicinal
CH
C H4 to 10 Hz
H C C CH
0 to 3 Hz four bond 4J
three bond 3J
C CC H
H0 to 3 Hz four bond 4J
H
H
cis
trans
6 to 12 Hz
4 to 8 Hzthree bond 3J
Couplings that occur at distances greater than three bonds arecalled long-range couplings and they are usually small (<3 Hz)
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 C=O: 2.0 - 2.5 ppm
• Methylene groups directly attached to oxygen appear near 4.0 ppm
CH3 C
O
CH2CH3
2-Butanone (Methyl Ethyl Ketone)60 MHz Spectrum
1
WWU Chemistry
2-butanone, 300 MHz spectrum
Ethyl Acetate2
CH3 C
O
O CH2CH3
Compare the methylene shift to that of Methyl Ethyl Ketone (previous slide).
t-Butyl Methyl Ketone3
C
O
CCH3 CH3
CH3
CH3
(3,3-dimethyl-2-butanone)
Phenylethyl Acetate4
CH2CH2 O C
O
CH3
Ethyl Succinate5
O C
O
CH2CH2 C
O
OCH3CH2 CH2CH3
-Chloropropionic Acid6
CH C
O
OH
Cl
CH3
13.12 and 13.1313.12 and 13.13Alkenes, Alkynes Alkenes, Alkynes and Aromatic and Aromatic CompoundsCompounds
• 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
CHEMICAL SHIFTS
Alkenes and alkynes
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.
BENZENE RING HYDROGENSBENZENE RING HYDROGENS
NMR Spectrum of Toluene
CH35
3
C
HH
ROC
RO
HH
Only the o- protons are in range for this effect.
When a carbonyl group is attached to the ring theo- and p- protons are deshielded by the anisotropicfield of C=O
THE EFFECT OF CARBONYL SUBSTITUENTSTHE EFFECT OF CARBONYL SUBSTITUENTS
CCH3O
HH
Acetophenone (90 MHz)
2 3
3
deshielded
NMR Spectrum of 1-iodo-4-methoxybenzene
OCH3I
CHCl3 impurity
2 2
3
NMR Spectrum of 1-bromo-4-ethoxybenzene
OCH2CH3Br
4
2
3
X
Y
X
X'
X
X
X = Y X ~ X’ X = X
THE p-DISUBSTITUTED PATTERN CHANGES AS THETHE p-DISUBSTITUTED PATTERN CHANGES AS THE TWO GROUPS BECOME MORE AND MORE SIMILARTWO GROUPS BECOME MORE AND MORE SIMILAR
all Hequivalent
All peaks move closer.Outer peaks get smaller …………………..… and finally disappear.Inner peaks get taller…………………………. and finally merge.
same groups
NMR Spectrum of 1-amino-4-ethoxybenzene
OCH2CH3H2N4
2 2
3
NMR Spectrum of p-Xylene
(1,4-dimethylbenzene)
CH3CH3
4
6
13.14 Hydroxyl 13.14 Hydroxyl and Amino and Amino ProtonsProtons
Hydroxyl and Amino Protons
Carboxylic acid protons generally appear fardownfield near 11 to 12 ppm.
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.
SPIN-SPIN DECOUPLING BY EXCHANGESPIN-SPIN DECOUPLING BY EXCHANGE
In alcohols coupling between the O-H hydrogen andthose on adjacent carbon atoms is usually not seen.
C O
H H
This is due to rapid exchange ofOH protons between the various alcohol molecules in the solution. The OH peak is usually broad.
In ultrapure alcohols, however,coupling will sometimes be seen.
NMR Spectrum of Ethanol
CH3CH2 OH
2 1
3
1-propanol
CH3CH2CH2 OH
13.16 Unequal Couplings13.16 Unequal Couplings Tree DiagramsTree Diagrams
WHERE DOES THE N+1 RULE WORK ?WHERE DOES THE N+1 RULE WORK ?The n+1 rule works only for protons in aliphatic chainsand rings, and then under special conditions.
1) All 3J values must be the same all along the chain.
There are two requirements for the n+1 rule to work:
2) There must be free rotation or inversion (rings) to make all of the hydrogens on a single carbon be nearly equivalent.
CH
HCH
HCH
H3Ja = 3Jb
The typical situationwhere the n+1 ruleapplies.
Hydrogens can interchange theirpositions byrotations aboutthe C-C bonds.
WHAT HAPPENS WHEN THE J VALUES ARE NOT EQUAL ?
CH
HCH
HCH
H3Ja
3Jb
3Ja = 3Jb
In this situation each coupling must be consideredindependently of the other.
A “splitting tree” is constructed as shown on thenext slide.
CH
HCH
HCH
H3Ja = 7
-CH2-CH2-CH2-
CONSTRUCTING A TREE DIAGRAMCONSTRUCTING A TREE DIAGRAM( SUPPOSE 3Ja = 7 Hz and 3Jb = 3 Hz )
The largest J value is usually used first.
CH
HCH
HCH
H3Jb = 3
triplet of triplets
WHEN BOTH 3J VALUES ARE THE SAME
-CH2-CH2-CH2-
….. because of overlapping legsyou get the quintet predicted bythe n+1 rule.
The n+1 rule is followed
n+1 = (4 + 1) = 5
2-PHENYLPROPANAL2-PHENYLPROPANAL
A case where there are unequal J values.
Spectrum of 2-Phenylpropanal
J = 2 Hz
J = 7 Hz
a
b
c
d
CHCH3 CHO
a b d
c TMS
CHCH3 CHO
3J1 = 7 Hz
7 Hz 2 Hz
3J2 = 2 Hz
the methine hydrogen is split by two different3J values.
Rather than the expectedquintet …..
ANALYSISANALYSISOF METHINEOF METHINEHYDROGEN’SHYDROGEN’SSPLITTINGSPLITTING
quartet by -CH3
doubletby -CHO
quartet of doublets
• proton b is a quartet of doublets
• 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
2-PHENYLPROPANAL2-PHENYLPROPANAL
VINYL ACETATEVINYL ACETATE
• 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
PROTONS ON C=C DOUBLE BONDS
H
H
HH
H
H
PROTONS ON C=C DOUBLE BONDSPROTONS ON C=C DOUBLE BONDSCOUPLING CONSTANTS
NMR Spectrum of Vinyl Acetate
CH3 C
O
O CH CH2
Analysis of Vinyl AcetateAnalysis of Vinyl Acetate
HC HB HA
CCHH33 CC
OO
OOCC
HHCC
CCHHAA
HHBB
3JBC
3JAC
3JAC3JBC
2JAB2JAB
trans trans
cis
cis
gem gem
3J-trans > 3J-cis > 2J-gem
OVERVIEW
TYPES OF INFORMATION TYPES OF INFORMATION FROM THE NMR SPECTRUMFROM THE NMR SPECTRUM
1. Each different type of hydrogen gives a peak or group of peaks (multiplet).
3. The integral gives the relative numbers of each type of hydrogen.
2. The chemical shift (in ppm) gives a clue as to the type of hydrogen generating the peak (alkane, alkene, benzene, aldehyde, etc.)
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.
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 completestructure of an unknown molecule.
SPECTROSCOPY IS A POWERFUL TOOLSPECTROSCOPY IS A POWERFUL TOOL
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 hydrogenand the relative numbers of each type.
EACH TECHNIQUE YIELDS VALUABLE DATAEACH TECHNIQUE YIELDS VALUABLE DATA