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SPIN-SPIN SPLITTINGSPIN-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
1,1,2-Trichloroethane
• Two types of H
C CH
Cl
Cl H
H
Cl
C CH
Cl
Cl H
H
Cl
integral = 2
integral = 1
triplet doublet
1,1,2-Trichloroethane1,1,2-TrichloroethaneThe two kinds of hydrogens do not appear as single peaks,rather there is a “triplet” and a “doublet”.
The subpeaks are due tospin-spin splitting and are predicted by the n+1 rule.
nn + 1 RULE + 1 RULE
1,1,2-Trichloroethane1,1,2-Trichloroethane
C CH
Cl
Cl H
H
Cl
integral = 2
integral = 1
Where do these multiplets come from ? ….. interaction with neighbors
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
1,1,2-Trichloroethane1,1,2-Trichloroethane
C CH
Cl
Cl H
H
Cl
integral = 2
integral = 1
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
moredetaillater
3) The n+1 rule applies principally to protons in aliphatic (saturated) chains or on saturated rings.
EXCEPTIONS TO THE N+1 RULEEXCEPTIONS TO THE N+1 RULE
CH2CH2CH2CH2CH3
CH3Hor
but does not apply (in the simple way shown here) to protons on double bonds or on benzene rings.
CH3
H
H
H
CH3
NONO NONO
YESYES YESYES
SOME COMMON PATTERNSSOME COMMON PATTERNS
SOME COMMON SPLITTING PATTERNSSOME COMMON SPLITTING PATTERNS
CH2 CH2X Y
CH CHX Y
CH2 CH
CH3 CH
CH3 CH2
CH3
CHCH3
( X = Y )
( X = Y )
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
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 +½ and the other half will have spin -½.
aligned with Bo opposed to Bo
neighbor aligned neighbor opposed
+½ -½
C C
H H
C C
H H
one neighbor n+1 = 2 doublet
one neighbor n+1 = 2 doublet
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 neighbors n+1 = 3 triplet
one neighbor n+1 = 2 doublet
SPIN ARRANGEMENTSSPIN ARRANGEMENTS
methylene spinsmethine spins
three neighbors n+1 = 4 quartet
two neighbors n+1 = 3 triplet
SPIN ARRANGEMENTSSPIN ARRANGEMENTS
C C
H H
H
H
H
C C
H H
H
H
H
methyl spinsmethylene spins
THE COUPLING CONSTANTTHE 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
100 MHz
200 MHz
123456
123
100 Hz
200 Hz
J = 7.5 Hz
J =7.5 Hz
ppm4
200 Hz
400 Hz
56
J = 7.5 Hz
Note the compression ofmultiplets in the 200 MHzspectrum when it is plotted on the same scale as the 100 MHz spectruminstead of on a chart whichis twice as wide.
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.
Couplings larger than 3J (e.g., 4J, 5J, etc) are usually called “long-range coupling.”
CC
CH H
4J , for instance, occurs mainlywhen the hydrogens are forcedto adopt this “W” conformation(as in bicyclic compounds).
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
Hax
Hax
Heq
Heq
Hax,Hax = 8 to 14
Hax,Heq = 0 to 7
Heq,Heq = 0 to 5
three bond 3J
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• they are usually small (<3 Hz) and frequently nonexistent (0 Hz).
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 enviroment of each hydrogenand the relative numbers of each type.
EACH TECHNIQUE YIELDS VALUABLE DATAEACH TECHNIQUE YIELDS VALUABLE DATA
