nmr 4 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. 
SPIN-SPIN SPLITTING 
Multiplets are named as follows: 
Singlet Quintet 
Doublet Septet 
Triplet Octet 
Quartet 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-Trichloroethane 
The two kinds of hydrogens do not appear as 
a “triplet” and a “doublet”. 
The subpeaks are due to 
spin-spin splitting and are 
predicted by the n+1 rule. 
n + 1 RULE 
1,1,2-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 neighbors 
n+1 = 3 
triplet 
one neighbor 
n+1 = 2 
doublet 
singlet 
doublet 
triplet 
quartet 
quintet 
sextet 
septet 
MULTIPLETS this hydrogen’s peak 
is split by its two neighbors 
these hydrogens are 
split by their single 
neighbor 
EXCEPTIONS TO THE N+1 RULE 
IMPORTANT ! 
Protons that are equivalent by symmetry 
usually 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
3) The n+1 rule applies principally to protons in 
aliphatic (saturated) chains or on saturated rings. 
EXCEPTIONS TO THE N+1 RULE 
CH
2
CH
2
CH
2
CH
2
CH
3
CH
3
H
or 
but does not apply (in the simple way shown here) 
to protons on double bonds or on benzene rings. 
CH
3
H
H
H
CH
3
NO 
NO 
YES YES 
SOME COMMON PATTERNS 
SOME COMMON SPLITTING PATTERNS 
CH2 CH2X Y
CH CHX Y
CH2 CH
CH3 CH
CH3 CH2
CH3
CH
CH3
( x = y ) 
( x = y ) 
SOME EXAMPLE SPECTRA 
 WITH 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 peaks 
are often nearly lost in the baseline 
NMR Spectrum of Acetaldehyde 
offset = 2.0 ppm 
CCH3
O
H
 INTENSITIES OF 
MULTIPLET PEAKS 
PASCAL’S TRIANGLE 
1 2 1 
PASCAL’S TRIANGLE 
1 
1 1 
1 3 3 1 
1 4 6 4 1 
1 5 10 10 5 1 
1 6 15 20 15 6 1 
1 7 21 35 35 21 7 1 
singlet 
doublet 
triplet 
quartet 
quintet 
sextet 
septet 
octet 
The interior 
entries are 
the sums of 
the two 
numbers 
immediately 
above. 
Intensities of 
multiplet peaks 
 THE ORIGIN OF 
SPIN-SPIN SPLITTING 
HOW IT HAPPENS 
C C 
H H 
C C 
H H A A 
upfield downfield 
Bo 
THE CHEMICAL SHIFT OF PROTON HA IS 
AFFECTED BY THE SPIN OF ITS NEIGHBORS 
50 % of 
molecules 
50 % of 
molecules 
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. 
aligned with Bo opposed to Bo 
neighbor aligned neighbor opposed 
+1/2 -1/2 
C C 
H H 
C C 
H H 
one neighbor 
 n+1 = 2 
 doublet 
one neighbor 
 n+1 = 2 
 doublet 
SPIN 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 ARRANGEMENTS 
methylene spins 
methine spins 
three neighbors 
 n+1 = 4 
 quartet 
two neighbors 
 n+1 = 3 
 triplet 
SPIN ARRANGEMENTS 
C C 
H H 
H 
H 
H 
C C 
H H 
H 
H 
H 
methyl spins 
methylene spins 
THE COUPLING CONSTANT 
J J 
J 
J J 
THE 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 
1 2 3 4 5 6 
1 2 3 
100 Hz 
200 Hz 
200 Hz 
400 Hz 
J = 7.5 Hz 
J = 7.5 Hz 
7.5 Hz 
7.5 Hz 
Coupling constants are 
constant - they do not 
change at different 
field strengths 
The shift is 
dependant 
on the field 
ppm 
FIELD COMPARISON 
Separation 
is larger 
100 MHz 
200 MHz 
1 2 3 4 5 6 
1 2 3 
100 Hz 
200 Hz 
J = 7.5 Hz 
J = 
7.5 Hz 
ppm 4 
200 Hz 
400 Hz 
5 6 
J = 7.5 Hz 
Note the compression of 
multiplets in the 200 MHz 
spectrum when it is 
plotted on the same scale 
as the 100 MHz spectrum 
instead of on a chart which 
is twice as wide. 
Separation 
is larger 
1 2 3 
1 2 3 
100 MHz 
200 MHz 
Why buy a higher 
field instrument? 
Spectra are 
simplified! 
Overlapping 
multiplets are 
separated. 
Second-order 
effects are 
minimized. 
1 2 3 
 50 MHz 
J = 7.5 Hz 
J = 7.5 Hz 
J = 7.5 Hz 
NOTATION FOR COUPLING CONSTANTS 
The 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 bonds 
intervene between the two hydrogens. 
Another type of coupling that can also occur in special 
cases is 
C H
H
2J or geminal coupling 
Geminal coupling does not occur when 
the two hydrogens are equivalent due to 
rotations 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.” 
C 
C 
C 
H H 
4J , for instance, occurs mainly 
when the hydrogens are forced 
to adopt this “W” conformation 
(as in bicyclic compounds). 
LONG RANGE COUPLINGS 
C C
H H
C C
H
H
C C
HH
C
H
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 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 
C
H
C H
4 to 10 Hz 
H C C C
H
0 to 3 Hz four bond 4J 
three bond 3J 
C C
C H
H 0 to 3 Hz four bond 4J 
H
H
cis 
trans 
6 to 12 Hz 
4 to 8 Hz 
three bond 3J 
Couplings that occur at distances greater than three bonds are 
called long-range couplings and they are usually small (<3 Hz) 
and frequently nonexistent (0 Hz). 
OVERVIEW 
 TYPES OF INFORMATION 
FROM 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 complete 
structure of an unknown molecule. 
SPECTROSCOPYIS 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 hydrogen 
and the relative numbers of each type. 
EACH TECHNIQUE YIELDS VALUABLE DATA