1. SPIN-SPIN SPLITTING

2. 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
Quartet Nonet
This happens because of interaction with neighboring
hydrogens and is called
SPIN-SPIN SPLITTING.

3. 1,1,2-Trichloroethane
The two kinds of hydrogens do not appear as single peaks,
rather there is a “triplet” and a “doublet”.
integral = 2
integral = 1
The subpeaks are due to
spin-spin splitting and are
predicted by the n+1 rule.
triplet
doublet

4. n + 1 RULE

5. 1,1,2-Trichloroethane Where do these multiplets come from ?
integral = 2
integral = 1
Where do these multiplets come from ?
….. interaction with neighbors

6. 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
MULTIPLETS
singlet
doublet
triplet
quartet
quintet
sextet
septet
two neighbors
n+1 = 3
triplet
one neighbor
n+1 = 2
doublet

7. EXCEPTIONS TO THE N+1 RULE
IMPORTANT ! 1)
Protons that are equivalent by symmetry
usually do not split one another
no splitting if x=y
no splitting if x=y 2)
Protons in the same group
usually do not split one another
more
detail
later or

8. EXCEPTIONS TO THE N+1 RULE3)
The n+1 rule applies principally to protons in
aliphatic (saturated) chains or on saturated rings. or
YES
YES
but does not apply (in the simple way shown here)
to protons on double bonds or on benzene rings. NO NO

9. SOME COMMON PATTERNS

10. SOME COMMON SPLITTING PATTERNS
( x = y )
( x = y )

11. SOME EXAMPLE SPECTRA
WITH SPLITTING

12. NMR Spectrum of Bromoethane

13. NMR Spectrum of 2-Nitropropane
1:6:15:20:16:6:1
in higher multiplets the outer peaks
are often nearly lost in the baseline

14. NMR Spectrum of Acetaldehyde
offset = 2.0 ppm

15. INTENSITIES OF
MULTIPLET PEAKS
PASCAL’S TRIANGLE

16. PASCAL’S TRIANGLE
Intensities of
multiplet peaks 1
singlet
1 1
The interior
entries are
the sums of
the two
numbers
immediately
above.
doublet
triplet
quartet
quintet
sextet
septet
octet

17. THE ORIGIN OF
SPIN-SPIN SPLITTING
HOW IT HAPPENS

18. 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
downfield
upfield
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.

19. SPIN ARRANGEMENTS H H H H C C C C one neighbor one neighbor n+1 = 2
doublet
one neighbor
n+1 = 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.

20. SPIN ARRANGEMENTS two neighbors n+1 = 3 triplet one neighbor n+1 = 2
doublet
methine spins
methylene spins

21. SPIN ARRANGEMENTS C H C H three neighbors n+1 = 4 quartet
two neighbors
n+1 = 3
triplet C H C H
methylene spins
methyl spins

22. THE COUPLING CONSTANT

23. THE COUPLING CONSTANT 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.

24. 100 MHz 6 5 4 3 2 1 200 MHz 3 2 1 FIELD COMPARISON 200 Hz 100 Hz
Coupling constants are
constant - they do not
change at different
field strengths
7.5 Hz
J = 7.5 Hz 6 5 4 3 2 1
200 MHz
400 Hz
Separation
is larger
200 Hz
7.5 Hz
The shift is
dependant
on the field
J = 7.5 Hz 3 2 1
ppm

25. 100 MHz
200 Hz
100 Hz
J =
7.5 Hz
J = 7.5 Hz 6 5 4 3 2 1
200 MHz
Separation
is larger
400 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.
200 Hz
J = 7.5 Hz 6 5 4 3 2 1
ppm

26. 50 MHz
J = 7.5 Hz
Why buy a higher
field instrument? 3 2 1
Spectra are
simplified!
100 MHz
J = 7.5 Hz
Overlapping
multiplets are
separated. 3 2 1
200 MHz
J = 7.5 Hz
Second-order
effects are
minimized. 3 2 1

27. 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
( most often 3J = 7.5 )
Another type of coupling that can also occur in special
cases is
2J or geminal coupling
( most often 2J = 0 ) 2J
Geminal coupling does not occur when
the two hydrogens are equivalent due to
rotations around the other two bonds.

28. LONG RANGE COUPLINGS C C C
Couplings larger than 2J or 3J also exist, but operate
only in special situations. H C H C C
4J , for instance, occurs mainly
when the hydrogens are forced
to adopt this “W” conformation
(as in bicyclic compounds).
Couplings larger than 3J (e.g., 4J, 5J, etc) are usually
called “long-range coupling.”

29. SOME REPRESENTATIVE COUPLING CONSTANTS
6 to 8 Hz
three bond 3J
vicinal
11 to 18 Hz
three bond 3J
trans
cis
6 to 15 Hz
three bond 3J
geminal
0 to 5 Hz
two bond 2J
Hax,Hax = 8 to 14
Hax,Heq = 0 to 7
three bond 3J
Heq,Heq = 0 to 5

30. cis 6 to 12 Hz three bond 3J trans 4 to 8 Hz 4 to 10 Hz three bond 3J
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 (<3 Hz)
and frequently nonexistent (0 Hz).

31. OVERVIEW

32. 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.

33. 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.

34. 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 enviroment of each hydrogen
and the relative numbers of each type.