1. 13.9 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
integral = 2
integral = 1
triplet
doublet

4. n + 1 RULE

5. 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

6. Some Common Patterns

7. SOME COMMON SPLITTING PATTERNS
CH-CH3
( x = y )
CH-CH2
-CH2-CH3
CH3
( x = y ) CH
CH3

8. tert-butyl group

9. 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 or

10. SOME EXAMPLE SPECTRA
WITH SPLITTING

11. NMR Spectrum of Bromoethane

12. 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

13. NMR Spectrum of Acetaldehyde
offset = 2.0 ppm

14. The propyl group

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 n+1 = 2 doublet
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. 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

26. 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

27. 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.”

28. 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

29. 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)

30. 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: ppm
Methylene groups directly attached to oxygen appear near 4.0 ppm

31. 1
2-Butanone (Methyl Ethyl Ketone)
60 MHz Spectrum

32. 2-butanone, 300 MHz spectrum
WWU Chemistry

33. 2
Ethyl Acetate
Compare the methylene shift to that of Methyl Ethyl Ketone (previous slide).

34. 3
t-Butyl Methyl Ketone
(3,3-dimethyl-2-butanone)

35. 4
Phenylethyl Acetate

36. 5
Ethyl Succinate

37. a-Chloropropionic Acid6

38. 13.12 and 13.13
Alkenes, Alkynes and Aromatic Compounds

39. Alkenes and alkynes CHEMICAL SHIFTS 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

40. 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.

41. NMR Spectrum of Toluene5 3

42. 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 C=O
Only the o- protons are in range for this effect.

43. Acetophenone (90 MHz) 3 2 3
deshielded

44. NMR Spectrum of 1-iodo-4-methoxybenzene3
CHCl3 impurity 2 2

45. NMR Spectrum of 1-bromo-4-ethoxybenzene3 4 2

46. 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

47. NMR Spectrum of 1-amino-4-ethoxybenzene3 4 2 2

48. NMR Spectrum of p-Xylene (1,4-dimethylbenzene)6 4

49. Hydroxyl
and Amino Protons

50. 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.

51. C O H H 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.

52. NMR Spectrum of Ethanol3 2 1

53. 1-propanol

54. 13.16 Unequal Couplings
Tree Diagrams

55. WHERE DOES THE N+1 RULE WORK ?
The n+1 rule works only for protons in aliphatic chains
and rings, and then under special conditions.
There are two requirements for the n+1 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. C H
3Ja = 3Jb
Hydrogens can
interchange their
positions by
rotations about
the C-C bonds.
The typical situation
where the n+1 rule
applies.

56. WHAT HAPPENS WHEN THE J VALUES ARE NOT EQUAL ?
3Ja = 3Jb C C C H H H
3Ja Jb
In this situation each coupling must be considered
independently of the other.
A “splitting tree” is constructed as shown on the
next slide.

57. 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 C H
3Jb = 3
triplet of triplets H H H
3Ja = 7

58. WHEN BOTH 3J VALUES ARE THE SAME
The n+1 rule is followed
-CH2-CH2-CH2-
n+1 = (4 + 1) = 5
….. because of overlapping legs
you get the quintet predicted by
the n+1 rule.

59. 2-PHENYLPROPANAL
A case where there are unequal J values.

60. Spectrum of 2-Phenylpropanalb d
J = 7 Hz c
TMS a c
J = 2 Hz d b

61. 3J1 = 7 Hz 3J2 = 2 Hz ANALYSIS OF METHINE HYDROGEN’S SPLITTING 7 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
HYDROGEN’S
SPLITTING
quartet of doublets

62. 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

63. VINYL ACETATE

64. PROTONS ON C=C DOUBLE BONDS
COUPLING CONSTANTS
PROTONS ON C=C DOUBLE BONDS
PROTONS ON C=C 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

65. NMR Spectrum of Vinyl Acetate

66. Analysis of Vinyl AcetateH 3 O HC HA HB
3J-trans > 3J-cis > 2J-gem HC HB HA
3JAC
3JBC
3JBC
cis
trans
trans
3JAC
2JAB
2JAB
cis
gem
gem

67. OVERVIEW

68. 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.

69. 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.

70. 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.