The document discusses spin-spin splitting in NMR spectroscopy, explaining the n+1 rule for predicting peak multiplicities based on neighboring hydrogen atoms. It also covers the coupling constant, which measures the interaction between hydrogen sets, and the effects of molecular structure on chemical shifts, particularly for alkenes, alkynes, and carbonyl compounds. Additionally, it highlights the significance of various spectral characteristics in deducing the molecular structure of unknown compounds.

1. 13.9 Spin-Spin Splitting

2. 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.

3. C C
H
Cl
Cl H
H
Cl
integral = 2
integral = 1
triplet doublet
1,1,2-Trichloroethane

4. n + 1 RULE

5. 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

6. Some Common Patterns

7. SOME COMMON SPLITTING PATTERNS
CH2 CH2
X Y
CH CH
X Y
( x = y )
( x = y )
CH3
CH
CH
3
-CH2-CH3
CH-CH3
CH-CH2

8. tert-butyl group
CH3
C Cl
CH3
H3C 9 equivalent protons = singlet

9. EXCEPTIONS TO THE N+1 RULE
IMPORTANT !
Protons that are equivalent by symmetry
usually do not split one another
CH CH
X Y CH2 CH2
X 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

10. SOME EXAMPLE SPECTRA
WITH SPLITTING

11. NMR Spectrum of Bromoethane
CH2CH3
Br

12. NMR Spectrum of 2-Nitropropane
C
CH3 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

13. NMR Spectrum of Acetaldehyde
offset = 2.0 ppm
C
CH3
O
H

14. The propyl group
CH3-CH2-CH2-X
Can you predict the splitting patterns for this compound?

15. INTENSITIES OF
MULTIPLET PEAKS
PASCAL’S TRIANGLE

16. 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

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

18. 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

19. 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.

20. 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

21. 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

22. 13.10 The Coupling Constant

23. 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

24. 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

25. 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

26. NOTATION FOR COUPLING CONSTANTS
The most commonly encountered type of coupling is
between hydrogens on adjacent carbon atoms.
C C
H
H 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

27. 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.”
LONG RANGE COUPLINGS

28. C C
H H
C C
H
H
C C
H
H
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
trans
cis
geminal
vicinal

29. 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)

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: 2.0 - 2.5 ppm
• Methylene groups directly attached to
oxygen appear near 4.0 ppm

31. CH3 C
O
CH2CH3
2-Butanone (Methyl Ethyl Ketone)
60 MHz Spectrum
1

32. WWU Chemistry
2-butanone, 300 MHz
spectrum

33. Ethyl Acetate
2
CH3 C
O
O CH2CH3
Compare the methylene shift to that of Methyl Ethyl Ketone (previous slide).

34. t-Butyl Methyl Ketone
3
C
O
C
CH3 CH3
CH3
CH3
(3,3-dimethyl-2-butanone)

35. Phenylethyl Acetate
4
CH2CH2 O C
O
CH3

36. Ethyl Succinate
5
O C
O
CH2CH2 C
O
O
CH3CH2 CH2CH3

37. a-Chloropropionic Acid
6
CH C
O
OH
Cl
CH3

38. 13.12 and 13.13
Alkenes, Alkynes and
Aromatic Compounds

39. • 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

40. 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 HYDROGENS

41. NMR Spectrum of Toluene
CH3
5
3

42. C
H
H
R
O
C
R
O
H
H
Only the o- protons are in range for this effect.
When a carbonyl group is attached to the ring the
o- and p- protons are deshielded by the anisotropic
field of C=O
THE EFFECT OF CARBONYL SUBSTITUENTS

43. C
CH3
O
H
H
Acetophenone (90 MHz)
2 3
3
deshielded

44. NMR Spectrum of
1-iodo-4-methoxybenzene
OCH3
I
CHCl3 impurity
2 2
3

45. NMR Spectrum of
1-bromo-4-ethoxybenzene
OCH2CH3
Br
4
2
3

46. X
Y
X
X'
X
X
X = Y X ~ X’ X = X
THE p-DISUBSTITUTED PATTERN CHANGES AS THE
TWO GROUPS BECOME MORE AND MORE SIMILAR
all H
equivalent
All peaks move closer.
Outer peaks get smaller …………………..… and finally disappear.
Inner peaks get taller…………………………. and finally merge.
same groups

47. NMR Spectrum of
1-amino-4-ethoxybenzene
OCH2CH3
H2N
4
2 2
3

48. NMR Spectrum of p-Xylene
(1,4-dimethylbenzene)
CH3
CH3
4
6

49. 13.14 Hydroxyl
and Amino Protons

50. Hydroxyl and Amino Protons
Carboxylic acid protons generally appear far
downfield 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.

51. SPIN-SPIN DECOUPLING BY EXCHANGE
In alcohols coupling between the O-H hydrogen and
those on adjacent carbon atoms is usually not seen.
C O
H H
This is due to rapid exchange of
OH protons between the various
alcohol molecules in the solution.
The OH peak is usually broad.
In ultrapure alcohols, however,
coupling will sometimes be seen.

52. NMR Spectrum of Ethanol
CH3CH2 OH
2 1
3

53. 1-propanol
CH3CH2CH2 OH

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.
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.
C
H
H
C
H
H
C
H
H
3Ja = 3Jb
The typical situation
where the n+1 rule
applies.
Hydrogens can
interchange their
positions by
rotations about
the C-C bonds.

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

57. C
H
H
C
H
H
C
H
H
3Ja = 7
-CH2-CH2-CH2-
CONSTRUCTING A TREE DIAGRAM
( SUPPOSE 3Ja = 7 Hz and 3Jb = 3 Hz )
The largest J value
is usually used first.
C
H
H
C
H
H
C
H
H
3Jb = 3
triplet of triplets

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

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

60. Spectrum of 2-Phenylpropanal
J = 2 Hz
J = 7 Hz
a
b
c
d
CH
CH3 CHO
a b d
c TMS

61. CH
CH3 CHO
3J1 = 7 Hz
7 Hz 2 Hz
3J2 = 2 Hz
the methine hydrogen
is split by two different
3J values.
Rather than the expected
quintet …..
ANALYSIS
OF METHINE
HYDROGEN’S
SPLITTING
quartet
by -CH3
doublet
by -CHO
quartet of doublets

62. • 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-PHENYLPROPANAL

63. VINYL ACETATE

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

65. NMR Spectrum of Vinyl
Acetate
CH3 C
O
O CH CH2

66. Analysis of Vinyl Acetate
HC HB HA
CH3 C
O
O
C
HC
C
HA
HB
3JBC
3JAC
3JAC
3JBC
2JAB
2JAB
trans trans
cis
cis
gem gem
3J-trans > 3J-cis > 2J-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).
3. The integral gives the relative numbers of each
type of hydrogen.
2. The chemical shift (d, 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.

69. 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.
SPECTROSCOPY IS A POWERFUL TOOL

70. 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.
EACH TECHNIQUE YIELDS VALUABLE DATA