SPIN-SPIN SPLITTING
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
Doublet
Triplet
Quartet
Quintet
Septet
Octet
Nonet
This happens because of interaction with neighboring
hydrogens and is called
SPIN-SPIN SPLITTING.
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
Cl H
H C C Cl
integral = 1
triplet
Cl H
doublet
The subpeaks are due to
spin-spin splitting and are
predicted by the n+1 rule.
n + 1 RULE
1,1,2-Trichloroethane
integral = 2
Cl H
H C C Cl
integral = 1
Cl H
Where do these multiplets come from ?
….. interaction with neighbors
this hydrogen’s peak
is split by its two neighbors
these hydrogens are
split by their single
neighbor
H
H
H
H
C
C
C
C
H
two neighbors
n+1 = 3
triplet
H
one neighbor
n+1 = 2
doublet
MULTIPLETS
singlet
doublet
triplet
quartet
quintet
sextet
septet
EXCEPTIONS TO THE N+1 RULE
IMPORTANT !
1)
2)
Protons that are equivalent by symmetry
usually do not split one another
X CH CH Y
X CH2 CH2 Y
no splitting if x=y
no splitting if x=y
Protons in the same group
usually do not split one another
H
C H
H
H
or
C
H
more
detail
later
EXCEPTIONS TO THE N+1 RULE
3)
The n+1 rule applies principally to protons in
aliphatic (saturated) chains or on saturated rings.
CH3
CH2CH2CH2CH2CH3
or
H
YES
YES
but does not apply (in the simple way shown here)
to protons on double bonds or on benzene rings.
H
CH3
H
H
CH3
NO
NO
SOME COMMON PATTERNS
SOME COMMON SPLITTING PATTERNS
X CH CH Y
CH3 CH
(x=y)
CH2 CH
X CH2 CH2 Y
(x=y)
CH3 CH2
CH3
CH
CH3
SOME EXAMPLE SPECTRA
WITH SPLITTING
NMR Spectrum of Bromoethane
Br CH2CH3
NMR Spectrum of 2-Nitropropane
H
CH3
C
CH3
+
N
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
O
CH3 C
offset = 2.0 ppm
H
INTENSITIES OF
MULTIPLET PEAKS
PASCAL’S TRIANGLE
PASCAL’S TRIANGLE
Intensities of
multiplet peaks
1
The interior
1 1
entries are
the sums of
1 2 1
the two
numbers
immediately
1
3
3
1
above.
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 ORIGIN OF
SPIN-SPIN SPLITTING
HOW IT HAPPENS
THE CHEMICAL SHIFT OF PROTON HA IS
AFFECTED BY THE SPIN OF ITS NEIGHBORS
aligned with Bo
+1/2
50 % of
molecules
opposed to Bo
-1/2
H
HA
H
HA
C
C
C
C
50 % of
molecules
Bo
downfield
neighbor aligned
upfield
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.
SPIN ARRANGEMENTS
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.
SPIN ARRANGEMENTS
two neighbors
n+1 = 3
triplet
one neighbor
n+1 = 2
doublet
H
H
H
H
C
C
C
C
H
methylene spins
H
methine spins
SPIN ARRANGEMENTS
three neighbors
n+1 = 4
quartet
H
H
C
C
H
H
methyl spins
H
two neighbors
n+1 = 3
triplet
H
H
C
C
H
H
H
methylene spins
THE COUPLING CONSTANT
THE COUPLING CONSTANT
H H
J
J
C C H
J
H H
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.
FIELD COMPARISON
100 MHz
100 Hz
Coupling constants are
constant - they do not
change at different
field strengths
6
5
4
3
2
1
400 Hz
Separation
is larger
3
7.5 Hz
J = 7.5 Hz
200 MHz
The shift is
dependant
on the field
200 Hz
200 Hz
7.5 Hz
J = 7.5 Hz
2
1
ppm
100 MHz
200 Hz
100 Hz
J=
7.5 Hz
J = 7.5 Hz
6
5
4
200 MHz
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.
6
5
4
3
2
Separation
is larger
1
400 Hz
200 Hz
J = 7.5 Hz
3
2
1
ppm
Why buy a higher
field instrument?
Spectra are
simplified!
Overlapping
multiplets are
separated.
Second-order
effects are
minimized.
50 MHz
J = 7.5 Hz
3
2
1
2
1
2
1
100 MHz
J = 7.5 Hz
3
200 MHz
J = 7.5 Hz
3
NOTATION FOR COUPLING CONSTANTS
The most commonly encountered type of coupling is
between hydrogens on adjacent carbon atoms.
3J
H H
C C
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
2J or geminal coupling
H
( most often 2J = 0 )
C H
Geminal coupling does not occur when
2J
the two hydrogens are equivalent due to
rotations around the other two bonds.
LONG RANGE COUPLINGS
Couplings larger than 2J or 3J also exist, but operate
only in special situations.
H
C
C
C
H
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.”
SOME REPRESENTATIVE COUPLING CONSTANTS
H H
vicinal
C C
6 to 8 Hz
three bond
3J
11 to 18 Hz
three bond
3J
6 to 15 Hz
three bond
3J
0 to 5 Hz
two bond
H
C C
trans
H
H
cis
H
C C
H
C
geminal
H
Hax
Hax,Hax = 8 to 14
Heq
Heq
2J
Hax
Hax,Heq = 0 to 7
Heq,Heq = 0 to 5
three bond
3J
H
H
cis
6 to 12 Hz
trans
4 to 8 Hz
three bond
3J
4 to 10 Hz
three bond
3J
0 to 3 Hz
four bond
4J
0 to 3 Hz
four bond
4J
H
C
C H
H
C C
C H
H C C C
H
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).
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.
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.
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.