TYPES OF INFORMATION
FROM NMR SPECTRUM
1. Each different type of hydrogen gives a peak
or group of peaks (multiplet).
2. The chemical shift (δ, 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.
CHEMICAL SHIFT
DIAMAGNETIC ANISOTROPY
SHIELDING BY VALENCE ELECTRONS
Diamagnetic Anisotropy
Applied magnetic
field induces
circulation
of valence
electrons - this
generates a
magnetic field
that opposes the
applied field.
valence electrons
shield the nucleus
from the full effect
of the applied field
magnetic field lines
Bo applied
B induced (opposes Bo)
PROTONS DIFFER IN THEIR SHIELDING
All different types of protons in a molecule
have a different amounts of shielding.
They all respond differently to the applied magnetic
field and appear at different places in the spectrum.
DOWNFIELD
deshielded protons
appear here.
SPECTRUM
UPFIELD
shielded
protons appear here.
PEAKS ARE MEASURED RELATIVE TO TMS
Rather than measure exact position of a peak,
we measure how far downfield it is shifted from TMS.
reference compound
tetramethylsilane
“TMS”
CH3
CH3 Si CH3
CH3
Highly shielded
protons appear
way upfield.
TMS
shift in Hz
downfield
n
0
THE CHEMICAL SHIFT
The shifts from TMS in Hz are bigger in higher field
instruments (300 MHz, 500 MHz) than they are in the
lower field instruments (100 MHz, 60 MHz).
We can adjust the shift to a field-independent value,
the “chemical shift” in the following way:
parts per
million
chemical =
shift
δ
shift in Hz
=
= ppm
spectrometer frequency in MHz
This division gives a number independent
of the instrument used.
A particular proton in a given molecule will always have
the same chemical shift (constant value).
NMR Correlation Chart
-OH -NH
DOWNFIELD
DESHIELDED
UPFIELD
SHIELDED
CHCl3 , H
TMS
12
11
10
9
8
7
6
H
RCOOH
RCHO
C=C
5
4
CH2F
CH2Cl
CH2Br
CH2I
CH2O
CH2NO2
3
2
CH2Ar
CH2NR2
CH2S
C C-H
C=C-CH2
CH2-CO
1
0
δ (ppm)
C-CH-C
C
C-CH2-C
C-CH3
Ranges can be defined for different general types of protons.
This chart is general, the next slide is more definite.
DESHIELDING AND ANISOTROPY
Three major factors account for the resonance
positions (on the ppm scale) of most protons.
1. Deshielding by electronegative elements.
2. Anisotropic fields usually due to pi-bonded
electrons in the molecule.
3. Deshielding due to hydrogen bonding.
DESHIELDING BY
ELECTRONEGATIVE ELEMENTS
DESHIELDING BY AN ELECTRONEGATIVE ELEMENT
δ-
Cl
δ+
C
δ-
electronegative
element
H
δ+
Chlorine “deshields” the proton,
it takes valence electron
density away from carbon, which
in turn takes more density from
hydrogen deshielding the proton.
NMR CHART
“deshielded”
protons appear
downfield
More shielded
protons appear
upfield
deshielding moves proton
resonance to lower field
Electronegativity Dependence
of Chemical Shift
Dependence of the Chemical Shift of CH3X on the Element X
Compound CH3X
Element X
Electronegativity of X
Chemical shift
δ
most
deshielded
CH3F CH3OH
CH3Cl
CH3Br CH3I
CH4
(CH3)4Si
F
O
Cl
Br
I
H
Si
4.0
3.5
3.1
2.8
2.5
2.1
1.8
4.26
3.40
3.05
2.68
2.16
0.23
0
TMS
deshielding increases with the
electronegativity of atom X
Substitution Effects on
Chemical Shift
most
deshielded
most
deshielded
CHCl3 CH2Cl2 CH3Cl
7.27 5.30
3.05
-CH2-Br
3.30
ppm
-CH2-CH2Br
1.69
The effect
increases with
greater numbers
of electronegative
atoms.
-CH2-CH2CH2Br
1.25
ppm
The effect decreases
with incresing distance.
ANISOTROPIC FIELDS
DUE TO THE PRESENCE OF PI BONDS
The presence of a nearby pi bond or pi system
greatly affects the chemical shift.
Benzene rings have the greatest effect.
Ring Current in
in Benzene
Circulating
H
Bo
H
π electrons
Deshielded
fields add together
Secondary magnetic field
generated by circulating π
electrons deshields aromatic
protons
ANISOTROPIC FIELD IN AN ALKENE
protons are
deshielded
Deshielded
fields add
H
H
shifted
downfield
C=C
H
Bo
H
secondary
magnetic
(anisotropic)
field lines
HYDROGEN BONDING
HYDROGEN BONDING DESHIELDS PROTONS
R
O
H
H
O
R
H
O R
The chemical shift depends
on how much hydrogen bonding
is taking place.
Alcohols vary in chemical shift
from 0.5 ppm (free OH) to about
5.0 ppm (lots of H bonding).
Hydrogen bonding lengthens the
O-H bond and reduces the valence
electron density around the proton
- it is deshielded and shifted
downfield in the NMR spectrum.
SOME MORE EXTREME EXAMPLES
O
H
O
C R
R C
O
H
O
Carboxylic acids have strong
hydrogen bonding - they
form dimers.
With carboxylic acids the O-H
absorptions are found between
10 and 12 ppm very far downfield.
H3C O
O
H
O
In methyl salicylate, which has strong
internal hydrogen bonding, the NMR
absortion for O-H is at about 14 ppm,
way, way downfield.
Notice that a 6-membered ring is formed.
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
Cl H
integral = 1
triplet
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 ?
Ö.. 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)
Protons that are equivalent by symmetry
usually do not split one another
X CH2 CH2 Y
X CH CH Y
no splitting if x=y
2)
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
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
PASCALíS TRIANGLE
Intensities of
multiplet peaks
1
The interior
1 1
entries are
the sums of
1 2 1
the two
numbers
1 3 3 1
immediately
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 COUPLING CONSTANT
THE COUPLING CONSTANT
H H
J
C C H
J
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.
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.
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
H
cis
C C
H
C
geminal
H
Ha x
Hax,Hax = 8 to 14
He q
He q
2J
Ha x
Hax,Heq = 0 to 7
Heq,Heq = 0 to 5
three bond
3J
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 (δ, 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.