Proton NMR
Four Questions
•
•
•
•
How many signals? Equivalence
Where on spectrum? Chemical Shift
How big? Integration
Shape? Splitting (coupling)
Proton NMR Shifts
Basic Correlation Chart
• How many 1H signals?
• Shift?
Proton NMR
alkane CH3
alkane CH2
alkane CH
X=C-C-H
(Ph-C-H or
O=C-C-H)
O-C-H
X-C-H
(sp2) C=C-H
Ph-H
O=C-H
O=C-O-H
O-H, N-H
Chemical Shift (ppm)
0.9
1.3
1.7
2-3
3-4
5-6
7-8
9-10
10-12
1-5
Common “exceptions”
• Phenyl protons give one “signal” even if
nonequivalent
Common “exceptions”
• Exchangeable protons are broad and small
• Washed away with D2O
Four Questions
•
•
•
•
How many signals? Equivalence
Where on spectrum? Chemical Shift
How big? Integration
Shape? Splitting (coupling)
Integration
• Area under signal is proportional to the number
of protons in the set
• RELATIVE AREA is calculated by computer
Integration
Which proton
set is this?
• Trace of spectrum is cut into sections above
each signal to give integration values
Integration
• You need to “set” the values
• Here is a more realistic outcome
434.2
72.3
143.5
72.1
This is the NMR for a
C5H10O compound.
What are the
integration values?
Product Distribution by Integration
• In a mixture, areas are also proportional to
number of protons
• But they can by fractions
A
57%
B
D
B
C
E
F
43%
F
C
G
G
B F
D
E
A
Four Questions
•
•
•
•
How many signals? Equivalence
Where on spectrum? Chemical Shift
How big? Integration
Shape? Splitting (coupling)
Splitting
Splitting
• Shielding is also affected by magnetic fields
of nearby nuclei
N + 1 Rule
• The signal for protons b is a
quartet
• Signal b is “coupled” to the
three protons labeled “a”
• Four possible affects on signal b
• Summarized by this
simplification: n+1, where n =
number of adjacent,
nonequivalent protons
Splitting
Exchangeable protons
• If protons exchange, they are not coupled
• If protons are not coupled, they will no show
splitting in signal
• Typical for alcohols and acids
Label splitting for each proton set
N+1 is a Simplification
• We learn it first because it
applies to many molecules
• Assumes that all adjacent
protons couple to an identical,
measureable degree
• One exception: Aldehyde
• We will learn a more robust
treatment after we master
first principles
Very small coupling
between these protons,
even though they are
adjacent
Problems: n+1 Spectra
Predicting Spectra based on Structure
• Step 1: Identify number of signals
Predicting Spectra
• Step 2: Table of data
Signal
Shift (ppm)
Integration
Splitting
A
2-3
3
singlet
B
2-3
2
triplet
C
3-4
2
triplet
D
3-4
3
singlet
Predicting Spectra
• Step 3: Draw spectrum
Practice
• Make problems for yourself: ChemDraw
• Don’t do chiral compounds
6H, d
5H, multiplet
2H, doublet
1H, multiplet
Interpreting Spectra
• Reverse Process
– Make table
– Assign possible “pieces”
– Predict structure
C6H10Cl2O2
2.3
4.6
4.5
4.6
7.0
• Make a table
Peak
Shift (ppm)
Int
Split
A
5.45
1H
T, n=2
B
4.1
2H
T, n=2
C
2.3
2H
Qt, n=3
D
2.1
2H
Qt, n=3
E
1.1
3H
T, n=2
C6H10Cl2O2
2.3
4.6
4.5
4.6
7.0
• Assign “pieces”
Peak
Shift(ppm) Int
A
5.45
B
Split
Piece
Adjacent
1 H T, n=2
CH
Electronegative, CH2
4.1
2 H T, n=2
CH2
Oxygen? CH2
C
2.3
2 H Qt, n=3
CH2
CH3 or CH, CH2
D
2.1
2 H Qt, n=3
CH2
CH3 or CH, CH2
E
1.1
3 H T, n=2
CH3
CH2
-Cl, -Cl, C=O
C6H10Cl2O2
2.3
4.6
4.5
4.6
7.0
• Predict structure
Peak
Shift(ppm) Int
A
5.45
B
Split
Piece
Adjacent
1 H T, n=2
CH
Electronegative, CH2
4.1
2 H T, n=2
CH2
Oxygen? CH2
C
2.3
2 H Qt, n=3
CH2
CH3 or CH, CH2
D
2.1
2 H Qt, n=3
CH2
CH3 or CH, CH2
E
1.1
3 H T, n=2
CH3
CH2
-Cl, -Cl, C=O
C6H10Cl2O2
2.3
4.6
4.5
4.6
7.0
Coupling Constants
Review of Splitting
• Caused by shift due to magnetic fields of
adjacent protons
• We say that these protons are “coupled”
•
•
•
•
Protons may be coupled to different degrees
Coupling constant
Typically 7 Hz for adjacent sp3 carbons
Tree diagram
Basis of n+1 Rule
• Shortcut: N+1 if
all protons
coupled with
same constant
• Look at tree
diagram
• Coupling constant
is 7.1-7.2 Hz
Tree diagram
• Draw the tree
diagram that
shows why signal
A is a triplet
• What is the
coupling constant
for signal A
Coupling Constants
• Coupling
constants are
not all 7Hz
• In this class,
we will need
to know other
J values
Typical Constants
• Use the
table to
predict
typical
coupling
constants
Example: Cinnamic Acid
• Can the trans and cis isomers be differentiated
using proton NMR?
• Yes—with coupling constants
400 MHz NMR:
Doublet at 6.310ppm and
6.355ppm
400 MHz NMR:
Doublet at 5.925ppm and
5.950ppm
Calculate the coupling constants for these doublets
Spectra that are not N+1
• Consider the allylic methyl
group
• Coupled to two protons—
but not with the same
coupling constant
• Not N+1
• Split into a doublet by Ha
• That doublet is split into
doublet by Hb
• It is doublet of doublets
Why not N+1?
• Jac = 1.7Hz (typical
0 Hz)
• Jbc = 6.9 Hz
(typical 4-10 Hz)
• If Jac = Jbc = 6.9Hz,
what would we
observe?
Predict the Splitting
• What signals would be
observed for proton A?
– Proton A is coupled to
one proton B (doublet)
– Proton A is coupled to
three proton C (quartet)
– Doublet of quartets with
J = 15.6 Hz and J = 1.7Hz
Predict the Splitting
• Do the same for
proton B
• How is signal
same/different than
proton A signal?
Proton B
• Also a doublet of
quartets
• But coupling
constants are 15.6
and 6.9Hz
Exercise
• Explain these
two observed
signals
– How would
you describe
them?
– Which
proton(s) do
they belong
to?
Diastereotopic Protons
Peak
multiplicity
J (Hz)
A
m
B
sx
7.2
C
dd
7.2, 14
D
dd
7.2, 14
E
d
7.2
A
E
B
C D
Types of problems
•
•
•
•
Know typical coupling constants
Describe expected signal (dd, dt, etc)
Draw expected proton NMR
Interpret proton NMR given coupling
constants
Predict Structure
• C4H8O ether
• IR: 1650 cm-1