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
O
O
OH
OCH3
Cl
OCH3
Br
O
OH
Br
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
H
(of ten singlet)
O
H
H
(of ten doublet)
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
B
A
O
C
O
D
Predicting Spectra
• Step 2: Table of data
B
A
O
C
O
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
D
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
1 H
T, n=2
B
4.1
2 H
T, n=2
C
2.3
2 H
Qt, n=3
D
2.1
2 H
Qt, n=3
E
1.1
3 H
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
ab
OCH3
Ha
ac
ad
bc
Hb
H3C
bd
cd
ab
H3C
CH3
ac
ad
bc
H
H
bd
cd
Example: Cinnamic Acid
• Can the trans and cis isomers be differentiated using proton NMR?
• Yes—with coupling constants
7.45
H
O
7.60
7.40
OH 11.0
7.33
400 MHz NMR:
Doublet at 6.310ppm and
6.355ppm
H
7.60
6.33
7.40
7.35
H
7.60
H 5.98
7.40
7.33
7.60
7.40
11.0 HO
O
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
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•
•
•
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