2/20/2011 Comparing H to C NMR:

2/20/2011
Nuclear
Magnetic
Resonance
Comparing 1H to 13C NMR:
In the
Nucleus
Involves
Magnets
In the
Nucleus
All spectra were thankfully obtained from SDBSWeb :
http://www.aist.go.jp/RIODB/SDBS (National Institute of Advanced Industrial
Science and Technology). Without their dedication to cataloging spectra, educators
would have an extremely hard time to teach this material effectively.
This website at Michigan State is the best site on Spectroscopy online. Please use it
as a reference!
http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/spectro.htm#contnt
1H
SPECTROSCOPY
Read the 1H NMR chapter in your organic textbook
and Techniques book. Practice the problems!!
Interpreting 1H-NMR Spectra
How many types of H? Indicated by how many groups of
signals there are in the spectra
What types of H?
Indicated by the chemical shift of
each group
How many H of each
type are there?
Indicated by the integration
(relative area) of the signal for each
group.
1H
Abundance
13C
99%
1.1%
Chemical shift 0-15 ppm
0-220 ppm
Reference
TMS
TMS
Number of
signals
Number of proton
environments
Number of carbon
environments
Deshielding
Electron withdrawing effects Electron withdrawing effects
observed and cumulative
observed and cumulative
Coupling
Yes, signals show
splitting
No splitting due to low
abundance (1%) of 13C
Integration
Yes, area under peak
relates to H #
No, too long relaxation
times
1H
NMR provides information about:
• Splitting – Which atoms are attached to one another?
• Integration – How many protons of a certain type?
• Shift – Which atoms are more deshielded?
O
What is the
connectivity?
a
b
c
2-butanone
Look at the coupling patterns. This
tells you what is next to each group.
Splitting patterns
Pascal’s triangle
The relative intensities of the lines in a coupling pattern is
given by a binomial expansion or more conveniently
called Pascal's triangle.
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2/20/2011
Typical Coupling Constants (J)
Coupling constants, J




Distance between the peaks in a multiplet, measured in Hz
(J = 2 – 15 Hz typical) but not dependent on strength of
the external field
Multiplets with the identical coupling constants most likely
arise from protons that split each other.
Complex splitting = Signals may be split by adjacent but
different protons with different coupling constants.
a
c
e.g. Trans and cis J are different.
H
H
 Ja-b
 Ja-c
 Jb-c
trans coupling constant
cis coupling constant
geminal coupling constant
C C

Hb
Why does splitting arise?
Why does splitting arise?
Analyzing an NMR spectrum
Remember to look for repeatable patterns…
1. Look at the molecular formula (if available), and look
at the total integration of the spectrum.
2. Determine how many H’s are causing each signal.
3. Look at the splitting pattern so you get an idea of
what H’s are around that group.
4. Try to assemble the molecule with the clues you have
gathered. (Think of it as a puzzle…)
5. If you have other spectroscopic information (like IR or
13C NMR), make sure your analysis is consistent with
all available data.

-CH3
singlet, 3H

-CH2CH3
triplet, 3 H
quartet, 2H

-CH(CH3)2
doublet, 6 H
septet, 1H

-C(CH3)3
singlet, 9H
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2/20/2011
O
C
There are trends to learn…
A
B
D
Integration data: 2 3 2
3
Electronegativity environments
B
C
Compound CH3F CH3OH CH3Cl CH3Br CH3I CH4 (CH3)4Si
CH3X
A
X
1 3
6
F
O
Cl
Br
I
H
Si
Electroneg. 4.0
of X
3.5
3.1
2.8
2.5
2.1
1.8
Chemical
Shift
3.4
3.05
2.68
2.16
.23
0
4.26
Cumulative effects
Compound CH4
CH3Cl
CH2Cl2
CHCl3
/ppm
3.05
5.30
7.26
0.23
Aromatic Protons, 7-8
Acetylenic Protons, 2.5
These inductive effects at not just felt by the immediately
adjacent protons as the disruption of electron density has an
influence further down the chain.
1.69
 1.25
 3.30
Br
Vinyl Protons, 5-6
Aldehyde Proton, 9-10
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2/20/2011
Type of Proton
Cyclopropane
Primary
Secondary
Tertiary
Vinylic
Acetylenic
Aromatic
Benzylic
Allylic
Structure
C3 H 6
R-CH3
R2-CH2
R3-C-H
C=C-H
triple bond,CC-H
Ar-H
Ar-C-H
C=C-CH3
Chemical Shift, ppm
0.2
0.9
1.3
1.5
4.6-5.9
2-3
6-8.5
2.2-3
1.7
Alcohols
H-C-OH
3.4-4
Ethers
Esters
Esters
Acids
Carbonyl Compounds
Aldehydic
Hydroxylic
Phenolic
Carboxylic
Amino
H-C-OR
RCOO-C-H
H-C-COOR
H-C-COOH
H-C-C=O
R-(H-)C=O
R-C-OH
Ar-OH
RCOOH
RNH2
3.3-4
3.7-4.1
2-2.2
2-2.6
2-2.7
9-10
1-5.5
4-12
10.5-12
1-5
O
B
A
3
3
O
O
A
C
B
Br
2
2
3
4
2/20/2011
Alcohols – Ultrapure samples of ethanol show splitting.
Ethanol with a small amount of acidic or basic impurities will
not show splitting. Compare:
A
C
B
OH
D
2
N—H peaks may be broad


Chemical shift will depend on concentration and solvent.
To verify an O-H or N-H peak, shake the sample with
D2O. Deuterium will exchange with the O-H or N-H
protons and be absent in 2nd spectrum.
1H
NMR for Styrene
1
2
3
Complex Splitting Patterns
a
H
H
C C
c
Hb
What would Hc look like?
Splitting Tree
Complex Splitting Patterns
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2/20/2011
Non-equivalent protons
a
H
H
C C




c
Hb
Proton Chemical Shift Ranges (in CDCl3)
c
H OHa
CH3
H
aH
Cl
Hb
dH
Hb
Cl
Molecules are tumbling relative to the magnetic field, so NMR is an
averaged spectrum of all the orientations.
If protons have different environments, they will show different
signals (e.g.
).
Axial and equatorial protons on cyclohexane interconvert so rapidly
that they give a single signal.
H-transfers for OH and NH may occur so quickly that the proton is
not split by adjacent H’s in the molecule.
Evaluate the 1H NMR of the following
Proton Chemical Shift Ranges (in CDCl3)
1H, t; 1H, s; 1H, d; 1H,d
Evaluate the 1H NMR of the following
2H, q;
3H, s
3H, t
Evaluate the 1H NMR of the following
1H, t
1H, s 2H,t
2H, quartet; 2H, quintet
1H, t; 2H, d
6H, s
3H,s
10
8
6
PPM
4
2
0
6
2/20/2011
Evaluate the
1H
Shift (ppm)
9.78
7.75
6.90
3.77
NMR of the following
Area
1.00
2.02
2.01
3.17
singlet
C8H8O2
singlet
doublet
1H, q 2H, q
doublet
1H, s
12
Shift (ppm)
6.05
4.01
1.08
3H, t
10
8
Area
1.00
2.00
3.12
6
PPM
4
2
3H,d
0
C8H12O4
triplet
Shift (ppm)
7.09
6.83
5.41
2.35
Area
1.93
1.85
0.80
2.98
C7H8O
singlet
singlet
quartet
doublet
doublet
singlet
Shift (ppm)
7.82
7.72
7.40
7.32
2.48
Area
0.1
0.1
0.1
0.1
0.31
C8H7OCl
singlet
Shift (ppm)
9.51
7.37
7.26
6.59
6.45
2.96
Area
1.00
2.00
2.10
1.11
1.09
6.00
C11H13NO
7