NMR – Effect of Magnetic Field

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Transcript NMR – Effect of Magnetic Field

Common 1H NMR Patterns
1. triplet (3H) + quartet (2H)
-CH2CH3
2. doublet (1H) + doublet (1H)
-CH-CH-
3. large singlet (9H)
t-butyl group
4. singlet 3.5 ppm (3H)
-OCH3 group
5. large double (6H) + muliplet (1H)
isopropyl
O
6. singlet 2.1 ppm (3H)
CH3
methyl ketone
Common 1H NMR Patterns
7. multiplet ~7.2 ppm (5H)
aromatic ring,
monosubstituted
8. multiplet ~7.2 ppm (4H)
aromatic ring,
disubstituted
9. broad singlet, variable
chemical shift
-OH or –NH
(H on heteratom)
Solving NMR Problems
1. Check the molecular formula and degree of
unsaturation. How many rings/double bonds?
2. Make sure that the integration adds up to the
total number of H’s in the formula.
3. Are there any signals in the double bond region?
4. Check each signal and write down a possible
sub-structure for each one.
5. Try to put the sub-structures together to find
the structure of the compound.
Proton NMR Spectrum: C9H12
aromatic,
disubst.
H
H
H
H
D. of Unsat = 4
CH3
CH3-CH2CH3-CH2-
1H
NMR Spectrum: C4H7O2Br
s
3H
H
H
O
C
C
C
H
H
O CH3
5.0
Br
O
4.0
t
t
2H
2H
3.0
Br
2.0
O
1.0
0
Electronegative Substituents: Shift Left
d 0.9 d 1.3
small
effect
~no
effect
d 4.3 d 2.0
d 1.0
heteroatom
region
Propane:
d 0.9
H3C—CH2—CH3
O2N—CH2—CH2—CH3
Effect is cumulative
– CH3Cl
– CH2Cl2
– CHCl3
3.1
5.3
7.3
(one Cl)
(two Cl’s)
(three Cl’s)
Hydrogens on Heteroatoms
Chemical shifts for protons on heteroatoms are variable,
and signals are often broad (not generally useful).
Chemical shift (ppm) Type of proton
far
left
1-3
H
NR
0.5-5
H
OR
6-8
H
OAr
O
10-13
HO
C
may be
useful
13C
NMR Spectroscopy
• Carbon-13: only carbon isotope with a nuclear spin
natural abundance of 13C is only 1.1%
(99% of carbon atoms are 12C, with no NMR signal)
• All signals are obtained simultaneously using a broad pulse of energy.
The resulting “mass signal” changed into an NMR spectrum
mathematically using the operation of Fourier transform (FT-NMR)
• Frequent repeated pulses give many data sets that are averaged to
eliminate noise
13C
13C
NMR Spectroscopy
signals go from 0 to 240 ppm.
(wider range than in 1H NMR)
13C
signals: always sharp singlets.
(1H signals: broad multiplets)
These two facts mean that in carbon-13 NMR, each separate signal is
usually visible, and you can accurately count the number of different
carbons in the molecule.
No signal overlap!
Chemical shift affected by electronegativity of nearby atoms:
alkane-like range:
0 – 40 ppm
(R-CH2-R)
heteroatom range:
50 – 100 ppm
(O-CH2-R)
double bond range:
100 – 220 ppm
(sp2 carbons)
NMR: Scanning for All Nuclei
13C
area is
much wider
1H
area
is small
To see both proton and C-13
nuclei, a very wide region
would have to be scanned.
An instrument can only
examine one area at a time.
Why does 13C NMR give singlets?
13C
is only 1.1% natural abundant, so most carbons are 12C,
and give no NMR signal.
No splitting seen with carbon, because carbons next to the
13C are likely to be carbon-12:
Sample of 1-Propanol:
12CH -12CH -12CH -OH
3
2
2
12CH -13CH -12CH -OH
3
2
2
12CH
3-
12CH -12CH -12CH -OH
3
2
2
13CH -12CH -12CH -OH
3
2
2
12CH -12CH -OH
2
2
12CH -13CH -13CH -OH
3
2
2
12CH -12CH -12CH -OH
3
2
2
12CH -12CH -12CH -OH
3
2
2
NMR: Number of Signals for 13C NMR
How many signals should appear in the carbon-13
NMR spectrum for these compounds?
O
octane
In theory:
10
4
Signals actually
resolved:
10
4
13C
NMR Example
Note the wide spectral width and the sharp singlets in the
spectrum below.
Also note that there is no integration with 13C NMR.
13C
NMR: smaller signal to noise ratio
Noise
13C
NMR Spectrum: C5H11Cl
Cl
5 signals
Cl
5 signals
3 signals
Cl
five 13C
signals
D. of Unsat = 0
13C
NMR Spectrum: C4H7O2Br
D. of Unsat = 1
O
C
200
CDCl3
150
double bond region
100
50
0