Common 1H NMR Patterns

1
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 6. singlet 2.1 ppm (3H)
methyl ketone
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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
-OH or NH chemical shift (H on
heteratom)
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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 Hs 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.
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Proton NMR Spectrum C9H12
aromatic, disubst.
D. of Unsat 4
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1H NMR Spectrum C4H7O2Br
s 3H
t 2H
t 2H
5.0 4.0 3.0 2.0 1.0
0
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Electronegative Substituents Shift Left
Propane
smalleffect
noeffect
heteroatomregion
d 0.9
d 0.9
d 1.3
d 1.0
d 4.3
d 2.0
H3CCH2CH3
O2NCH2CH2CH3
Effect is cumulative

• CH3Cl 3.1 (one Cl)
• CH2Cl2 5.3 (two Cls)
• CHCl3 7.3 (three Cls)

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Hydrogens on Heteroatoms
Chemical shifts for protons on heteroatoms are
variable, and signals are often broad (not
generally useful).
Type of proton
Chemical shift (ppm)
1-3
0.5-5
6-8
farleft
may beuseful
10-13
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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

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13C NMR Spectroscopy

• 13C signals go from 0 to 240 ppm. 13C
  signals always sharp singlets.
• (wider range than in 1H NMR) (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.
• 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)

No signal overlap!
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NMR Scanning for All Nuclei
13C area is much wider
1H area is small
An instrument can only examine one area at a
time.
To see both proton and C-13 nuclei, a very wide
region would have to be scanned.
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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 12CH3-12CH2-12CH2-OH
12CH3-12CH2-12CH2-OH
12CH3-13CH2-12CH2-OH 13CH3-12CH2-12CH2-OH 12C
H3-12CH2-12CH2-OH
12CH3-12CH2-12CH2-OH 12CH3-13CH2-13CH2-OH
12CH3-12CH2-12CH2-OH
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NMR Number of Signals for 13C NMR
How many signals should appear in the carbon-13
NMR spectrum for these compounds?
In theory 10 4 Signals
actually resolved 10 4
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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.
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13C NMR smaller signal to noise ratio
Noise
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13C NMR Spectrum C5H11Cl
D. of Unsat 0
five 13Csignals
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13C NMR Spectrum C4H7O2Br
D. of Unsat 1
CDCl3
200 150 100 50 0
double bond region