CARBON13 NMR

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CARBON-13 NMR
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Exercise N.4 13C NMR

• Prelab
• Read lab text, p. 143-147
• Read Carey, Chapter 13, 13C NMR
• Lab notebook
• No entries required
• Practice problems
• Download from schedule page
• Datasheet
• Spectra handout (in class)

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TYPES OF INFORMATION FROM THE NMR SPECTRUM

• Each different type of carbon gives a unique
  signal (line).
• The chemical shift (d, in ppm) gives a clue as
  to the type of carbon generating the peak
  (alkane, alkene, ketone, etc.)

Every compound has a unique spectrum.
caffeine
Signals
Chemical shift
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Nuclear Spin (angular momentum)

• Spin
• A fundamental property of nature like electrical
  charge or mass
• Comes in multiples of ½ and can be or
• Protons, neutrons, and electrons all possess
  angular momentum (spin)
• and spins can pair up and eliminate the
  observed manifestation of spin
• When placed in a magnetic field, a nucleus with
  net spin can absorb a photon

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The Resonance Phenomenon
Resonance a transition between two energy
states by the absorption of a photon A particle
(electron, nucleus, molecule, etc) in the lower
energy state absorbs a photon and ends up in the
upper energy state. The energy of this photon
must exactly match the energy difference between
the two states. E h? Infrared spectroscopy
two vibrational states NMR two nuclear spin
states
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Nuclear Spin States - Carbon Nucleus
The two states are equivalent in energy in
the absence of a magnetic or an electric field.
1/2
- 1/2
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The Energy Separation Depends On Bo
- 1/2
DE
kBo hn
degenerate at Bo 0
1/2
Bo
increasing magnetic field strength
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Absorption of Energy
quantized
-1/2
-1/2
DE
DE hn
1/2
1/2
Bo
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N
Nuclei precess at frequency ? when placed in a
strong magnetic field.
hn
NUCLEAR MAGNETIC RESONANCE
If n w then energy will be absorbed and the
spin will invert.
S
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During an NMR Experiment

• Apply an external magnetic field
• This causes the carbon-13 nuclei to precess at an
  angular frequency ?
• Then apply a short pulse ( 5 microseconds) of
  radio frequency energy
• If the frequency of photons in the pulse match
  the precessional frequency of the spinning
  nuclei, the photon is absorbed and the nucleus
  moves from the low energy spin state to the high
  energy spin state. (Resonance)
• The spectrometer detects the energy absorption
  and converts it to a graphical display

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But Wont all the 13C carbons precess at the
same frequency? (Which would lead to the same
resonance frequency for all the carbons, which
leads to one peak in the spectrum) NO!! It
turns out the carbons precess at different
frequencies in most molecules. So they have
different resonance frequencies. Why is this the
case???
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Electron Shielding

• Nuclei are surrounded by an electron cloud
• Shields the nucleus from full force of Bo
• So the magnetic field experienced by the nucleus
  is less than the applied field Bo
• Bnucleus Bapplied - Bshielding
• Amount of shielding varies depending on the
  particular electronic environment of the nucleus

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Electron Shielding

• Electronegative atoms such as halogens or oxygen
  pull electron density a way from the carbons,
  leaving them more exposed
• Deshielding
• Changes the electronic environment of the carbons
• Leads to different precessional frequencies in a
  magnetic field
• Leads to different resonance frequencies
• Bottom line 13C nuclei in different electronic
  environments will have different resonance
  frequencies due to varying degrees of shielding.

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THE CHEMICAL SHIFT ?
the shift observed depends on the frequency of
the instrument used
TMS
shift in Hz
parts per million
0
n
Position of signal position of TMS peak
?

ppm
x 106
spectrometer frequency in MHz
this division gives a number independent of the
instrument used
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APPROXIMATE 13C CHEMICAL SHIFT RANGES FOR
SELECTED TYPES OF CARBON (ppm)
R-CH3 8 - 30
C C 65 - 90
R2CH2 15 - 55
CC 100 - 150
R3CH 20 - 60
C N 110 - 140
110 - 175
C-I 0 - 40
C-Br 25 - 65
O
O
C-Cl 35 - 80
R-C-OR
R-C-OH
155 - 185
O
C-N 30 - 65
R-C-NH2
155 - 185
O
O
C-O 40 - 80
R-C-H
R-C-R
185 - 220
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Assume we had 500 molecules of 2-pentanol (remembe
r that 13C is only 1 of all carbon)
495 of these
1 of these
1 of these
1 of these
1 of these
1 of these
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Spin-spin Coupling

• Definition the communication of nuclear spin
  information via the electrons in the bonds
  between nuclei.
• Coupling is greatest when the number of
  intervening bonds is small (2 or 3).
• So IF we had two carbon 13s next to each other,
  each one would see the two spin states of the
  other and each carbon signal would show up as a
  doublet.

X-13C-13C-Y
X-13C
13C-Y
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But because of its low natural abundance (0.0108)
there is a low probability of finding two 13C
atoms next to each other in a single molecule.
(Spectra are determined by many molecules
contributing to the spectrum, each having only
one 13C atom.)
However, 13C does couple to hydrogen atoms (I
1/2)
very common
13C - 1H
coupling
YES!
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DECOUPLING THE PROTON SPINS
PROTON-DECOUPLED SPECTRA
A common method used in determining a
carbon-13 NMR spectrum is to irradiate all of the
hydrogen nuclei in the molecule at the same time
the carbon resonances are being measured.
This requires a second radiofrequency (RF) source
(the decoupler) tuned to the frequency of the
hydrogen nuclei, while the primary RF source is
tuned to the 13C frequency.
RF source 1
RF source 2
1H-13C
pulse tuned to carbon-13
the decoupler
continuously saturates hydrogens
13C signal measured
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In this method the hydrogen nuclei are
saturated, a situation where there are as many
downward as there are upward transitions, all
occuring rapidly.
During the time the carbon-13 spectrum is
being determined, the hydrogen nuclei cycle
rapidly between their two spin states (1/2 and
-1/2) and the carbon nuclei see an average
coupling (i.e., zero) to the hydrogens.
The hydrogens are said to be decoupled from
the carbon-13 nuclei.
You no longer see multiplets for the 13C
resonances. Each carbon gives a singlet, and the
spectrum is easier to interpret.
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General Approach to NMR Problems

• If the formula is known, calculate the degree of
  unsaturation (p. 146 in lab text)
• U ((2C2)-H)/2
• Examine the spectrum and look for the number of
  signals
• Tells you the number of types of carbons
• Use the chemical shift of each signal to get an
  idea of the functional groups attached to each
  carbon
• Needs to be consistent with formula!!
• Put it all together find a structure consistent
  with all the data

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diethyl ether
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pentane
pentane
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hexane
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2-bromohexane
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1-hexene
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1-hexyne
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1-hexanol
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2-hexanone
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hexanal
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hexanoic acid
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hexanamine
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hexanenitrile
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cyclohexane
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cyclohexene
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1,3-cyclohexadiene
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benzene
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toluene
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benzoic acid
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m-nitroaniline
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acetanilide