Spectroscopy

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Spectroscopy

• PART III Carbon-13 Nuclear Magnetic Resonance
  Spectroscopy
• Organic Chemistry 30B

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13C NMR Spectroscopy

• Carbon-12 nuclei have a spin of zero, so are not
  NMR active
• Carbon-13 nuclei (1.11 natural abundance) have
  a spin of 1/2, so are NMR active
• Absorption of radio waves in the wavelength range
  of 3.3 m (frequency of 100 MHz) causes nuclear
  spin flips
• The resonance frequencies are a function of the
  local magnetic field which is dependant on the
  molecular structure

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13C NMR Spectroscopy

• Since protons have a spin of 1/2 every carbon
  with protons attached should show 1J coupling and
  be split following the 2I 1 rule
• Thus a methyl carbon should appear as a quartet
  and a methine as a doublet
• And every carbon with protons attached to the
  next carbon should show 2J coupling and be
  further split following the 2I 1 rule
• Therefore 13C NMR spectra are acquired in a
  proton decoupled mode so only single lines are
  shown

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Chemical Shift

• The Chemical Shift range for 13C NMR is 0-200
  ppm.
• Tetramethylsilane is again the reference standard
  at 0 ppm.

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13C NMR Spectrum
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Chemical Shift

• Carbon-13 NMR signals near 20 ppm are for nuclei
  that are shielded
• Carbon-13 NMR signals near 200 ppm are for nuclei
  that are very deshielded

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13C NMR Spectra

• Information obtained from an 13C NMR spectrum
• The number of signals indicates the number of
  different types of carbons
• The chemical shift indicates the type of carbon
  and proximal functional groups

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13C NMR Spectra

• Information NOT generally obtained from a
  standard 13C NMR spectrum
• Integration for the ratios of numbers of carbons
• Signal shape indicating environment dynamics
• Signal splitting indicating the number of nearby
  protons or carbons
• But different types of 13C NMR, such as DEPT, can
  indicate the number of attached protons

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Equivalency of Atoms or Groups
Topicity

• Homotopic atoms or groups are equivalent
• and will present one NMR signal
• Enantiotopic atoms or groups are equivalent
• in an achiral environment and will present
• one NMR signal
• Diastereotopic atoms or groups are different
• and will present separate NMR signals

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Integration

• Integration is not typically accurate for
  indicating the ratios of the number of carbon
  atoms in 13C NMR

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Chemical Shift
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Chemical Shift
20 ppm Alkane carbons R3CH 50 ppm Carbons with
heteroatoms X-CR3 80 ppm Alkyne
carbons R-CC-R 120 ppm Alkene, Aromatic
carbons R2CCHR ArH 170 ppm Acid, amide,
ester carbons RXCO 200 ppm Aldehyde, ketone
carbons RRCO
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Example 13C NMR Spectrum
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Example 13C NMR Spectrum
In CDCl3 solvent
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Experimental Detail
13C NMR spectra are usually obtained with the
compound of interest in solution, so cannot avoid
seeing carbons from solvent. Usually use
deuterated solvents such as CDCl3 for both 1H NMR
and 13C NMR spectra. Since deuterium has a spin
of I 1, see a 111 triplet signal (splitting
to 2I1 lines) at 77 ppm for CDCl3.
In CDCl3 solvent
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Example 13C NMR Spectrum
In CDCl3 solvent
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Example 13C NMR Spectrum
In CDCl3 solvent
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Example 13C NMR Spectrum
In CDCl3 solvent
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Attached Protons

• Can the number of attached protons be found?
• 1) Generally, carbons with protons attached give
  taller signals
• 2) DEPT Spectra

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Attached Protons
Six short signals in 13C NMR spectrum for the
six carbons without protons attached
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DEPT 13C NMR Spectrum

• DEPT Carbon-13 NMR spectra directly indicate the
  number of attached protons
• Distortionless Enhancement by Polarization
  Transfer

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Example DEPT 13C NMR Spectrum
135o
Normal
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Example DEPT 13C NMR Spectrum
Normal
135o
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Example DEPT 13C NMR Spectrum
135o
Normal
CDCl3 signal disappears in DEPT spectra, since no
protons attached to carbon.
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Example DEPT 13C NMR Spectrum
135o
90o
45o
Do not see quaternary carbons in DEPT spectra, so
do not see signals at 150 and 175 ppm.
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NMR Spectral Problems
WebSpectra NMR spectral problems are available
at http//www.chem.ucla.edu/webspectra
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Interpreting 13C NMR Spectra

• Number of Signals - of types of carbons
• Chemical Shifts - Carbon environment
• DEPT spectra - of attached protons
• Combined with other information such as Mass
  Spectra, Infrared Spectra, and 1H NMR Spectra,
  the goal is complete determination of the
  molecular structure

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End