13CNMR, 2DNMR, and MRI

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13C-NMR, 2D-NMR, and MRI

• Lecture Supplement
• Take one handout from the stage

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Midterm Exam 2

• Date Monday May 21
• Time 500-650 PM
• Topics All of spectroscopy (mass spectrometry ?
  today)
• Location last name A-La in Haines 39
• last name Le-Z in Moore 100
• Calculators not allowed
• Question and Answer Session
• Lecture time, Monday May 21
• Submit questions to harding_at_chem.ucla.edu
• Label as Question for QA
• Deadline for possible inclusion noon Sunday May
  20
• Extra Office Hours
• Saturday 3-5 PM, Young Hall 3077F (Steve Joiner)
• Sunday ???

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13C-NMR

• Is NMR limited to 1H?
• Any nucleus with I ? 0 can be observed
• I ? 0 when nucleus has odd number of protons or
  odd number of neutrons
• Includes 1H, 2H, 13C, 19F, 29Si, 31P, 127I, etc.
• Examples
• 19F 9 protons, 10 neutrons 100 natural
  abundance
• 31P 15 protons, 16 neutrons 100 natural
  abundance

• 13C-NMR
• Carbon is backbone of organic molecules so
  13C-NMR has high potential, but...
• Low natural abundance 13C 1.1 (12C 98.9
  but has 6 protons and 6 neutrons)
• Low probability that photon absorption causes
  spin flip 1.6 compared to 1H
• Result 13C spin flip much harder to observe than
  1H spin flip
• Modern NMR spectrometers have overcome these
  problems 13C-NMR now routine

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13C-NMR

• What can we deduce about molecular structure from
  13C-NMR spectrum?
• NMR fundamentals are the same regardless of
  nucleus
• Information from carbon NMR spectrum
• Number of signals equivalent carbons and
  molecular symmetry
• Chemical shift presence of high EN atoms or pi
  electron clouds
• Integration ratios of equivalent carbons
• Coupling number of neighbors

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13C-NMR Number of Signals

• Number of 13C-NMR signals reveals equivalent
  carbons
• One signal per unique carbon type
• Reveals molecular symmetry
• Examples

CH3CH2CH2CH2OH
CH3CH2OCH2CH3
2 x CH3 equivalent
No equivalent carbons Four 13C-NMR signals
2 x CH2 equivalent
Two 13C-NMR signals
Symmetry exists when of 13C-NMR signals lt of
carbons in formula
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13C-NMR Position of Signals

• Position of signal relative to reference
  chemical shift
• 13C-NMR reference TMS 0.00 ppm
• 13C-NMR chemical shift range 0 - 250 ppm
• Downfield shifts caused by electronegative atoms
  and pi electron clouds

Example
HOCH2CH2CH2CH3
OH does not have carbon ? no 13C-NMR OH signal
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13C-NMR Position of Signals

• Trends
• RCH3 lt R2CH2 lt R3CH
• EN atoms cause downfield shift
• Pi bonds cause downfield shift
• CO 160-210 ppm

It is not necessary to memorize this table. It
will be given on an exam if necessary.
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13C-NMR Integration

• 1H-NMR Integration reveals relative number of
  hydrogens per signal
• 13C-NMR Integration reveals relative number of
  carbons per signal
• Rarely useful due to slow relaxation time for 13C

• Relaxation time important phenomenon for MRI

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13C-NMR Spin-Spin Coupling

• Spin-spin coupling of nuclei causes splitting of
  NMR signal
• Only nuclei with I ? 0 can couple
• Examples 1H with 1H, 1H with 13C, 13C with 13C
• 1H NMR splitting reveals number of H neighbors
• 13C-NMR limited to nuclei separated by just one
  sigma bond no pi bond free spacers

• Conclusions
• Carbon signal split by attached hydrogens (one
  bond coupling)
• No other coupling important

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13C-NMR Spin-Spin Coupling
1H-13C Splitting Patterns

• Carbon signal split by attached hydrogens
• N1 splitting rule obeyed

Triplet
Doublet
Quartet
Singlet
Example
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13C-NMR Spin-Spin Coupling
Simplification of Complex Splitting Patterns

• Broadband decoupling all C-H coupling is
  suppressed
• All split signals become singlets
• Signal intensity increases less time required to
  obtain spectrum

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13C-NMR Spin-Spin Coupling
Distortionless Enhancement by Polarization
Transfer (DEPT)

• Assigns each 13C-NMR signal as CH3, CH2, CH, or C

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Two-Dimensional NMR (2D-NMR)

• Basis interaction of nuclear spins (1H with 1H,
  1H with 13C, etc.) plotted in two dimensions
• Applications
• Simplifies analysis of more complex or ambiguous
  cases such as proteins
• Obtain structural information not accessible by
  one-dimensional NMR methods
• Techniques include

Correlation Spectroscopy (COSY) Heteronuclear
Correlation Spectroscopy (HETCOR) Heteronuclear
Multiple-Quantum Coherence (HMQC) Nuclear
Overhauser Effect Spectroscopy (NOESY) Incredible
Natural Abundance Double Quantum Transfer
Experiment (INADEQUATE) Many others
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2D-NMR
COSY Correlation of 1H-1H coupling
Sucrose 1H-NMR

• Dots correlations

Sucrose 1H-NMR

• Ignore dots on diagonal

• Examples
• H6 and H5 are coupled

• Identify H8 by its coupling with H9

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2D-NMR
HMQC Correlation of spin-spin coupling between
1H and nuclei other than 1H such as 13C
Sucrose 1H-NMR

• No diagonal
• Example
• Which carbon bears H6?

Sucrose 13C-NMR
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Magnetic Resonance Imaging (MRI)

• Basis Spin-excited nuclei relax at a rate
  dependent on their environment
• Environmental factors bonding to other atoms,
  solvent viscosity, etc.
• Photons released upon excitation are detected
• 1H relaxation times varies with tissue type
  (brain, bone, etc.)
• Therefore tissues may be differentiated by NMR
• Timeline
• 1971 First MRI publication Tumor Detection by
  Nuclear Magnetic Resonance
• Science, 1971, 171, 1151
• 2002 22,000 MRI instruments in use 6 x 107 MRI
  exams performed
• 2003 Nobel Prize in Physiology or Medicine to
  Paul Lauterbur and Peter Mansfield
• for their discoveries concerning
  magnetic resonance imaging
• http//nobelprize.org

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Magnetic Resonance Imaging (MRI)NMR and MRI Use
Similar Instruments
Powerful magnets
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Magnetic Resonance Imaging (MRI)MRI Images
Quite Different from NMR Spectra!