Chapter 13 Nuclear Magnetic Resonance Spectroscopy

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Chapter 13Nuclear Magnetic Resonance
Spectroscopy
The slides used in this presentation borrow
heavily from the great downloadable
Organic Chemistry, 5th EditionL. G. Wade, Jr.
Jo Blackburn Richland College, Dallas, TX Dallas
County Community College District ã 2003,
Prentice Hall
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1 GHz machinein France!Wow !
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4 Essential points Re NMR

1. Number of peaks No of H environments(a peak
   may be split into many smaller peaks called a
   splitting pattern)
2. Position of the peak depends of chemical
   environment (actually, magnetic environment).
3. Area under the peak relative number of H in
   that chemical environment
4. Splitting patters reveals info about adjacent Hs

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The NMR Spectrometer
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The NMR output
Two H environments, so two peaks
TMS reference
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Tetramethylsilane

• TMS is added to the sample. Functions as a
  reference
• Since silicon is less electronegative than
  carbon, TMS protons are highly shielded. Signal
  defined as zero.
• Organic protons absorb downfield (to the left) of
  the TMS signal.
  gt

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1) Number of Signals

• Equivalent hydrogens have the same chemical
  shift. Each H in the methyl group at a are
  equivalent to each other. (etc)

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2) Peak position (location of signals)shielding
/ deshielding

• More electronegative atoms deshield adjacent
  protons more and give larger shift values.
• (Effect decreases with distance)
• Additional electronegative atoms cause increase
  in chemical shift gt

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Delta Scale
This slide basically says peak position is
independent of electromagnetic frequency
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Typical Values you will have to use the ones in
your databook
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e.g. Ethanol
All Protons relativelyshielded. So, Peaks
lietowards upfield region
upfield?
Take note The peaks are split.(discuss later)
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Aromatic Protons appear downfield, ?7-?8
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Vinyl (terminal alkenes) Protons, ?5-?6
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Acetylenic (C??C) Protons, ?2.5
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Aldehyde Proton, ?9-?10
Electronegative oxygen atom
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Carboxylic Acid proton, ?10
9.3 2.5 11.8 ?
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Aromatic Hs 7-8 ppm (downfield)
Notice the splittingAromatic Hs complicated
multiplet,Aliphatic Hs splitting more clear
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O-H and N-H Signals

• Chemical shift depends on concentration.
• Hydrogen bonding in concentrated solutions
  deshield the protons, so signal is around ?3.5
  for N-H and ?4.5 for O-H.
• Proton exchanges between the molecules broaden
  the peak.
  gt

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3) Area intensity of signals

• The area under each peak is proportional to the
  number of protons.
• Shown by integral trace.

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Summary

• Flv Video Instrumentation

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How many (envionment) types of hydrogen?

• When the molecular formula is known, each
  integral rise can be assigned to a particular
  number of hydrogens.

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Spin-Spin Splitting/Coupling

• Nonequivalent protons on adjacent carbons have
  magnetic fields that may align with or oppose the
  external field.
• This magnetic coupling causes the proton to
  absorb slightly downfield when the external field
  is reinforced and slightly upfield when the
  external field is opposed.
• All possibilities exist, so signal is split. gt

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Range of Magnetic Coupling

• Protons on adjacent carbons normally will couple.
• Equivalent protons do not split each other.
• Protons separated by four or more bonds will not
  couple.
• Protons bonded to the same carbon will split each
  other only if they are not equivalent. (e.g CH2
  next to a C-H)
• 
  gt

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Simple spectra

• Stolen from http//www.organicchemistryreview.com/
  spectroscopy.html

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• Stolen from http//www.organicchemistryreview.com/
  spectroscopy.html

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Doublet 1 Adjacent Proton
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Triplet 2 Adjacent Protons
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No splitting no adjacent H
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1,1,2-Tribromoethane
Nonequivalent protons on adjacent carbons.
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ethanol
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The N 1 Rule
If a signal is split by N equivalent protons, it
is split into N 1 peaks.
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Pascal's triangle

• Helps you interpret the
• splitting Pattern.
• OR gives the
• splitting pattern
• (prediction)

http//www.lincoln.k12.nv.us/alamo/high/Department
s/Math/Pascal/Pascal's_Triangle_Webquest.html
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(No Transcript)
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Splitting for Ethyl Groups
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Splitting for Isopropyl Groups
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http//www.absoluteastronomy.com/topics/Proton_NMR
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Double bond equivalents

• Satd organic molecule has 2n2 Hs for every C.
• Each double bond takes 2 H away No Hs 2n
  e.g. ethene C2H4
• A ring also takes 2 Hs away cyclehexane C6H12

From molecular formula

• For every halogen, add 1 H
• For every N take away 1 H
• For every O, do nothing.
• Ethanol C2H5OH gtC2H6
• Compare it to satd formula for 2Cs No difference
  therefore moleculeC2H5OH has no double bond
  equivalents.
• Ethanal ? Methylbenzene ?

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• 1. NMR spectroscopy - Introduction to proton
  nuclear magnetic resonance - James Mungall -
  flv.avi
• 2. NMR spectroscopy - Integration - James Mungall
  - flv.avi
• 3. NMR spectroscopy - Chemical shift and regions
  of the spectrum - James Mungall - flv.avi
• 4i. NMR spectroscopy - Coupling - James Mungall -
  flv.avi
• 4ii. NMR spectroscopy - Coupling - James Mungall
  - flv.avi
• 5i. NMR spectroscopy - Examples of NMR spectra -
  James Mungall - flv.avi
• 5ii. NMR spectroscopy - Examples of NMR spectra -
  James Mungall - flv.avi
• MRI
• Dangers of MRI

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Functional group region gt1400 cm-1

• Stolen from http//www.organicchemistryreview.com/
  spectroscopy.html

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Quiz

• Answer?

Sure?
2-methylpropan-1-ol http//science.widener.edu/svb
/nmr/seminar/isobutanol.html
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High Performance (High Pressure)Liquid
Chromatography (HPLC)
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HPLC schematic
http//www.idex-hs.com/support/upchurch/i/hplcDiag
ram.gif
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http//www.goehler-hplc.de/images/parts.jpg
The column
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High Performance Liquid Chromatography
(HPLC)(High Pressure)

• High pressure gives separation much faster than
  say gravity based chromatography (paper or column
  chromatography)
• Detector uses UV light to detect presence of
  chromophores

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High Performance Liquid Chromatography
(HPLC)(High Pressure)

• Stationary phase in the form of a steel encased
  column (size of a straw) silica gel, Al2O3 or
  C18 hydrocarbon giving different polarities.
• Mobile phase solvent
• Area under peak measure of the relative
  abundance of the compound

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• Add a standard of known concentration to get its
  peak area.
• The same species have the same retention time (if
  all other factors are kept constant e.g.
  pressure, solvent, column type)
• Can separate chiral compounds ?

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Amino acids are poor at absorbing UV so they Have
a UV chromophore bonded to them for the purpose
of being HPLCd
UV detector set at ?338nm
http//www.biocompare.com/Articles/ApplicationNote
/1508/ANALYSIS-OF-AMINO-ACIDS-BY-REVERSED-PHASE-CH
ROMATOGRAPHY-WITH-PRECOLUMN-DERIVATISATION-AND-UV-
VISIBLE-DETECTION.html
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http//www.aldbot.com/HPLC-Hawaii.gif
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Sample will pass quickly through the column IF

• It has similar polarity to mobile phase
• Stationary phase in the column has a large
  particle size.
• Pressure of HPLC is increased
• Temp not really a factor as HPLC tends not to
  have a heater added
• Column is short.
• IN GENERAL LONGER COLUMN TIMES ARE BETTER (GIVE
  BETTER SEPARATION IN MISTURES SO REVERSE THE
  ABOVE CONDITIONS.