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Chapter 13 Nuclear Magnetic Resonance Spectroscopy

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Title: Chapter 13 Nuclear Magnetic Resonance Spectroscopy


1
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
2
1 GHz machinein France!Wow !
3
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

4
The NMR Spectrometer
gt
5
The NMR output
Two H environments, so two peaks
TMS reference
gt
6
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

7
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)

gt
8
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

9
Delta Scale
This slide basically says peak position is
independent of electromagnetic frequency
gt
10
Typical Values you will have to use the ones in
your databook
gt
11
e.g. Ethanol
All Protons relativelyshielded. So, Peaks
lietowards upfield region
upfield?
Take note The peaks are split.(discuss later)
12
Aromatic Protons appear downfield, ?7-?8
gt
13
Vinyl (terminal alkenes) Protons, ?5-?6
gt
14
Acetylenic (C??C) Protons, ?2.5
gt
15
Aldehyde Proton, ?9-?10
Electronegative oxygen atom
gt
16
Carboxylic Acid proton, ?10
9.3 2.5 11.8 ?
gt
17
Aromatic Hs 7-8 ppm (downfield)
Notice the splittingAromatic Hs complicated
multiplet,Aliphatic Hs splitting more clear
18
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

19
3) Area intensity of signals
  • The area under each peak is proportional to the
    number of protons.
  • Shown by integral trace.

20
Summary
  • Flv Video Instrumentation

21
How many (envionment) types of hydrogen?
  • When the molecular formula is known, each
    integral rise can be assigned to a particular
    number of hydrogens.

22
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

23
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

24
Simple spectra
  • Stolen from http//www.organicchemistryreview.com/
    spectroscopy.html

25
  • Stolen from http//www.organicchemistryreview.com/
    spectroscopy.html

26
Doublet 1 Adjacent Proton
gt
27
Triplet 2 Adjacent Protons
gt
28
No splitting no adjacent H
29
1,1,2-Tribromoethane
Nonequivalent protons on adjacent carbons.
gt
30
ethanol
31
The N 1 Rule
If a signal is split by N equivalent protons, it
is split into N 1 peaks.
gt
32
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
33
(No Transcript)
34
Splitting for Ethyl Groups
gt
35
Splitting for Isopropyl Groups
gt
36
http//www.absoluteastronomy.com/topics/Proton_NMR
37
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 ?

38
  • 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

39
Functional group region gt1400 cm-1
  • Stolen from http//www.organicchemistryreview.com/
    spectroscopy.html

40
Quiz
  • Answer?

Sure?
2-methylpropan-1-ol http//science.widener.edu/svb
/nmr/seminar/isobutanol.html
41
High Performance (High Pressure)Liquid
Chromatography (HPLC)
42
HPLC schematic
http//www.idex-hs.com/support/upchurch/i/hplcDiag
ram.gif
43
http//www.goehler-hplc.de/images/parts.jpg
The column
44
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

45
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

46
  • 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 ?

47
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
48
http//www.aldbot.com/HPLC-Hawaii.gif
49
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.
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