Transcript 1H NMR - INTEC Chemistry Blog

The slides used in this presentation are
borrowed heavily from the great downloadable:
Chapter 13
Nuclear Magnetic
Resonance Spectroscopy
Organic Chemistry, 5th Edition
L. G. Wade, Jr.
Jo Blackburn
Richland College, Dallas, TX
Dallas County Community College District
2003, Prentice Hall
1H
NMR
1 GHz machine in France!
4 Essential points Re: NMR
1. Number of peaks = No of H environments
(a peak may be split into many smaller peaks)
2. Position of the peak depends of ‘chemical’ (actually
magnetic) environment.
3. Area under the peak = relative number of H in that
chemical environment
4. Splitting patters reveals info about adjacent H’s
The NMR Spectrometer
=>
The NMR Graph
=>
C H3
H3C
S i C H3
Tetramethylsilane
C H3
• TMS is added to the sample.
• 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.
=>
Number of Signals
Equivalent hydrogens have the same chemical
shift. Each H in the methyl group at ‘a’ are
equivalent to each other. (etc)
=>
Location of Signals - deshielding
• More electronegative
atoms deshield more and
give larger shift values.
• Effect decreases with
distance.
• Additional electronegative
atoms cause increase in
chemical shift.
=>
This slide basically says
peak position is independent
of electromagnetic frequency
Delta Scale
 OLD MACHINE
 NEWER MACHINE
=>
Typical Values
=>
Ethanol
Aromatic Protons, 7-8
=>
Vinyl Protons, 5-6
=>
Acetylenic Protons, 2.5
=>
Aldehyde Proton, 9-10
Electronegative
oxygen atom
=>
Carboxylic Acid
Proton, 10+
=>
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.
=>
Intensity of Signals
• The area under each peak is
proportional to the number of protons.
• Shown by integral trace.
=>
• Flv Video: Instrumentation
How Many Hydrogens?
When the molecular
formula is known,
each integral rise can
be assigned to a
particular number of
hydrogens.
=>
Spin-Spin Splitting
• 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. =>
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)
=>
Simple spectra
Stolen from http://www.organicchemistryreview.com/spectroscopy.html
Stolen from http://www.organicchemistryreview.com/spectroscopy.html
Doublet: 1 Adjacent Proton
=>
Triplet: 2 Adjacent Protons
=>
No splitting – no adjacent H
1,1,2-Tribromoethane
Nonequivalent protons on adjacent carbons.
=>
ethanol
The N + 1 Rule
If a signal is split by N equivalent protons,
it is split into N + 1 peaks.
=>
Pascal's triangle
Helps you interpret the
splitting Pattern.
OR gives the
splitting pattern
(prediction)
http://www.lincoln.k12.nv.us/alamo/high/Departments/Math/Pascal/Pascal's_Triangle_Webquest.html
Splitting for Ethyl Groups
=>
Splitting for
Isopropyl Groups
=>
http://www.absoluteastronomy.com/topics/Proton_NMR
Double bond equivalents
• Satd organic molecule has 2n+2 H’s for every C.
• Each double bond ‘takes 2 H away’ No H’s =2n e.g.
ethene C2H4
• A ring also ‘takes 2 H’s 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 =>C2H6
Compare it to satd formula for 2C’s No difference therefore molecule
C2H5OH has no double bond equivalents.
Ethanal = ? Methylbenzene = ?
• MRI
• Dangers of MRI
Functional group region >1400 cm-1
Stolen from http://www.organicchemistryreview.com/spectroscopy.html
Answer?
Sure?
2-methylpropan-1-ol
http://science.widener.edu/svb/nmr/seminar/isobutanol.html
Quiz
High Performance (High Pressure)
Liquid Chromatography (HPLC)
HPLC schematic
http://www.idex-hs.com/support/upchurch/i/hplcDiagram.gif
http://www.goehler-hplc.de/images/parts.jpg
The
column
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
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
• Add a standard of known concentration to
get it’s 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 
Amino acids are poor at absorbing UV so they
Have a UV chromophore bonded to them for
the purpose of being HPLC’d
UV detector set at λ=338nm
http://www.biocompare.com/Articles/ApplicationNote/1508/ANALYSIS-OF-AMINO-ACIDS-BY-REVERSED-PHASECHROMATOGRAPHY-WITH-PRECOLUMN-DERIVATISATION-AND-UV-VISIBLE-DETECTION.html
http://www.aldbot.com/HPLC-Hawaii.gif
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.