chapter 13 nuclear magnetic resonance spectroscopy

Chapter 13Nuclear Magnetic

Resonance Spectroscopy

Jo BlackburnRichland College, Dallas, TX

Dallas County Community College District2003,Prentice Hall

Organic Chemistry, 5th EditionL. G. Wade, Jr.

The slides used in this presentation borrow The slides used in this presentation borrow heavily from the great downloadable:heavily from the great downloadable:

1 GHz machine1 GHz machinein France!in France!

Wow !Wow !

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 patternsplitting 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 H’s

The NMR Spectrometer

=>

The NMR output

=>

Two H Two H environmenenvironments,ts,so two so two peakspeaks TMSTMS

referencereference

Tetramethylsilane

• TMS is added to the sample. Functions as a reference

• Since silicon is less electronegative than carbon, TMS protons are highly shieldedshielded. Signal defined as zero.

• Organic protons absorb downfielddownfield (to the left) of the TMS signal.

=>

Si

CH3

CH3

CH3

H3C

1) Number of Signals1) Number of Signals

=>

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

2) Peak position 2) Peak position (location of signals)(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 =>

Delta ScaleDelta Scale

=>This slide basically says peak position is independent of electromagnetic frequencyThis slide basically says peak position is independent of electromagnetic frequency

OLD MACHINE

NEWER MACHINE

Typical Values – you will have to use the ones in your databook

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Take note: The peaks are split.Take note: The peaks are split.(discuss later)(discuss later)

AllAllProtonsProtonsrelativelyrelativelyshielded.shielded. So, So, Peaks Peaks lielietowards towards ‘upfield’ ‘upfield’ regionregion

upfieldupfield

e.g. Ethanol

Aromatic Protons appear ‘downfield’, 7-8

=>

Vinyl (terminal alkenes) Protons, 5-6

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Acetylenic (CC) Protons, 2.5

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AldehydeAldehyde Proton, 9-9-1010

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Electronegativeoxygen atom

Carboxylic Acid proton, 10+

=>9.3 + 2.5 = 11.8 11.8

Aromatic H’s = 7-8 ppm (downfield)

Notice the splitting:Notice the splitting:AromaticAromatic H’s = complicated multiplet, H’s = complicated multiplet,

AliphaticAliphatic H’s =H’s = splitting more clearsplitting more clear

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. =>

3) 3) AreaArea intensity of signals intensity of signals• The area under each peak is

proportional to the number of protons.

• Shown by integral trace.

=>

Summary

• Flv Video: Instrumentation

How many (envionment) types of hydrogen?

=>

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

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. =>

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)

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Simple spectra

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

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

Doublet: 1 Adjacent Proton

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Triplet: 2 Adjacent Protons

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No splitting – no adjacent H

1,1,2-TribromoethaneNonequivalent 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

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Splitting for Isopropyl Groups

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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

• 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 moleculeC2H5OH has no double bond equivalents.

• Ethanal = ? Methylbenzene = ?

From molecular formula…

• 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

Functional group region >1400 cm-1

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

Answer? Sure?

2-methylpropan-1-olhttp://science.widener.edu/svb/nmr/seminar/isobutanol.html

Quiz

High Performance (High Pressure)High Performance (High Pressure)Liquid Chromatography (HPLC)Liquid Chromatography (HPLC)

http://www.idex-hs.com/support/upchurch/i/hplcDiagram.gif

HPLC schematic

The column

http://www.goehler-hplc.de/images/parts.jpg

High Performance Liquid Chromatography (HPLC)

(High Pressure)

High pressure gives separation much faster than High pressure gives separation much faster than say gravity based chromatography (paper or say gravity based chromatography (paper or column chromatography)column chromatography)

Detector uses UV light to detect presence of Detector uses UV light to detect presence of chromophoreschromophores

High Performance Liquid Chromatography (HPLC)

(High Pressure)

• Stationary phase in the form of a steel Stationary phase in the form of a steel encased column (size of a straw) = silica encased column (size of a straw) = silica gel, Algel, Al22OO33 or C or C1818 hydrocarbon – giving hydrocarbon – giving

different polarities.different polarities.

• Mobile phase = solventMobile phase = solvent

• Area under peak = measure of the Area under peak = measure of the relativerelative abundance of the compoundabundance of the compound

• Add a standard of known concentration to Add a standard of known concentration to get it’s peak area.get it’s peak area.

• The same species have the same The same species have the same retention time (if all other factors are kept retention time (if all other factors are kept constant e.g. pressure, solvent, column constant e.g. pressure, solvent, column type)type)

• Can separate Can separate chiralchiral compounds compounds

http://www.biocompare.com/Articles/ApplicationNote/1508/ANALYSIS-OF-AMINO-ACIDS-BY-REVERSED-PHASE-CHROMATOGRAPHY-WITH-PRECOLUMN-DERIVATISATION-AND-UV-VISIBLE-DETECTION.html

UV detector set at λ=338nmAmino acids are poor at absorbing UV so theyHave a UV chromophore bonded to them for the purpose of being HPLC’d

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.