The slides used in this presentation borrow heavily

The slides used in this presentation borrow heavily from the great downloadable: Chapter 13 Nuclear Magnetic Resonance Spectroscopy Organic Chemistry, 5 th Edition L. G. Wade, Jr. Jo Blackburn Richland College, Dallas, TX Dallas County Community College District ã 2003, Prentice Hall

1 GHz machine in France! 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 pattern) 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 environments, so two peaks TMS reference =>

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. =>

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) =>

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 =>

Delta Scale OLD MACHINE NEWER MACHINE This slide basically says peak position is independent of electromagnetic frequency =>

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

e. g. Ethanol All Protons relatively shielded. So, Peaks lie towards ‘upfield’ region upfield Take note: The peaks are split. (discuss later)

Aromatic Protons appear ‘downfield’, 7 - 8 =>

Vinyl (terminal alkenes) Protons, 5 - 6 =>

Acetylenic (C C) Protons, 2. 5 =>

Aldehyde Proton, 9 - 10 Electronegative oxygen atom =>

Carboxylic Acid proton, 10+ 9. 3 + 2. 5 = 11. 8 =>

Aromatic H’s = 7 -8 ppm (downfield) Notice the splitting: Aromatic H’s = complicated multiplet, Aliphatic H’s = splitting 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) Area 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 CH 2 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. k 12. nv. us/alamo/high/Departments/Math/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 2 n+2 H’s for every C. • Each double bond ‘takes 2 H away’ No H’s =2 n e. g. ethene C 2 H 4 • A ring also ‘takes 2 H’s away’ cyclehexane C 6 H 12 From molecular formula… • • • For every halogen, add 1 H For every N take away 1 H For every O, do nothing. Ethanol = C 2 H 5 OH =>C 2 H 6 Compare it to satd formula for 2 C’s No difference therefore molecule C 2 H 5 OH has no double bond equivalents. Ethanal = ? Methylbenzene = ?

• 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 • 4 i. NMR spectroscopy - Coupling - James Mungall - flv. avi • 4 ii. NMR spectroscopy - Coupling - James Mungall - flv. avi • 5 i. NMR spectroscopy - Examples of NMR spectra - James Mungall - flv. avi • 5 ii. 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 -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/hplc. Diagram. 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, Al 2 O 3 or C 18 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=338 nm http: //www. biocompare. com/Articles/Application. Note/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.