Chapter 13 Nuclear Magnetic Resonance Spectroscopy

1. The slides used in this presentation borrow heavily from the great downloadable: Chapter 13Nuclear Magnetic Resonance Spectroscopy 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 • Number of peaks = No of H environments(a peak may be split into many smaller peaks called a splitting pattern) • Position of the peak depends of ‘chemical’ environment (actually, magnetic environment). • Area under the peak = relative number of H in that chemical environment • Splitting patters reveals info about adjacent H’s

4. The NMR Spectrometer =>

5. The NMR output Two H environments, so two peaks TMS reference =>

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. =>

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) =>

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 =>

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

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

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 =>

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

14. Acetylenic (CC) Protons, 2.5 =>

15. Aldehyde Proton, 9-10 Electronegative oxygen atom =>

16. Carboxylic Acid proton, 10+ 9.3 + 2.5 = 11.8  =>

17. Aromatic H’s = 7-8 ppm (downfield) Notice the splitting:Aromatic H’s = complicated multiplet,AliphaticH’s =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. =>

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 molecularformula 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. =>

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) =>

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 =>

27. Triplet: 2 Adjacent Protons =>

28. No splitting – no adjacent H

29. 1,1,2-Tribromoethane Nonequivalent protons on adjacent carbons. =>

30. ethanol

31. The N + 1 Rule If a signal is split by N equivalent protons, it is split into N + 1 peaks. =>

32. 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

34. Splitting for Ethyl Groups =>

35. Splitting for Isopropyl Groups =>

36. http://www.absoluteastronomy.com/topics/Proton_NMR

37. 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 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 >1400 cm-1 Stolen from http://www.organicchemistryreview.com/spectroscopy.html

40. Quiz Sure? 2-methylpropan-1-ol http://science.widener.edu/svb/nmr/seminar/isobutanol.html Answer?

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

42. HPLC schematic http://www.idex-hs.com/support/upchurch/i/hplcDiagram.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 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 

47. 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-PHASE-CHROMATOGRAPHY-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.