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Interpreting 1 H (Proton) NMR Spectra

Interpreting 1 H (Proton) NMR Spectra. Information contained in an NMR spectrum includes:. 1. number of signals 2. their intensity (as measured by area under peak) 3. splitting pattern (multiplicity). Number of Signals.

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Interpreting 1 H (Proton) NMR Spectra

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  1. Interpreting 1H (Proton) NMR Spectra

  2. Information contained in an NMRspectrum includes: 1. number of signals 2. their intensity (as measured by area under peak) 3. splitting pattern (multiplicity)

  3. Number of Signals protons that have different chemical shifts are chemically nonequivalent exist in different molecular environment

  4. N CCH2OCH3 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 NMR OCH3 NCCH2O Chemical shift (d, ppm)

  5. Chemically equivalent protons are in identical environments have same chemical shift replacement test: replace Hs with an arbitrary "test group"generates the same compound H3CCH2CH3 chemically equivalent

  6. Chemically equivalent protons Replacing protons at C-1 and C-3 gives same compound (1-chloropropane) C-1 and C-3 protons are chemically equivalent and have the same chemical shift ClCH2CH2CH3 CH3CH2CH2Cl H3CCH2CH3 chemically equivalent

  7. Chemical Shift: Chemically equivalent protons Each set of chemically equivalent protons in a compound gives rise to a signal in an 1H NMR spectrum of that compound

  8. Question • How many chemically non-equivalent kinds of protons are there in 2,2-dimethylbutane? • A) 2 • B) 3 • C) 4 • D) 5

  9. 1H NMR spectrum of 1-bromo-2,2-dimethylpropane

  10. Question • How many proton signals would you expect to find in the 1H-NMR spectrum of 2-chloropentane? • A) 2 • B) 3 • C) 4 • D) 5

  11. Br H C C H3C H Diastereotopic protons replacement by some arbitrary test group generates diastereomers: non-superimposable, non-mirror images; multiple chiral carbon atoms diastereotopic protons can have differentchemical shifts d 5.3 ppm d 5.5 ppm

  12. Enantiotopic protons are in mirror-image environments replacement by some arbitrary test group generates enantiomers enantiotopic protons have the samechemical shift

  13. H H C CH2OH H3C H Cl Cl H C C CH2OH CH2OH H3C H3C Enantiotopicprotons

  14. http://chemconnections.org/organic/Movies%20Org%20Flash/nmr-chem-equivalence-Flash/W13_06v3.swfhttp://chemconnections.org/organic/Movies%20Org%20Flash/nmr-chem-equivalence-Flash/W13_06v3.swf

  15. Question • How many proton signals would you expect to possibly find in the 1H-NMR spectrum of 2-chloropentane? • A) 6 • B) 7 • C) 8 • D) 9 • E) More than 9

  16. Spin-Spin SplittinginNMR Spectroscopy not all peaks are singlets signals can be split by coupling of nuclear spins

  17. 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Cl2CHCH3 NMR 4 lines; quartet 2 lines; doublet CH3 CH Chemical shift (d, ppm)

  18. C C C C Two-bond and three-bond coupling H H H H protons separated bytwo bonds(geminal relationship) protons separated bythree bonds(vicinal relationship)

  19. C C C C Two-bond and three-bond coupling H in order to observe splitting, protons cannot have same chemical shift coupling constant (2J or 3J) is independent of field strength H H H

  20. 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 Cl2CHCH3 NMR 4 lines; quartet 2 lines; doublet coupled protons are vicinal (three-bond coupling) CH splits CH3 into a doublet CH3 splits CH into a quartet CH3 CH Chemical shift (d, ppm)

  21. H Cl H H C C H Cl Why do the methyl protons of1,1-dichloroethane appear as a doublet? To explain the splitting of the protons at C-2, we first focus on the two possible spin orientations of the proton at C-1 signal for methyl protons is split into a doublet

  22. H Cl H H C C H Cl Why do the methyl protons of1,1-dichloroethane appear as a doublet? There are two orientations of the nuclear spin for the proton at C-1. One orientation shields the protons at C-2; the other deshields the C-2 protons. signal for methyl protons is split into a doublet

  23. H Cl H H C C H Cl Why do the methyl protons of1,1-dichloroethane appear as a doublet? The protons at C-2 "feel" the effect of both the applied magnetic field and the local field resulting from the spin of the C-1 proton. signal for methyl protons is split into a doublet

  24. H Cl H H C C H Cl Why do the methyl protons of1,1-dichloroethane appear as a doublet? "true" chemicalshift of methylprotons (no coupling) this line correspondsto molecules in which the nuclear spin of the proton at C-1 reinforcesthe applied field this line correspondsto molecules in which the nuclear spin of the proton at C-1 opposesthe applied field

  25. H Cl H H C C H Cl Why does the methine proton of1,1-dichloroethane appear as a quartet? The proton at C-1 "feels" the effect of the applied magnetic field and the local fields resulting from the spin states of the three methyl protons. The possible combinations are shown on the next slide. signal for methine proton is split into a quartet

  26. H Cl H H C C H Cl Why does the methine proton of1,1-dichloroethane appear as a quartet? There are eight combinations of nuclear spins for the three methyl protons. These 8 combinations split the signal into a 1:3:3:1 quartet.

  27. The splitting rule for 1H NMR For simple cases, the multiplicity of a signalfor a particular proton is equal to the number of chemically equivalent vicinal protons + 1.

  28. Table Number of equivalent Appearance Intensities of linesprotons to which H of multiplet in multipletis coupled 1 Doublet 1:1 2 Triplet 1:2:1 3 Quartet 1:3:3:1 4 Pentet 1:4:6:4:1 5 Sextet 1:5:10:10:5:1 6 Septet 1:6:15:20:15:6:1 Splitting Patterns of Common Multiplets

  29. 1H NMR Splitting

  30. Question • What is the multiplicity (spin-spin splitting) of the protons of 1,2-dichloroethane? • A) one singlet • B) two singlets • C) one doublet • D) one triplet

  31. Splitting Patterns:The Ethyl Group CH3CH2X is characterized by a triplet-quartet pattern (quartet at lower field than the triplet)

  32. 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 BrCH2CH3 NMR 4 lines; quartet 3 lines; triplet CH3 CH2 Chemical shift (d, ppm)

  33. Splitting Patterns:The Isopropyl Group (CH3)2CHX is characterized by a doublet-septet pattern (septet at lower field than the doublet)

  34. 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 BrCH(CH3)2 NMR 2 lines; doublet 7 lines; septet CH3 CH Chemical shift (d, ppm)

  35. Question • Which isomer of C4H9Br has only one peak in its 1H-NMR spectrum having the chemical shift d 1.8? • A) 1-bromobutane • B) 2-bromobutane • C) 1-bromo-2-methylpropane • D) 2-bromo-2-methylpropane

  36. Question Which compound is the most reasonable for the NMR spectrum above? • Ethyl pentanoate • Propyl isobutyrate • Butyl propanoate • Isopropyl butyrate • Isopentyl acetate

  37. Splitting Patterns:Pairs of Doublets Splitting patterns are not always symmetrical, but lean in one direction or the other when “coupled”.

  38. C C Pairs of Doublets Consider coupling between two vicinal protons. If the protons have different chemical shifts, each will split the signal of the other into a doublet. H H

  39. C C Pairs of Doublets Let Dn be the difference in chemical shift in Hz between the two hydrogens. Let J be the coupling constant between them in Hz. H H

  40. The Difference between a Quartet and a Doublet of Doublets

  41. A Splitting Diagram for a Doublet of Doublets

  42. Coupling Constants The coupling constant (J) is the distance between two adjacent peaks of a split NMR signal in hertz Coupled protons have the same coupling constant

  43. A trans coupling constant is greater than a cis coupling constant

  44. To observe well-defined splitting patterns, the difference in the chemical shifts (in Hz) must be 10 times the coupling constant values

  45. H H OCH3 Cl H H 10.0 9.0 8.0 7.0 6.0 5.0 4.0 3.0 2.0 1.0 0 NMR skewed doublets OCH3 Chemical shift (d, ppm)

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