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Proton Nuclear Magnetic Resonance ( 1 H-NMR) Spectroscopy - Part 1 Lecture Supplement page 133

Proton Nuclear Magnetic Resonance ( 1 H-NMR) Spectroscopy - Part 1 Lecture Supplement page 133

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Proton Nuclear Magnetic Resonance ( 1 H-NMR) Spectroscopy - Part 1 Lecture Supplement page 133

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  1. Proton Nuclear Magnetic Resonance (1H-NMR) Spectroscopy - Part 1Lecture Supplement page 133

  2. Organic Structure Analysis Crews, Rodriguez, Jaspers 1998 p.5-6

  3. Bo add magnetic field No external magnetic field Spin alignment random With external magnetic field Spins aligned 1H-NMR SpectroscopyBackground and Theory Fundamental principle The energy required to cause nuclear spin flip is a function of the magnetic environment of the nucleus. • Protons, electrons, neutrons have “spin” (I) • Motion of charged particle creates magnetic field • In absence of external influence, magnetic poles (spin axis) randomly oriented • Add external magnetic field (Bo)  spins align

  4. Absorb energy (excitation) Release energy (relaxation) Background and TheoryNuclear Spin Flip • I = +1/2 parallel to Bo (lower energy); I = -1/2 antiparallel to Bo (higher energy) • Addition of energy results in nuclear spin flip I = -1/2 DE ~ 0.02 cal mol-1 = radio wave photons Increasing energy I = +1/2 Excited state Nuclear spin antiparallel to Bo Higher energy Ground state Nuclear spin parallel to Bo Lower energy Contrast this with absorption of infrared light (p. 114 of lecture supplement)

  5. I = -1/2 Spin state energy I = +1/2 Magnetic field strength at nucleus Background and TheoryMagnetic Field Controls DE • DE influenced by magnetic field strength at nucleus Small magnetic field  small DE DE DE Large magnetic field  large DE Energy required for spin flip (DE)  Information about magnetic field strength at nucleus  Information about chemical structure

  6. NMR signal Intensity of signal (photon quantity) Spin flip energy (photon energy) Background and TheoryThe NMR Spectrum • Spectrum = plot of photon energy versus photon quantity

  7. Resonance: Tendency of a system to oscillate at maximum amplitude at a certain frequency NMR Background and TheoryThe NMR Spectrum Nuclear: Manipulation of nuclear spin Magnetic: Magnetic field strength influences DE X 1H nucleus = a proton  1H-NMR = proton NMR

  8. Background and TheorySpectrum  Structure How do we deduce structure from NMR spectrum? • Information from NMR spectrum: • Number of signals  number of nonequivalent proton groups in molecule • Position of signals (chemical shift)  magnetic environment of protons • Relative intensity of signals (integration)  ratio of equivalent proton types • Splitting of signals (spin-spin coupling)  proton neighbors

  9. Protons equivalent One NMR signal Protons not equivalent Two NMR signals Number of SignalsProton Equivalency • NMR signal due to photon absorption • Photon energy controlled by magnetic environment of nucleus • Nuclei in same magnetic environment = equivalent • Multiple magnetic environments  multiple signals • Number of signals = number of equivalent proton sets

  10. Number of SignalsProton Equivalency How to test for equivalency? • Equivalent = proton magnetic environments identical in every way • Nonequivalent = proton magnetic environments not identical in one or more ways • Easier to test for nonequivalency than for equivalency Useful vocabulary CH3 = methyl CH2 = methylene CH = methine

  11. Hb, Ha not equivalent Ha, Hc not equivalent rapid equilibrium Ha, Hc equivalent Hb, Hc equivalent Number of SignalsProton Equivalency Proton equivalency examples One signal Two signals ? • NMR = camera with slow shutter speed • NMR detects only average when rotation is fast • Thousands of 360o bond rotations per second • Therefore Ha, Hb, Hc appear equivalent Single bond rotation in acyclic molecules often allows equivalency

  12. Hb, Ha not equivalent Ha, Hc not equivalent rapid equilibrium Ha, Hc equivalent Hb, Hc equivalent Number of SignalsProton Equivalency Proton equivalency examples One signal Two signals ? vs.

  13. mirror plane Number of SignalsProton Equivalency More proton equivalency examples Three signals Two signals One signal Four signals One signal

  14. Number of SignalsProton Equivalency Sample spectra • Verify what we have learned about equivalent protons • Which spectrum belongs to this molecule? Three proton sets  three signals

  15. Number of SignalsProton Equivalency Which spectrum belongs to this molecule? Two proton sets  two signals

  16. Proton Nuclear Magnetic Resonance (1H-NMR) Spectroscopy - Part 2Lecture Supplement page 139

  17. 1H-NMR Spectroscopy Part 1 Summary • Atomic nucleus has spin, and therefore generates a magnetic field • Nuclear spin axis can be parallel or antiparallel to external magnetic field (Bo) • Spin parallel to Bo (I = +1/2) lower energy than spin antiparallel to Bo (I = -1/2) • Energy difference between spin states (DE) controlled by magnetic field at nucleus • Absorption of radio wave photon with energy = DE causes nuclear spin flip • NMR spectrum = plot of photon energy (spin flip energy) versus photon quantity • Information from NMR spectrum • Number of signals reveals number of sets of equivalent protons Equivalency: Protons must be identical in all ways to be equivalent Nonequivalency: Protons can be different in just one way Example: 1H-NMR spectrum of CH3CH2OH has three signals • Position of signal (chemical shift) • Relative intensity of signals (integration) • Splitting of signals (spin-spin coupling) Vollhardt, 10-2

  18. I = -1/2 Spin state energy I = +1/2 Magnetic field strength at nucleus Position of SignalsThe Chemical Shift How does spin flip energy relate to molecular structure? • Spin flip energy depends on magnetic field strength: Small magnetic field  small DE DE DE Large magnetic field  large DE • Magnetic field strength varies between NMR spectrometers • High magnetic field = higher spectral resolution (more spectral detail) • Need a spin flip energy scale that is independent of magnetic field strength • Chemical shift: Spin flip energy scale normalized to be independent of field strength

  19. Position of SignalsThe Chemical Shift • How does molecular structure influence chemical shift? • Chemical shift controlled byDE which is controlled by magnetic field at nucleus What contributes to magnetic field at nucleus? Earth’s magnetic field Weak; 0.3-0.6 gauss Spectrometer’s magnetic field Strong; typically 94 kilogauss Other electrons and nuclei in the molecule • Electron cloud shields atomic nucleus from external magnetic fields • Shielded: Nucleus feels weaker magnetic field • Deshielded: Nucleus feels stronger magnetic field

  20. 0.00 ppm (CH3)4Si Tetramethylsilane (TMS) Position of SignalsThe Chemical Shift Reference point? Intensity of signal (photon quantity) 15 ppm Spin flip energy (photon energy) Chemical shift scale (ppm) 0 ppm Deshielded Shielded Low Magnetic Field Strength High Magnetic Field Strength

  21. CH3CH3 0.86 ppm CH4 0.23 ppm CH3I 2.16 ppm CH3F 4.26 ppm CH3Cl 3.05 ppm CH3Br 2.68 ppm CH3OH 3.42 ppm (CH3)4Si 0.00 ppm EN of X in CH3-X Si 1.8 H 2.1 I 2.5 C 2.5 Br 2.8 Cl 3.0 O 3.5 F 4.0 Position of SignalsThe Chemical Shift • How does molecular structure influence chemical shift? Chemical shifts for H3C-X: Conclusion:  EN of atoms near H  chemical shift

  22. Electron cloud shields atomic nucleus from external magnetic fields • Shielded: Nucleus feels weaker magnetic field • Deshielded: Nucleus feels stronger magnetic field shielded deshielded Vollhardt, Fig 10-9

  23. Position of SignalsThe Chemical Shift How does electronegativity influence chemical shift? • Chemical shift related to magnetic field strength at nucleus • Electron cloud shields nucleus from effects of Bo H Br H I H Cl H F Iodine has low EN Fluorine has high EN Electron density at H is high Electron density at H is low H more shielded H is less shielded H has lower chemical shift H has higher chemical shift Metaphor: Ozone layer shields Earth

  24. Position of Signals Do not memorize chemical shifts. Table given on exams.

  25. 2.01 ppm Typically 2.0-2.6 ppm 2.59 ppm 3.59 ppm 1.53 ppm 0.23 ppm 3.39 ppm 1.18 ppm 0.93 ppm 3.49 ppm Position of SignalsNotes On Characteristic Chemical Shifts Table • Characteristic shifts are typical proton averages. Actual shifts may lie outside given range. • Useful chemical shift trends • RCH3 < RCH2R < R3CH EN of C (in R) > EN of H • EN effects decrease with distance: CH4 CH3OH CH3CH2OH CH3CH2CH2OH

  26. 2.3 ppm not always ArCH3 3.8 ppm not always ROCH3 Common exception 6.5-8.0 ppm usually benzene ring protons C=O stretch Position of Signals Avoid this common misconception: “NMR peaks can be assigned based on chemical shift alone” • Example:

  27. Relative Intensity of PeaksIntegration • Information from NMR spectrum • Number of signals  number of nonequivalent proton groups in molecule • Position of signals (chemical shift)  magnetic environment of protons • Relative intensity of signals (integration)  ratio of equivalent proton types • Splitting of signals (spin-spin coupling)  proton neighbors

  28. Relative Intensity of PeaksIntegration • Beer’s Law: Amount of energy absorbed or released proportional to moles of stuff present • NMR: Amount of radio wave energy proportional to peak area • Measurement of peak areas = integration ∫ir I∫aac Newton Gottfried Leibniz Inventor∫ of calculu∫ Peak area Peak height • Relative intensities of NMR signals proportional to relative number of equivalent protons • Integrals do not always correspond to exact number of protons • Example: Integrals of 2:1 might be 2H:1H or 4H:2H or...

  29. Sample Spectra • Verify what we have learned about equivalent protons, chemical shifts, and integration • Assign peaks to corresponding hydrogens: 4.19 ppm: integral = 1.0 3.41 ppm: integral = 3.0 (1 H) (3 H) CH3OH has 4 H

  30. Sample Spectra Assign peaks to corresponding hydrogens: 3.19 ppm: integral = 1.0 1.33 ppm: integral = 1.0 (6 H) (6 H) C5H12O2 has 12 H Two equal integrals Two groups of equivalent H Smallest integral often set = 1 Integration gives proton ratio

  31. Sample Spectra Assign peaks to corresponding hydrogens: 3.55 ppm: integral = 1.0 3.39 ppm: integral = 1.5 (4 H) (6 H) CH3OCH2CH2OCH3 Two groups of equivalent H Two unequal integrals C4H10O2 has 10 H 10 H / (1.0 + 1.5) = 4 H per unit

  32. 1 1 2 CH3OCH2CH2OCH3 2 2 2 CH3CH2Br 2 3 2 Lecture Supplement p. 138, 139, 146

  33. Three peaks! Four peaks! Sample Spectra • Assign peaks to corresponding hydrogens: CH3CH2Br • Why the extra peaks? Hint: Think about spin and magnetic fields

  34. Proton Nuclear Magnetic Resonance (1H-NMR) Spectroscopy - Part 3Lecture Supplement page 147 CH3CH2Br

  35. 1H-NMR Spectroscopy Part 2 Summary Information from NMR Spectrum • 1. Number of signals  how many sets of equivalent protons • 2. Position of signals (chemical shift)  magnetic environment of nucleus • Deshielding by electronegative atoms  higher chemical shift • 3. Relative intensity of signals (integration)  how many hydrogens per signal • Integrals give proton ratio; not always equal to absolute proton count (i.e., 1.5:1) • 4. Splitting of signals (spin-spin coupling) • Example: 3.55 ppm: integral = 1.0 3.39 ppm: integral = 1.5 4 H 6 H CH3OCH2CH2OCH3 Two groups of equivalent H Two unequal integrals C4H10O2 has 10 H 10 H / (1.0 + 1.5) = 4 H per unit

  36. Signal Splitting 1H-NMR spectrum of CH3CH2Br has more details... 3.43 ppm: integral = 1.0 1.68 ppm: integral = 1.5 2 H 3 H Two unequal integrals 5 H / (1.0 + 1.5) = 2 H per unit CH3CH2Br Three lines A triplet Four lines A quartet Signals are split

  37. Signal Splitting What is the origin of signal splitting? Each line in signal... ...has slightly different chemical shift ...represents slightly different spin flip energy ...represents nucleus with slightly different magnetic environment A nucleus with only one magnetic environment causes a singlet A nucleus with two magnetic environments causes a doublet

  38. Bo Signal Splitting How can one nucleus have different magnetic environments? • Caused by spin direction of adjacent nuclei Ha feels Bo + Hb Larger DE Ha feels Bo - Hb Smaller DE NMR signal for Ha • Ha feels two different magnetic environments • Ha has two different spin flip DE • Ha has two different (but very similar) chemical shifts • Ha signal is split into a doublet

  39. Signal SplittingSome Useful Terms Spin-spin coupling: One nuclear spin influences spin of another nucleus Splitting: Effect on NMR signal caused by spin-spin coupling Coupling constant (J): Spacing between lines in a splitting pattern J

  40. } Bo equal energy Signal SplittingMore Than One Neighbor What is splitting when there is more than one neighbor? Ha feels Bo + Hb + Hc Ha feels Bo - Hb + Hc = Bo Ha feels Bo + Hb - Hc = Bo NMR signal for Ha Ha feels Bo - Hb - Hc 1:2:1 because of energy state population probabilities • Ha has three different (but very similar) chemical shifts • Ha signal is split into a triplet

  41. General rule: The signal for a proton with n neighbors is split into n+1 lines Signal SplittingRules and Restrictions Rules and Restrictions for Proton-Proton Spin-Spin Coupling 1. Only nonequivalent protons couple. X • Hb couples with Hc X • Hb and Ha do not couple because they are equivalent • Hc and Hd do not couple because they are equivalent

  42. Free spacer Signal SplittingRules and Restrictions 2. Protons separated by more than three single bonds usually do not couple. • Ha and Hb _____ bonds apart Can couple Cannot couple 2 * *Assuming Ha and Hb are not equivalent • Ha and Hc _____ bonds apart Can couple Cannot couple 3 X • Ha and Hd _____ bonds apart Can couple Cannot couple 4 Pi bonds do not count toward this bond limit, but J may be too small to observe. • Ha and Hb _____ bonds apart Can couple Cannot couple 2 2 (+1) • Ha and Hc _____ bonds apart Can couple Cannot couple 3 (+1) • Ha and Hd _____ bonds apart Can couple Cannot couple * *But Jad may be very small

  43. X X Signal SplittingRules and Restrictions 2. Protons separated by more than three single bonds usually do not couple. • Benzene ring = one big free spacer • All benzene ring protons may couple with each other but J may be small • Ha, Hb, Hc, and Hd all couple with each other • Jad may be too small to observe Benzene ring is a “gated community”; it blocks some coupling that we expect to observe.

  44. singlet singlet Signal SplittingRules and Restrictions 3. Signals for O-H and N-H are usually singlets triplet triplet • Splitting of O-H or N-H protons may be observed in rare circumstances

  45. Sample Spectra • Verify what we have learned about equivalency, chemical shifts, integration, and splitting • Assign peaks to corresponding hydrogens in structure BrCH2CH2CH3 3.39 ppm (triplet; integral = 1.0) 1.87 ppm (sextet; integral = 1.0) 1.03 ppm (triplet; integral = 1.5) 2 H 2 H 3 H 7 H / (1.0 + 1.0 + 1.5) = 2 H per unit

  46. Sample Spectra • Assign peaks to corresponding hydrogens in structure 3.78 ppm (septet; integral = 1.0) 1.31 ppm (doublet; integral = 6.0) 1 H 6 H 7 H / (1.0 + 6.0) = 1 H per unit

  47. CH3OCH2CH2OCH3 CH3CH2Br Lecture Supplement p. 138, 139, 146

  48. Sample Spectra For next lecture: • Assign peaks to corresponding hydrogens in structure • Explain splitting patterns 5.66 ppm (multiplet; integral = 1.0) 1.98 ppm (multiplet; integral = 2.0) 1.61 ppm (multiplet; integral = 2.0)

  49. Proton Nuclear Magnetic Resonance (1H-NMR) Spectroscopy - Part 4Lecture Supplement page 154