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Understanding 1H-NMR Spectroscopy: Nuclear Spin & Magnetic Field Control

Explore the fundamentals of 1H-NMR spectroscopy, including nuclear spin flip, magnetic field influence, and interpreting NMR spectra. Learn about equivalent proton sets, signal integration, and spin coupling.

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Understanding 1H-NMR Spectroscopy: Nuclear Spin & Magnetic Field Control

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  1. Proton Nuclear Magnetic Resonance (1H-NMR) Spectroscopy Part 1 Lecture Supplement: Take one handout from the table

  2. 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

  3. I = -1/2 Absorb energy DE ~ 0.02 cal mol-1 = radio wave photons Increasing energy I = +1/2 Release energy (relaxation) Excited state Nuclear spin antiparallel to Bo Higher energy Ground state Nuclear spin parallel to Bo Lower energy 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

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

  5. 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 Deshielded (downfield) Low magnetic field strength Shielded (upfield) High magnetic field strength

  6. 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

  7. 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

  8. 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

  9. 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 • Models: Build two copies; label protons in question (replace H with D) Superimpose protons in question If rest of molecule superimposable then protons in question are equivalent Not superimposable

  10. not equivalent not equivalent rapid equilibrium equivalent equivalent Number of SignalsProton Equivalency Proton Equivalency Examples One signal Two signals ? • NMR = “slow camera” • NMR detects only average if rotation is fast • Thousands of H3C-CH3 rotations per second • Ha, Hb, Hc appear equivalent • In general single bond rotation in acyclic molecules allows equivalency

  11. Number of SignalsProton Equivalency More Proton Equivalency Examples: a plane of symmetry or axis of symmetry renders nuclei equivalent. Three signals Two signals One signal mirror plane Four signals One signal

  12. Number of SignalsProton Equivalency Sample Spectra • Verify what we have learned about equivalent protons • How many signals in 1H-NMR spectra of these molecules? Three proton sets  three signals Two proton sets  two signals

  13. 1H-NMR Spectroscopy: 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 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)

  14. I = -1/2 Small magnetic field  small DE DE DE Large magnetic field  large DE 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: • High magnetic field = higher spectral resolution (more spectral detail) • Magnetic field strength varies between NMR spectrometers • Need a scale that is independent of magnetic field strength • Chemical shift: spin flip energy scale normalized to be independent of field strength

  15. Position of SignalsThe Chemical Shift • How does molecular structure influence chemical shift? • Chemical shift  DE  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) (B0) • Other atoms in molecule • Electron cloud of nucleus in question shields it from external magnetic fields B0 causes these electrons to circulate; this generates a small magnetic field that opposes the spectrometer magnetic field; the nucleus is “shielded” from seeing the total B0; DE is different Bat nucleus = B0 - Bshielding Shielded: nucleus feels weaker magnetic field Deshielded: nucleus feels stronger magnetic field

  16. 0.00 ppm (CH3)4Si Tetramethylsilane (TMS) Position of SignalsThe Chemical Shift Reference point? Intensity of signal (photon quantity) 15 ppm Chemical shift scale (ppm) 0 ppm Deshielded (downfield) Shielded (upfield) Spin flip energy (photon energy)

  17. 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 1.8 2.1 2.5 2.5 2.8 3.0 3.5 4.0 Position of SignalsThe Chemical Shift • How does molecular structure influence chemical shift? Conclusion:  EN of atoms near H  chemical shift

  18. H X H X H X H X Increasing EN of X 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 Decreasing electron density around H Less shielding Higher chemical shift

  19. benzene ring Position of Signals Do not memorize chemical shifts. Table given on exams/quizzes.

  20. 2.01 ppm Typically 2.0-2.6 ppm 2.59 ppm 1.53 ppm 3.59 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 ENC = 2.5; ENH = 2.3 • EN effects decrease with distance: CH4 CH3OH CH3CH2OH CH3CH2CH2OH

  21. 2.2 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 SignalsAvoid This Common Misconception • Unlike IR peaks, we cannot assign NMR peaks based only on chemical shift • Example:

  22. ∫ir I∫aac Newton Inventor of calculu∫ Relative Intensity of PeaksIntegration • Beer’s Law: amount of energy absorbed or transmitted proportional to moles of stuff present • NMR: amount of radio wave energy proportional to peak area • Measurement of peak areas = integration • 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...

  23. 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

  24. 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

  25. 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

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

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