Chapter 9 — Key Takeaways

What you should leave Chapter 9 with

  1. NMR (Nuclear Magnetic Resonance) is the single most powerful structure-determination tool in organic chemistry. It reads individual nuclei (protons, carbons) in their chemical environments.

  2. The physics: certain nuclei (¹H, ¹³C, ¹⁹F, ³¹P) have spin and behave like tiny magnets. In a strong magnetic field ($B_0$ ≈ 11.7 T for 500 MHz), they resonate at frequencies determined by the field and their chemical environment.

  3. Chemical shift ($\delta$, in ppm) measures how a nucleus's resonance frequency differs from a reference standard (TMS at $\delta$ 0). Different chemical environments give different chemical shifts.

  4. ¹H NMR gives four pieces of information per peak: - Chemical shift (position): tells you the electronic environment of that proton. - Integration (peak area): tells you how many protons are at that environment. - Multiplicity (splitting pattern): tells you how many neighboring protons there are. - Coupling constant (J, in Hz): tells you about geometry (dihedral angle, cis/trans, etc).

  5. ¹H chemical shift ranges: - TMS: 0 - Alkyl CH₃: 0.7-1.0 - Alkyl CH₂: 1.2-1.5 - α to carbonyl: 2.0-2.5 - α to O (ether/alcohol): 3.3-4.0 - α to halogen: 3.0-4.0 - Vinyl (alkene H): 4.5-6.5 - Aromatic: 6.5-8.5 - Aldehyde H: 9-10 - Carboxylic acid H: 10-13

  6. The n+1 rule: a proton with $n$ non-equivalent neighbors shows $n+1$ peaks. Pascal's triangle gives intensities (1:1, 1:2:1, 1:3:3:1, etc).

  7. Coupling constants reveal geometry: - Vicinal sp³-sp³: ~6-10 Hz (depending on dihedral angle). - cis alkene ³J: ~7-10 Hz. - trans alkene ³J: ~14-18 Hz. - The difference distinguishes (E) from (Z) alkenes.

  8. Karplus equation: ³J (vicinal) depends on dihedral angle θ: - θ = 0° or 180°: maximum J (~8-12 Hz). - θ = 90°: minimum J (~0-2 Hz). - This lets NMR probe conformations.

  9. ¹³C NMR is complementary to ¹H NMR: - Chemical shift range: 0-220 ppm (much wider than ¹H). - Each carbon as a separate line (with broadband decoupling — no splitting). - Lower sensitivity (¹³C is only 1.1% abundant; spectra take longer). - Diagnostic ranges: alkyl 5-50; C-O sp³ 50-90; alkene 100-150; aromatic 110-150; ester C=O 165-180; aldehyde/ketone C=O 190-220.

  10. DEPT experiments classify carbons by attached H count:

    • DEPT 90: only CH peaks visible.
    • DEPT 135: CH and CH₃ up, CH₂ down, quaternary invisible.
    • Combined with regular ¹³C, gives full carbon classification.

  11. 2D NMR experiments:

    • COSY: ¹H-¹H couplings (which proton is coupled to which).
    • HSQC: ¹H-¹³C one-bond correlations (which proton is on which carbon).
    • HMBC: ¹H-¹³C multi-bond correlations (across heteroatoms; locates quaternary C).
    • NOESY: through-space proton correlations (≤5 Å); cis/trans on rings.
    • TOCSY: total correlation; reveals spin systems.

  12. Stereochemistry by NMR:

    • cis/trans alkenes: distinguish by coupling constants.
    • Diastereotopic protons: a -CH₂- next to a chiral center shows two separate peaks (AB pattern).
    • 3D structure: NOESY shows close protons.
    • Chiral shift reagents distinguish enantiomers; measure ee.

  13. Modern NMR instrumentation:

    • Superconducting magnets: 11.7-21 T (500-900 MHz for ¹H).
    • Sample: 1-10 mg in 0.5-0.7 mL deuterated solvent.
    • Common solvents: CDCl₃, D₂O, DMSO-d₆, methanol-d₄.
    • Acquisition time: minutes for ¹H; hours for ¹³C; longer for 2D.

  14. Practical tips:

    • Exchangeable protons (OH, NH) often appear as broad singlets and may not split adjacent CH.
    • Solvent residual peaks: CDCl₃ at δ 7.26 (¹H), 77 (¹³C); DMSO at δ 2.50 (¹H), 39 (¹³C).
    • Diastereotopic protons next to chirality appear as AB patterns.

  15. Combined IR + MS + NMR essentially always identifies a small organic molecule unambiguously. The standard structural elucidation toolkit.

  16. MRI is NMR's clinical descendant: same physics, with spatial encoding via gradient fields. Used for medical imaging (brain, spine, joints, cardiovascular). MRS extends to localized in vivo chemistry.

  17. Modern applications:

    • Pharmaceutical structure verification.
    • Natural product structure elucidation.
    • Reaction monitoring (real-time).
    • Metabolomics.
    • Protein structure determination.
    • Solid-state NMR of polymers and membranes.
    • Hyperpolarized NMR (DNP) for low-sensitivity nuclei.

  18. Nobel prizes for NMR-related chemistry:

    • 1952 Physics: Bloch & Purcell (NMR discovery).
    • 1991 Chemistry: Ernst (multidimensional NMR).
    • 2002 Chemistry: Wüthrich (protein NMR).
    • 2003 Physiology/Medicine: Lauterbur & Mansfield (MRI).

  19. Mastery of Chapter 9 lets you:

    • Identify any small organic compound from its spectra.
    • Distinguish stereoisomers.
    • Verify reaction products.
    • Read modern publications fluently.
    • Use NMR as a daily tool in research or pharmaceutical work.

  20. Spectroscopy is now your storyline. Every chapter from here on will use IR, MS, and NMR as the diagnostic tools for new functional groups and reactions. Build the habit of running the spectroscopic analysis in your head every time you see a new molecule.

Cross-references

  • Chapter 2 — Bonding (foundation for understanding chemical shifts via electron density).
  • Chapter 4 — Functional groups (each has characteristic NMR signatures).
  • Chapter 6 — IR and MS (work alongside NMR).
  • Chapter 7 — Stereochemistry (cis/trans, diastereotopic).
  • Chapter 8 — Stereochemistry of reactions (verify by NMR).
  • Chapter 19 — Conjugated systems (UV-Vis complements NMR).
  • Chapter 32 — Carbohydrates (anomeric NMR, Karplus for ring conformations).
  • Chapter 33 — Proteins and amino acids (NMR for sequence and structure; AlphaFold complement).
  • Chapter 35 — Drug design (verify drug structures).
  • Appendix D — Spectroscopy reference tables.

Study tip

For every new compound you encounter: 1. Predict the ¹H NMR: how many environments? Approximate shifts? Multiplicities? 2. Predict the ¹³C NMR: how many carbons? Approximate shifts? 3. Predict the IR: which functional groups give characteristic peaks? 4. Predict the MS: molecular ion mass? Common fragmentations?

Then compare to actual spectra (from SDBS or published data). 30 minutes per week with real spectra builds NMR fluency faster than any reading.

The habit to leave with: When you see a new molecule, immediately ask "how would this look in NMR?" and "what would be diagnostic?" This puts spectroscopy at the front of your mind, where it belongs in modern chemistry.

Part II ends here. Chapter 10 begins Part III with the first mechanism — $S_N2$. NMR will verify all the products.