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Chem-805/806

Chem-805/806. Structure determination. Strategies to assign resonances. Entry point: Try to find all the resonances that can be assign without ambiguity from 1D-NMR Predict chemical shifts (H and C) and compare with observed shifts.

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Chem-805/806

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  1. Chem-805/806 Structure determination

  2. Strategies to assign resonances • Entry point: Try to find all the resonances that can be assign without ambiguity from 1D-NMR • Predict chemical shifts (H and C) and compare with observed shifts. • Integration can be used to match number of protons in overlapping multiplets • Predict multiplets and see how they match

  3. Bookkeeping • Determine which signals belong to CH, CH2 and CH3. Integrate H-NMR • Create tables of proton and carbon shifts related to each other • Try to understand the shift (which type of oxygen do you have (carbonyl, ester, ether..) • Build substructure that make sense of multiplicity. • Propose a structure. Number the structure (UPAC rules). Assign H and C NMR.

  4. Example : H-NMRC8H15NO2 3H 2H + 1H 2H 1H 1H 1H 1H 1H 1H 1H

  5. C8H15NO2 Example COSY 3H, t 2H, q CH3 - CH2 - O

  6. C8H15NO2 Example HMQC CH2 CH2 CH2 CH2 O-CH2 CH3 CH  OH? NH?        

  7. Example : H-NMRC8H15NO2 In C13 NMR: 170 ppm ester 3H (CH3-CH2-O-C=O) 2H + 1H 2H OH or NH 1H 1H 1H 1H 1H 1H 1H         

  8. C8H15NO2 Example COSY 3H, t CH3-CH2-O-CO- NH 2H, q            All atoms are accounted for CH2   CH2   CH CH2 CH2 NH     Deshielded by N

  9. Example Jeffrey H. Simpson, “Orgamic structure determination using 2-D NMR spectroscopy”

  10. Example : H-NMRC8H15NO2      3H 2H + 1H 2H 5 NH 6’ 4’ 2’ 3 6 5’ 2 4

  11. Example

  12. Assignment of quaternary Carbons Quaternary carbons: 176.3, 154.8, 143.4, 125.2, 120.4 C4 = 176 C10 = 155, most deshielded due to oxygen C11 C10 C4

  13. HMBC 2 12 13 6 3 10 4 13 12 5 7 2 11

  14. Strategy for Unknown molecules • Consider solvent used: contribution to H and C NMR • Inspect H and C spectra for obvious clues Deshielded HOD Aromatic Solvent: D2O CHO

  15. EXPANSIONS CH – O Aromatic Ortho CHO

  16. C13 O CH-O Aromatic CH O 193.3 ppm CH=O Not shown

  17. Unknown: HMQC • CHO group: 10.2 ppm – 193.3 • 4 aromatic H: disubst., Ortho (2D, 2T) • Deshielded H at 5.2, coupled to C 100.1 ppm : deshielded by 2 O? CH-O O CH O Ortho aromatic CHO

  18. Unknown: HMQC expansion CHO CH2-O 4 x CH-O

  19. Unknown : good accounting

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