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Structure Determination Using 2D NMR Spectroscopy: Assigning Aromatic Protons

Learn how to assign aromatic protons using NMR without a structure, with guidance on interpreting HSQC, COSY, HMBC, and more for spectroscopic analysis.

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Structure Determination Using 2D NMR Spectroscopy: Assigning Aromatic Protons

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  1. Can we use 2D to assign this spectrum? 4 aromatic protons Structure first determined by Woodward in 1948 (without NMR!)

  2. HSQC CH2 Identify CH, CH2 and CH3 peaks on HSQC (use DEPT if necessary)

  3. COSY H-22 is a good place to start 22 23

  4. HSQC H-12 can be tentatively identified from C and H shifts 23 12 ? Transfer this information to the HSQC spectrum 22

  5. COSY 13 8 We can identify 11 as a CH2 and 13 as a CH from the HSQC H-8 can distinguished from H-14 by the doublet multiplicity and shift 13-14 correlation is weak, but is seen on one side of the diagonal 13, 14 11 13 12

  6. HSQC 14 11 13 8 23 12 Note that have a number of “dead ends” as far as COSY is concerned 22

  7. COSY The protons at C-15 can be identified by their correlation to H-14 14 15 15 13

  8. COSY Having identified the C-15 protons, we can now see the correlation to H-16 15 gem 16 15 16

  9. HSQC 15 14 11 13 16 8 23 12 This leaves 3 CH2s, at C-17, C-18 and C-20 22

  10. HSQC The C-20 protons can be identified as a pair of coupled (from COSY) doublets, not coupled to anything else

  11. COSY The CH2s at C-17 and C-18 are left. The HSQC shows an overlapping CH2 at 1.94 ppm. The chemical shift indicates that this must be the C-17 CH2. The CH2 at 2.92 and 3.35 ppm must be C-18. 18 18 17 (diagonal)

  12. HSQC 15 14 11 17 13 18 20 16 8 23 12 All protonated carbons have now been assigned 22

  13. HMBC We can now use HMBC to assign the non-protonated carbons (and confirm other assignments, if necessary) C-10 H-12 H-11ax H-11eq

  14. HMBC C-18 H-20 H-20 C-7 C-20 H-1 H-8 CH2-17 H-18 2 bond C-7 to H-16 correlation not observed? H-15

  15. COSY TOCSY 12 8 11 13 15 14 16 15 (diagonal) (diagonal) 17

  16. NOESY 8 16 12 15 13 15 13 15 NOESY Avance 300 Degassed sample Degassing removes oxygen (paramagnetic) from the sample which would otherwise provide alternate relaxation pathways. gem 15 8 12

  17. 15N-HMBC 15N spectroscopy is very difficult! However, we can measure 15N shifts and determine the location of the nitrogens with HMBC (AMX 500, 10 mg sample, overnight) N-19 H-18 H-18 H-16 H-15 CH2-17 N-9 H-13 H-8 H-11

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