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Below you'll find the structure of the disaccharide and three 2D NMR spectra - COSY, HSQC, HMBC. I have to match the signals in the spectra to the nuclei that caused them. Answers to any subset of the following questions would be very helpful, but please explain like I'm 5:

  1. How many protons would we expect the protons at A6 (highlighted in orange) to couple with?

  2. Would we expect any coupling between the protons at A1 and B4 (in yellow)?

  3. How many protons would we expect the protons at B6 (in green) to couple with? And are they considered axial or equatorial?

  4. How many protons would we expect the proton at B1 (in blue) to couple with?

  5. How do the heteronuclear spectra help us in this specific case? What is the crucial piece of information that they provide?

The structure:

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COSY:

enter image description here

HSQC:

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HMBC:

enter image description here

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    $\begingroup$ It is not helpful if an atom numbering is ambiguous (e.g., C4 left saccharide binds to an acetate, C4 right binds to an ether [glycoside]). A $|^3J_\text{H,H}|$ often is resolved (H6a, or H6b to chiral H5; either side), a $|^4J_\text{H,H}|$ (still across bonds) typically is much smaller; and both different to coupling through space (NOE). Does your instrument routinely resolve a $^4J_\text{H,H}$? Do you have «an anchor» to work through the space (Sudoku like), best if already known by an assigned precursor which was not altered by the last reaction from literature/own records (e.g., CHBr)? $\endgroup$
    – Buttonwood
    Commented Feb 15 at 13:29
  • $\begingroup$ @Buttonwood This is simply homework that I posted exactly as I have been given it (minus the questions and highlighting, these are my own). To be honest, I don't even understand your 2 questions as anything chemistry-related is quite foreign to me, hence the explain-like-I'm-5 plea. $\endgroup$
    – Dilara
    Commented Feb 17 at 8:29

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Answer time!

  1. A6a and A6b (in orange) actually give rise to two separate signals. A6a couples with A5 and A6b; A6b only couples with A6a.

  2. We do not expect any coupling between A1 and A4 because of the O in between.

  3. Again, B6a and B6b (in green) are two separate signals. B6a only couples with B6b; B6b couples with B6a and B5.

  4. B1 only couples with B2.

Below is the correct assignment for COSY, with the signals from ring A and ring B highlighted, resp. Ignore the highlighting if it makes things more confusing, and ask any questions that you might have and I'll try to answer.

enter image description here

enter image description here

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  • $\begingroup$ "How do the heteronuclear spectra help us in this specific case? What is the crucial piece of information that they provide?" Any comments? You did not show us the assignment for those spectra. $\endgroup$
    – Karsten ♦
    Commented Feb 17 at 21:03

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