[Question from NMRWiki Q&A forum] Odd 1D-ROESY Result

[nmrlearner]

Odd 1D-ROESY Result

Hello,

I've been trying to assign peaks in a proton spectrum of a highly dynamic ring, namely 2-deutero-diselenane. The structure is like 1,3-dioxane but with two seleniums instead:http://en.wikipedia.org/wiki/1,3-Dioxane

The experiment is at low temperature where exchange is slow on the chemical shift timescale but probably fast on the relaxation timescale. What I can't figure out is that when I irradiate the H2 signals in a 1D-ROESY, I see the following peaks:-Negative irradiated H2.-Negative peak for non-irradiated H2, due to exchange due to ring-flip.-Negative peaks for both H4/H6 axials and equatorials!

It's the last result I can't figure out - if it's a true ROE, those should be positive peaks. It can't be TOCSY contamination because there are intervening selenium atoms - so what could it be?

You see the same result if you irradiate the H4/H6s but with additional positive peaks from ROE to the H5s. If you irradiate H5s, you see a positive ROE to the H4/H6s and also a small positive peak to the H2s! This is a secondary ROE, so it should be negative, right? Something strange is going on...

Anyhow, help would be most appreciated ! Thank you.



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