First a few questions: What nucleus are you looking at 1H or Pt. What field and solvent are you working with and finally: What intensity and sign are the peaks in the NOESY and ROESY.

Thanks for answering.

I am looking at 1H spectra.

I obtained the same results on a 600 MHz and a 700 MHz instrument (Bruker). The solvent is a mixture of D2O/CD3OD (66/33, v/v). I need to add CD3OD to decrease the melting temperature.

The cross peaks in the NOESY spectra are in phase with the diagonal (both in the aromatic and aliphatic region) and quite intense. In the ROESY spectrum I observe NO cross peaks in the aromatic region and anti-phase peaks only in the aliphatic region (less intense with respect to the NOESY c.p.).

I hope I was clear enough.

Thank you in advance for your help.

Nico

So you have +ve cross peaks in the aliphatic region and you get -ve cross peaks in the ROESY for the same peaks. That looks like these are true NOE peaks. I do not know your molecule so I do not know if that is reasonable.

The aromatic region: you have + ve peaks in the NOESY and no peaks in the ROESY.

-> that COULD mean that you have exchange and NOE going in at the same time.... and it cancels in the ROESY. .....

I think you need to separate exchange from true NOE's. Depending on the exchange rate you will see exchange even at very small mixing times before there is any significant NOE buildup. It may be helpful to increase the temp if that is possible. That will minimize the NOE buildup and speed up the exchange. So if you run a NOESY at 10ms mixing time you may only get exchange. You can also see exchange peaks in a TOCSY. Do you see them there as well. (assuming the 1H are not J coupled or can have a TOCSY transfer)

The in phase cross peaks in NOESY mean negative NOE and possible exchange, whereas anti phase cross peaks in ROESY are from ROE only, since exchange is always negative and give in phase cross peaks in both NOESY and ROESY. This means the molecule gives negative NOE, and may have a non spheric shape with this small MW, heteroneous motion.

@Markus: thanks for your answer. I suppose that the signals that could eventually give an noe cannot give exchange between them. I will try to run a TOCSY: which sign do I have to expect for exchange peaks in a TOCSY?

Thank you.

@Yan: thanks for your answer.

Tocsy... same as the diagonal. Good luck!

@Markus: I recorded a NOESY at 10ms mixing time (at a temperature 5 K higher than previous ones) and observed only one of the several peaks previously observed in the aromatic region. I also recorded a TOCSY and I was able to observe the same exchange peaks of the short mix. time NOESY.

I am now able to distinguish between NOE and exchange cross-peaks.

It remains unclear why they both disappear in the ROESY.

Thanks again for your help.