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I have a simple, completely conjugated pyridinium salt with MW of ~300 g/mol. The molecule consists of regular non-labeled atoms (only C, H, N).

  • If the 1H T1 relaxation time for this molecule is measured in DMSO-d6 the T1 values of all of the protons are within the expected range (9 sets of protons with T1 values of 1.4-2.1 s).
  • However if the 1H T1 relaxation time for exacty the same molecule is measured in CDCl3 the relaxation times differ up to 20 times (9 sets of protons with T1 values of 0.04-1.9 s). The 1H relaxation is especially fast at the pyridinium ring (2 sets of pyridinium ring protons with T1 values of 0.04 and 0.07 s and further removed protons from the pyridinium have T1 values of 1.5-1.8s).

From what I have read in the literature the relaxation times usually differ up to 2-fold. Any ideas why this is the case?

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  • $\begingroup$ Can't help but wonder if aggregation is to blame. Unless you have a huge non-coordinating anion to go with your pyridinium compound, you can expect some pretty tightly bound ion pairs in chloroform, whereas DMSO would easily pull them apart. $\endgroup$ – Nicolau Saker Neto Jan 15 at 11:24
  • $\begingroup$ We performed dynamic light scattering experiments on the samples, and aggregate formation was not detected. I would imagine that a dimeric (or similar) structure could form in the weakly solvating solvent such as CHCl3, due to the formation of highly energetic pi+-pi+ or pi+-pi interactions. However even if an associates between the molecules form the relaxation time should not change in such a drastic way. $\endgroup$ – A small potato Jan 15 at 12:02

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