[Question from NMRWiki Q&A forum] Concentration-dependent signal fine splitting

[nmrlearner]

Concentration-dependent signal fine splitting

Hi,

I have synthesized coproporphyrin II tetramethyl ester and taken a 1H NMR in CDCl3:

http://illumina-chemie.de/upload/5_1...6157fa7971.png

Especially the peaks at 10.1 ppm, belonging to the CH groups connecting the pyrrole rings, show a strong concentration dependence. They should be two singlets, but in fact they are only singlets at very low concentrations and split up with increasing concentration:

http://illumina-chemie.de/upload/5_1...616989db39.png

To me that makes it look like it's an aggregation thing, probably monomer/dimer equilibrium, and that is in agreement with the literature (Abraham, R. J., et al., J. Chem. Soc. (B), 621 (1966)). It is, however, not in agreement with 1H DOSY spectra taken at 2.7 mM and 9.0 mM. These only show one porphyrin species, in both cases around D = 1E-5 m^2/s.

Could it be such an equilibrium despite no signs for it in the DOSY spectra (if so, why doesn't it show?) and if not, what else could cause the concentration-dependent fine structure of the signals?

Thanks for your help.

Jan



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