For example, the protons in a methyl group split each other. In comparison to J couplings in isotropic solutions, dipolar splittings in aligned media can be very complicated. The couplings are mutual, of course, so you should look for splittings elsewhere in the spectrum, particularly of the CH2 site "A" and the CH3 on the rings. In this system, the strongest couplings would be associated with the nearest protons. In the case of alignment, a peak may shift due to the chemical shift anisotropy, and it may split due to through-space dipole-dipole couplings. The midpoint of the integral is one easy way to quantify the first moment.) (Center of gravity is also known as the first moment. I noticed that the "center of gravity" of the 10 ppm peaks is concentration-dependent. What about magnetic alignment? A large planar conjugated system like this may have a tendency to align, and to some extent it may be cooperative and therefore concentration-dependent. These only show one porphyrin species, in both cases around D = 1E-5 m^2/s.Ĭould 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? It is, however, not in agreement with 1H DOSY spectra taken at 2.7 mM and 9.0 mM. 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. They should be two singlets, but in fact they are only singlets at very low concentrations and split up with increasing concentration:
I have synthesized coproporphyrin II tetramethyl ester and taken a 1H NMR in CDCl3:Įspecially the peaks at 10.1 ppm, belonging to the CH groups connecting the pyrrole rings, show a strong concentration dependence.
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