Hi,
I have encountered a strange behaviour of some signals (they are absent) in a series of polycyclic aromatic carboximidates spectra. We have X-Ray or mass spectroscopy data for these compounds, so we know how they look like. All signals in 1H and 13C spectra match up to structures, but some protons or carbons don't produce peaks at all.
(1) Here we have this problematic fragment. Red atoms are not visible on corresponding spectra. Imine proton can found near 14 ppm (CDCl3), but its intensity is like 0,05 in comparison to single aromatic proton's signal. Probably this peak belongs to 15N bonded protons. Red carbon signal is absent in 13C spectrum.
(2) Analogical coumarine was synthesised. In case of this compound all signals can be found and described on respective spectra.
(3) Trifluoromethanesulfonyl amide of 1st compund behaves like coumarine (2). All signals can be found and described.
(4) This compound is similar to (1) but instead of phenyl ring (twisted 90 deg in relation to the rest of molecule plane) we used here the ferrocenyl as an off-planar substituent (it's twisted approximately 40 deg). The second ferrocenyl is also incorporated in this structure, but its position is distant from this problematic region of molecule.
1H Spectrum: Imine proton is visible at 14 ppm with proper intensity (integral = 1).
Ferrocenyl (red one on illustration) signals are not visible at all. Signals from the second ferrocenyl (not included on ilustration) are present with proper intensities.
13C Spectrum: Imine carbon can be found at 189 ppm (CDCl3). 1st ferrocenyl signals (all !) are absent, 2nd ferrocenyl signals are present.
Does anyone know what is going here? Maybe this is an effect of strongly quadrupolar (14N) vicinity and/or CSA of C=N bond? Any idea, hint or literature would be a great help.
Best regards,
Arek
