The 1H-NMR spectra of the paraquat (1,1'-Dimethyl-4,4'-bipyridinium dichloride) in D2O is characterized by 2 doublets pecks in aromatic region: at 9.12 ppm, called "orto-protons" and respectively 8.58 ppm, called "meta-protons". After thermal treatment in D2O (90°C/3 days) the "orto-protons" practically desapear. What exactly caused diminishing of this protons at 9.11 ppm? For more information please see the attached document.
The protons are being replaced with deuterons. see http://ac.els-cdn.com/S0065272508604584/1-s2.0-S0065272508604584-main.pdf?_tid=11ebb2e6-3d97-11e3-b73b-00000aab0f6c&acdnat=1382720660_7fddc1e2856a7e7aafb524429ecd3b4b for a review. incidentally the integration on the second spectrum is probably wrong
The protons are being replaced with deuterons. see http://ac.els-cdn.com/S0065272508604584/1-s2.0-S0065272508604584-main.pdf?_tid=11ebb2e6-3d97-11e3-b73b-00000aab0f6c&acdnat=1382720660_7fddc1e2856a7e7aafb524429ecd3b4b for a review. incidentally the integration on the second spectrum is probably wrong
If it is possible to run a 13C-NMR on the same sample you should have triplets for carbons linked to deuterium (only one triplet due to the symmetry of the molecule), to confirm with an evidence the substitution.
Also ESI-MS spectra could be of help.
Yes, Wootton got it and Venditti gave you a hint to confirm
For a study of the deuteration of 1-methylpyridinium chloride and drevatives; see
--J. A. Zoltewicz, G. M. Kauffman, and C. L. Smith, J . Amer. Chem. SOC. 90, 6939 (1968).
--K. W. Ratts, R. K. Howe, and W. G. Phillips,J. Amer. Chem.Soc. 91,6115 (1969).
--J. A. Zoltewicz, C. L. Smith, and G. M. Kauffman, J . Heterocycl. Chem. 8, 337 (1971).
Good luck
I worked on the bromination and nitration of biphenyls and o-terphenyls, using electrophilic aromatic substitution as my model. What I found was that the nucleophile (o-terphenyl) would attack the electrophile with a bridgehead carbon, the carbon atom the bears no hydrogen and connects to the adjacent ring. From this transition state, the electrophile would "slide" over to the ortho-position. It sounds odd, but I calculated the energies transition states and the "sliding" and it all worked out. Loss of a proton would then give me my final product, and explained why I would always see more o-substitution than m-substitution.
This sounds to be the same thing that is happening in your case, the electrophile is D+, which is swapped for H+, becoming NMR silent.
Is there anything going on with the (integral of the) signal of the methyl protons? One more possibility would be that under such conditions a proton from the CH3 group gets abstracted by a base (although we do not have an apparent base strong enough for this, the experiment is run for a long time at 90 C), and result in a delocalized (partially neutralized, zwitterionic) species with the formal negative charge spread over the "ortho" positions as well. This would be followed by deuteration of the "ortho" positions but also of the methyl group leading to its decrease in integral value. Thus, if the ratio of the integrals of the "meta" protons to the methyl protons is higher than 2:3, this could be a possible mechanism explaining your findings as well. I would definitely recommend recording a 13C NMR, as suggested by Alessandro, for your reaction mixture, as it would give valuable info on the sites of deuteration.
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