Well, right during one of my training sessions, the first measurement problem surfaced. I want to monitor malonic acid degradation in aqeous solution and therefore I have prepared a couple of standards in D2O. Strangely, the singlet at 45 ppm always showed up as a quintett, pointing towards coupling with some other species other than H (the decoupler was working fine and the 1H spectrum was good too. All other 13C spectra that I recorded were fine too. The only other NMR active species present was deuterium, so I speculated that some exchange reactions must be going on, replacing the H in the molecule with D. The most acidic H was the one on the carboxylic acid group, but after discussion with the NMR facilities manager, we ruled out this process, because it takes place too quickly.
So the only other possibility was the C2 and its 2 (less) acidic H atoms. And this was indeed the case. Assuming this exchange, the resulting quintet makes sense and in diluted D2O (only 10%, with the rest being H2O, this rather weak effect was not observed. During the day of measurements, I have also posted the issue on sci.chem and got my thoughts and experimental results confirmed. Nice!
