Oddity #1
I’ve got a new category again. It’s called “Oddities”. I will put there chemical stuff that I find odd/weird/surprising/etc. They probably aren’t odd, weird or surprising to many of you but they are something that made me go “eh… wtf?”. This is the first oddities post.
The sulfone shown below is an intermediate I had during my project (sorry for the mystical “Ar” but I’m just being extra cautious with any unpublished results). The preparation of this intermediate was supposed to be fairly standard work with no surprises, and so there I was sitting in the NMR room running a standard H NMR to verify that it’s the right intermediate. And as soon as I see the spectrum I go “eh…wtf?”. Now pause for a moment and think how you would expect the H NMR (in d-chloroform) of that stuff to look like (you can ignore the Ar protons).
If you are thinking of something like two triplets (2H and 2H), then you are thinking exactly what I was thinking. However, there’s only one sharp singlet of 4 protons at around 4 ppm. I was, of course, quite surprised to see the singlet, so I ran a fast C NMR. Nothing weird there, one peak at 57 ppm and another at 35 ppm. After running some other experiments (e.g. HSQC) I was pretty convinced that the intermediate was ok. Eventually, I changed the solvent to d6-DMSO, and there they were, the triplets. I’m far from being an NMR expert so maybe one of you might want to explain the odd behavior in d-chloroform.
December 14, 2006 at 12:49 am
I had a 2,6- diaryl pyridine once that showed a singlet (3H) for the pyridine protons. I blamed it on one of those once-in-a-blue-moon coincident chemical shift events. Changing to d6 benzene caused them to look like the standard peaks I would expect.
December 14, 2006 at 5:11 am
Milo’s got it right. The most simplistic explanation is the correct one: in CDCl3 the protons have the same chemical shift, thus no splitting and a singlet. Different solvent, different pattern. Pretty common, actually.
December 15, 2006 at 8:36 pm
I would have guessed they were around 3 ppm
December 18, 2006 at 2:31 am
I have experienced this effect with the preparation of Leighton’s crotyl/allyl silation reagent. See Angew. Chem. Int. Ed. 2003 42(8) 946 for the prep, J. Organomet. Chem. 1998 v. 567 p. 199 for spectral data on the intermediate diimine. If anybody can explain what’s up with TMS-ethanol’s spectrum (http://www.sigmaaldrich.com/spectra/fnmr/FNMR003501.PDF), I would love to know why it looks so whacked.
December 19, 2006 at 1:11 am
Watson,
Have you ever checked out a detailed spectrum of THF? You’ll find peak shapes that are similar to TMS-EtOH. Although I cannot give a good explanation. I can give a good hand hand waving one, but that is not really scientific
December 19, 2006 at 6:08 am
It appears that 3,3-dimethylbutanol has a similar effect going on in one CH2. It must be a steric thing. Mercapto Ethanol, or Iodoethanol are just triplets, or dt depending on the dryness. SEMCl also the same effect.
December 20, 2006 at 6:37 am
Yeah, 2-nitroethanol shows a singlet for the four methylene protons as well. Never changed solvents to make sure, but I wasn’t bothered by it enough.
January 11, 2007 at 5:08 pm
Throw it on the 900 MHz to resolve those two chemical shifts.
January 29, 2007 at 2:17 pm
I had a 4-chloroarylsulfone where the 4 aryl protons were equivalent in CDCl3 at 400 MHz……
March 16, 2007 at 2:07 pm
My suggestion is do a variable temperature NMR to see whether there is any dynamic process occurring in CDCl3.
September 13, 2007 at 3:27 pm
I have an ‘out-there’ suggestion. The loss of DCl from your CDCl3 (often happens!) would result in a reactive carbene which could react with the chloro-alkane resulting in the loss of HCl and if your CDCl3 was a bit wet a sulphonic acid, and finally a dichloro-cyclopropane. The cyclopropane meets all of the spectroscopic requirements. Maybe a DEPT can tell you something?