[ashika]

Hello everyone,

I am in the posession of a compound which is capable of having both a cis and a trans form. However, I have only one form. I was wondering if anyone knew of any experimental methods to determine which form one has in this kind of situation (NMR wasn't useful).

[Dan]

(NMR wasn't useful).

Can you elaborate on this - why wasn't it useful?

While examination of the coupling constants in 1,2-disubstituted alkenes can be inconclusive, I would have thought NOE or NOESY experiments would provide compelling evidence (and would be informative for tri- or tetrasubstituted alkenes).

If you can give more detail about the structure, that would be useful.

[mjc123]

Cis alkenes typically have a CH wagging mode at ca. 680 cm^-1 in the IR; in the trans isomer this is at ca. 970 cm^-1. Depending on what else is in your molecule, this may or may not be sufficient to distinguish your isomers.

[Irlanur]

(NMR wasn't useful)

I suspect that the NMR user/interpreter wasn't useful

[ashika]

The compound I'm studying is not an alkene. It's a compound with a heteroatom doubly bound to a carbon with branching on both the carbon and the heteroatom. I cannot unfortunately fully disclose what kind of compound it is. But when I say NMR wasn't useful, I mean it and I don't mean NMR isn't useful as an absolute statement. There are no observable NOEs that allow to distinguish between the cis and trans isomers.

If anyone knows of any other method used for distinguishing between cis and trans isomers please tell me about it.

I wonder if anyone here has ever used a polarimeter to do this kind of thing? I'm not certain it's applicable.

[Dan]

It's a compound with a heteroatom doubly bound to a carbon with branching on both the carbon and the heteroatom. I cannot unfortunately fully disclose what kind of compound it is.

Ok, so like a secondary ketimine then?

It might be helpful if you can give the functional group (i.e. what the heteroatom is - I realise this may not be possible though).

[ashika]

I cannot give you a functional group sorry. All I can say is that it's not a classical functional group and that there are no observable NOEs between the two parts of the compound seperated by the double bond.

[Arkcon]

Perhaps you can achieve chiral separation on an HPLC system.  If you get two peaks, then (if possible) you can spike the sample with the cis or trans isomer to determine which is which.  I'm pretty much grasping at straws here.

By the by, I can understand the H-NMR may give you no information, but what about ¹³C-NMR, if its at all possible for you to make.

[Babcock_Hall]

If there is a ³J_HH in your molecule, the cis versus the trans form give very different values of coupling constants, all else held equal.

[Irlanur]

As always when it's about a specific compound, it would be helpful to have specific information. what kind of molecule is it? is it in solution? can you have it in a powder? what do you already know, etc.

[MOTOBALL]

Have you done a lit. search for MS or MS/MS to differentiate cis/trans ?

Using ESI-MS, I would be interested to see if

R1   H+     R3
      C=X           will lose  R1H; R2H; R3H and possibly R1-R3 all as neutrals
R2

whereas,

R2   H+    R3
      C=X          might be expected to lose  R1H; R2H; R3H and possibly R2-R3 all as neutrals
R1

especially if X = N.