Rotational isomers by HPLC

[djacqo]

Good day,

Does anyone have any experience with observing rotational isomers of amides by HPLC? I can understand a wide or split peak but I would think that they interconvert on timescales too small to allow them to separate into well defined peaks with good resolution.

I'm seeing two nice, separated peaks by UV at room temperature and they show the same UV/VIS and mass spectra. I cannot imagine they would be any constitutional isomer (stereoisomers are not possible).

Thanks!
D

[tom jupille]

The obvious question is "are you sure about the time constant for interconversion?".

One way to check would be to run the separation at increasing temperatures and see if you start to pick up the "batman" shape (two peaks with a sort of shelf bridging them).

[mattmullaney]

Hi Djacquo,

I have...DNPH (dinitrophenylhydrazine) derivatives of aldehydes and ketones as in the EPA TO-11A method. Using several polar-embedded stationary phases in RP-mode it is easy to separate E/Z isomers. There are a variety of other reported instances for separations of these types of isomers (Schiff Bases--diastereomers). Actually I think Famotidine "suffers" from a similar behavior, as well...one other pharmaceutical, too, but the name escapes me at the moment.

pH may also play a role in the formation of said isomers...in the case of the DNP-hydrazones, acid acts as a catalyst.

P.S.: The name of the pharmaceutical compound I couldn't remember earlier...an older drug...is captopril. The hindered rotation about the amide bond is strongly pH dependent.

[djacqo]

Thanks for the replies! I suppose I'll go ahead and do some temperature, possibly pH experiments.

D