May 25, 2019, 11:21:16 AM
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Topic: Salt vs Cocrystal  (Read 290 times)

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Offline jeffmoonchop

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Salt vs Cocrystal
« on: March 09, 2019, 04:27:00 PM »
Hi. Say you had done some cocrystal screening and found two new forms, a neutral cocrystal and a salt.

My coformers are an organic acid and some kind of charge acceptor like pyridine. I've done XRPD and SCXRD on both forms. Having acquired a CIF I've assigned one a salt and the other a cocrystal based on the position of the proton between the two coformers, one of which is closer to the acid (cocrystal) and one of which is closer to the base (salt).

I think the problem is the salt is the metastable form, based on DSC, melting points, phase transition to the cocrystal on heating and its the first form to appear (Ostwalds rule of stages) etc.  This is normally not the case, and its the first time from what I can see in the literature, that a salt is the metastable form over a cocrystal.

I'm being asked to prove even further than its a true salt. How can I do that without running an NMR?

I'm no longer on site for this so I cant run any more analysis on it. They have suggested using bond lengths to discuss this, however its hard to assign bond lengths as proof in ionic and hydrogen bonds, due to their variability. I could say the ionic bond is shorter between N-H than O-H and the opposite for the cocrystal but is that proof its a salt?

thanks.

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