Re: strange question
bergmann ( bergmann@Rcs1.urz.tu-dresden.de )
Wed, 15 Oct 1997 13:01:18 +0200 (MDT)
> Hello all,
>
> I have a question, maybe you can help me on this.
>
> I have small single crystals of an organic compound, A.
> By heating these crystals at about 300 C, a decomposition of A takes place
> (nothing wrong, yet) with about 20 percent mass loss. The obtained compound is
> in crystalline form, and it seems exactly like A (small needles, crystals-like);
> there is no melting or sublimation below 300 (the crystals are too small for a
> four-circle tentative).
> It this possible ?
> Thank you,
>
We have obtained similar problems while examining an organic trimer
(we have published it in Denver this year). At first, we have got
not the X-ray-density for the molecules in the known unit cell. So
we decided to put some crystal water into the Rietveld model. R values of
Rietveld refinement were much better, and the X-ray density now
equals the pycnometric measured one. But, in our case, after heating
(recristallization) the crystal was the same, indeed. And also the
XPD patterns. Thus, the crystal water either remains in the crystal while
heating or goes back into the crystal at room temperature. We have observed
some loss of substance while heating, but this was the solvent from the
reaction, which was sublimated on the surface of the micro crystals (also
small needles). There is a big surface on micro needles for sublimating
solvent!
Joerg Bergmann
bergmann@rcs.urz.tu-dresden.de