3.3- Solving the structure


The choice of the method (direct, Patterson) is suggested by the presence or not of "heavy" atoms.






OVERLAP is easy to run :


Results from different datasets.


The PATT option of SHELXS86 proposes two atoms :
Pd at 0,0,0 coordinates and Cr at 0,0,0.5.

Should be two palladium atoms sites.

A first refinement (with SHELX76), leads to a result quite unsatisfying with R = 0.62.

2 peaks clearly distinct in weight which could be associated with 2 Cr atoms.

This hypothesis is tested with SHELX76.

The reliability factor goes down to 35%.

Next Fourier difference synthesis.

And what would have given the direct methods ?
Application to the 1054 hkl,
of which probably no more than 250 are more or less well estimated,
proposes randomly the 4 heavy atom sites (2 Pd + 2 Cr)
at the head of the list and evenmore !


The brute force consisting in the SHELXS-97 application to the whole dataset gave a list of 15 atom sites.

Examining this list by a structure drawing program (STRUVIR) revealed immediately that the 11 first atom sites described completely a new MX3 corner sharing 3D network.

Where is the merit of the researcher ?
Thanks to G. Sheldrick !


    X in OVERLAP       Dataset      R(%)SHELX76
   0                1387            36
   0.01             1099            51
   0.02              866            39
   0.03              691            39
   0.04              522            46
   0.05              427            46
   0.06              338            42
Curiously, the best proposition (R = 36 %) is obtained
from the whole dataset.

SHELXS-97 proposition.

SHELX76 refinement result.

A structure drawing is very convincing that a large part of the beta-BaAlF5 structure has been obtained :


The next step will be obviously to apply the Rietveld method.

Solving the structure from the X-ray data was much easier.

The two barium atoms only had to be located.

Cimetidine C10H16N6S

The 924 extracted reflections are put into SHELXS-97,
applying the direct methods by the "brute force".

The resulting top 17 peaks are introduced into SHELXL
for a refinement against a reduced dataset

A very encouraging R1 ~ 18% was obtained allowing to go further.