COMPUTER PREDICTION OF ORGANIC CRYSTAL STRUCTURES USING PARTIAL X-RAY DIFFRACTION DATA. G. Filippini and A.Gavezzotti, Dipartimento di Chimica Strutturale e Stereochimica Inorganica and CSRSRC CNR, Universiita' di Milano, Milano, Italy.

We have recently developed a computer procedure by which many polymorphic crystal structures are obtained from a rigid molecular structure. A few applications, when only partial crystallographic information (cell and space group) is available, are described. First, five already determined structures selected as validation tests were reproduced, especially when obvious H-bonding patterns could be assigned. As expected, the lattice energies of computational polymorphs are quite similar, even if a preference for the observed structures appears. Next, literature cases were collected where the crystal structure of one polymorph was completely characterized, and mention was made of another polymorph for which only cell and space group were given. The calculations succesfully predicted the unknown crystal structure within very short CPU times (a few hours to a few minutes).

Our procedure applies to molecules containing C,H,N,O,S,Cl atoms, in the most frequent space groups for organics. The procedure may provide an alternative to direct methods, when they fail, for the solution of the phase problem. When cell parameters and space group only are available, it can give the full crystal structure with no X-ray diffraction intensity measurements, and if powder data are available, it will conceivably produce a good starting point for Rietveld refinement, reducing the need for well grown single crystals.

We believe that ab initio crystal structure prediction is still a faraway goal. In fact, in none of the above cases could the observed crystal structures have been predicted without the support of X-ray diffraction data.

Filippini, G.; Gavezzotti, A. J.Am.Chem.Soc. 117,12299(1995).

A. Gavezzotti, Accounts Chem.Res. 27, 309 (1994).