VIRTUAL EXPERIMENTS FOR THE BUILDUP OF ASPHERICAL ATOM SCATTERING FACTORS OBTAINED BY A NOVEL PARTITIONING SCHEME AND APPLICATION TO A TRIVALINE STRUCTURE

Birger Dittrich^a and Dylan Jayatilaka^b

^aFreie Universität Berlin, Takustr. 6, 14195 Berlin, Germany; ^bUniversity of Western Australia, 35 Stirling Highway, Crawley WA 6009, Australia (birger@chemie.fu‑berlin.de)

Following a route recently described by Koritsanszky et al. [1] we have calculated theoretical data sets for geometry optimized [2] model compounds. The theoretical structure factors are fitted with the Hansen and Coppens multipole model [3] using the XD program package [4]. In other words we perform a refinement on the theoretical Fc's, which are treated as Fo's. The goal is to obtain multipole parameters for transferable atoms and therefore to have a library of transferable aspherical atom electron density parameters.

Starting from [1] a generalized scheme for a partitioning of a structure into transferable atoms is proposed. These atoms are similar in a transfer process from one molecule to another because they mimic the same chemical environment with respect to the nearest neighbors of the atom of interest. To minimize the size of the model compounds the remaining valences of the nearest neighbors of the atom of interest are saturated with hydrogen atoms. These transferable atoms defined that way are called invarioms (from invariant atoms).

This partitioning scheme is used to build up a library of multipole parameters, in a similar manner as realized by Pichon Pesme et al. [5] and successors for experimental data. Amino acids and peptides consist of only 30 invarioms. The database used here contains all these 30 invarioms and an application to experimental data as a test of the method will be presented.

The test structure, VVV * EtOH * TFA, was recently determined in our laboratory. It crystallizes in the space group P21, a = 9.705, b = 19.270, c = 11.820 Å and b?= 100,26° with Z=2 and four independent molecules in the asymmetric unit. The spherical R-factor using the independent atom model is 4.4%. Although a good structure, it is not ideally suited for a high resolution data collection followed by a multipole refinement due to the four independent molecules in the asymmetric unit and the presence of the cocrystallizing trifluoracetic acid. On the other hand this makes the structure an ideal test case to appy a transfer of the theoretically obtained multipole parameters.

The expected outcome is not only a lower R-factor, so a better fit of the experimental data. From the transfered multipoles and the resulting aspherical electron density i.e. the electrostatic potential as well as dipole and multipole moments can be calculated. This approach could, if successfull, be generalized for any crystal structure without a limit in the numer of atoms and does not depend on experimental data.

References

[1] Koritsanszky, T., Volkov, A., and Coppens, P., Acta Cryst. A58, 2002, 464.

[2] Gaussian 98, Revision A.11.3, M. J. Frisch et al., Gaussian, Inc., Pittsburgh PA, 2002.

[3] Hansen, N., and Coppens, P., Acta Cryst. A34, 1978, 909.

[4] Koritsanszky, T., Howard, S., Richter, T., Su, Z. Mallinson, P., and Hansen, N. K., XD - A program package for Multipole Refinement and Analysis of Electron Densities from Diffraction Data. User Manual. Freie Universität Berlin, 1996.

[5] Pichon Pesme, V., Lecomte, C. And Lachekar, H., J. Phys. Chem. 99, 16, 1995.

Help from T. Koritsanszky for structure factor calculation is gratefully acknowledged.