Multi-Pole Approach to Structural Science

Chemistry Department, Warsaw University, Pasteura 1, 02093 Warszawa, Poland. *E-mail: kwozniak@chem.uw.edu.pl

ABSTRACT. Although everything seems to be already well known in the field of routine structural single crystal X-ray analysis and ca. 1.1 mln structures have been solved and refined so far, even commonly used approaches and models should be critically re-evaluated. It is incredible that the Independent Atom Model (IAM) of electron density effectively introduced a century ago is still the most common model of electron density used in structural analysis. One would even say that its success has dominated the whole field. On the other hand, when this model was introduced Max von Laue, the Braggs and their colleagues were using home-made pieces of equipment which could have hardly supplied qualitative information on diffraction spots. In consequence the errors associated with the model of electron density used were overshadowed by far larger diffraction hardware errors. However, within the past century there is an overwhelming progress in design and production of X-ray hardware which is made for needs of both small laboratories and large scale facilities. This progress in sophisticated X-ray hardware should also accelerate progress in the quality and complexity of models of electron density used to interpret experimental results. However, it is very surprising that although the quality of diffraction information collected in X-ray experiments in the XXI century allows these days for far more thorough structural data quality, almost all crystallographers keep using 100 years old models of electron density effectively proving that even with the most modern scientific tools, one can step backward and do ca. 100 years old crystallography :).

In my presentation, I will discuss the main ideas of experimental charge density studies, Hirshfeld Atom Refinement, and Transferable Aspherical Atom Model. I will present a detailed comparison of structural, thermal and electronic parameters obtained for the same diffraction data sets when different models of electron density (IAM, TAAM, HAR, MM) are refined against collected intensities of reflections. Accuracy and precision of structural data obtained from routine and charge density studies going beyond IAM will be discussed. Some practical suggestions will be presented how to estimate and improve the quality of single crystal X-ray diffraction structural results. As crystallography is also a matter of imagination, let us imagine that the Earth symbolizes an aspherical electron density of an atom. Then the consequences of application of the Independent Atom Model are illustrated below:

Fig. 1. Symbolic consequences of application of the Independent Atom Model.