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Charge Density
and
Chemistry
"...the experimental study of the scattering of atoms, in particular for light atoms, should get more attention, since along this way it should be possible to determine the arrangement of the electrons in the atoms..."
P. Debye, 1915 - Ann. Phys. 46 (1915) 809
The last 35 years have witnessed the steady blossoming of a new branch of crystallography: the field of charge-density [CD] analysis. It is the realization of a dream come true: The "visualization" of the bonds that hold atoms together in crystalline solid. A recent review has summarized its achievements to-date *). Careful analysis of X-ray diffraction data -often in combination with neutron data- can reveal subtleties in molecular environments, yield net atomic moments, bring forth the nature of the chemical bonds in the crystalline solid at hand, etc. In the best of cases, CD analys is is able to provide quantitative insights and help resolve controversial issues. E.g., the recent accurate CD study of urea was used to validate empirical ab-initio methods and the results underscored the need to considerably improve the level of theoretical calculations in this compound. However, the initial expectation -seeing CD analysis become a routine and powerful tool in the hands of non-specialists- was not fulfilled. The learning curve is just too steep in relation to the benefits the analysis routinely provides.
While the present trend in this field of research is towards a) the derivation of “experimental" wave functions from the observed electron densities and b) applying the methodology to larger molecules such as proteins, little effort has been spent to generalize the application of the method to crystalline matter subject to the whole range of p, T conditions. The only thermodynamic quantity varied so far has been the temperature, and then mostly to reduce the nettlesome effect of atomic thermal motion on the CD analysis. Pressure has never been used in CD experiments, mainly due to the difficulty in collecting diffraction data of a quality acceptable for such a study. We now propose to surmount this difficulty. Pressure alone is a thermodynamic variable of major importance in the study of magnetic systems, including molecular magnets ("pressure-tuning"), the latter a field of research in which we are particularly interested. Combining p with T will also, at last, open the field of CD to the study of polymorphs and of unquenchable phases. When investigating structure-property relationships, the existence of polymorphs often permit to trace the causes for a particular molecular packing - due to the often restricted number of variables involved. Ab-initio methods cannot easily take into account the effect of temperature, but have little difficulty in simulating the effect of pressure. Of particular interest is the fact that pressure drastically alters intermolecular (and sometimes interatomic) distances and thereby probes the interaction potential, allowing us to better parametrize it. Summed up a bit differently: Compared to temperature, pressure is a “clean” variable, with very few collateral effects!
*) Koritsanszky, T.S. and P. Coppens, Chemical Applications of X-ray Charge-Density Analysis, Chem. Rev. 101 (2001) 1583-1627