Experimental Charge Densities from Multipole Modeling: Moving into the Twenty-First Century

In this chapter, we intend to take a closer look at the capabilities and trends in experimental charge density studies based on single crystal X-ray diffraction, now that we are well and truly into the twenty-first century. Following a long and relatively dormant period since the fundamental discoveries of X-rays and their usage as probes for atomic arrangements in crystals (what we now call crystallography) about a century ago, great technological inventions on both the X-ray source and detector sides started to influence this scientific discipline in the 1980s. Ever since, scientists in this field have been required to constantly adapt to a long range of new developments. The increasing possibilities of the equipment enabled entirely new ways of preparing experiments and analyzing data. However, as skepticism is a prominent feature of most scientists’ DNA, the community at large has not embraced these new possibilities without hesitation, not least since commercial vendors appeared actively promoting their own products. Nevertheless, the last few decades have witnessed relentless progress in all hardware aspects of crystallography, and we are now at a fascinating point in time where all elements of diffraction are at an incredibly high level, enabling us to reach data quality of exceptional accuracy in very short time. In this chapter, we will therefore outline how we got here and where we may go from here. As a particular case, we explain in detail how the use of experimental charge density studies has had important impact in the field of single-molecule magnets – these fascinating materials being merely an example of the unlimited possibilities for coupling advanced structural studies with physical properties.