On the role of neutrons in quantum crystallography, and the role of quantum crystallography in chemical crystallography

Alison Edwards

Australian Centre for Neutron Scattering, ANSTO (Australian Centre for Nuclear Science and Technology)

e-mail: alison.ewards@ansto.au | alisonedwar@gmail.com

From the beginning of the User program (2009) on the KOALA single-crystal neutron diffractometer at ANSTO, we have had proposals seeking high quality neutron diffraction studies to validate or bench-mark results from X-ray quantum-crystallography studies. The crystals provided from this User cohort are typically of exceptional quality although the usual challenges regarding size can impede progress. There has also been a significant number of experiments undertaken seeking to explore the questions regarding whether a material is a salt or a co-crystal - making use of residual electron densities in X-ray studies to provide the initial choices of compounds alongside considerations such as pKa. Location of H atom nuclei whatever the formal oxidation state H+, H or H- is a typical application of KOALA and neutron single-crystal diffraction in general. It is important to note that it is not only the high resolution studies which provide significant insights into the chemistry under examination. Even relatively low resolution data can clearly differentiate the salt vs co-crystal question and at times results have not been “as expected”. At its most extreme, we have published our assessment of which of a group of calculated structures of a molecule (for which detailed spectroscopic data inform the calculations) fit the extremely limited single-crystal neutron diffraction data obtained on an Al6H8 cluster compound. At this extreme, the various structures calculated lay in energetically similar states, but one alone was found to fit the available data significantly better than the others. An important question in quantum crystallography is how to explore alternative possibilities - especially where no electrons are associated with an atom of the structure!

Beyond the rather obvious but nonetheless subtle questions regarding H atom locations, neutron diffraction gives a particular opportunity where isoelectronic situations exist - the differences in scattering power of many near neighbour elements can provide definitive identification - in cases where the same number of electrons are disordered over a number of sites - for example is it 4 copper atoms or 8 sulfur atoms distributed over the 12 sites of a cuboctahedron. In this instance, neutron diffraction was applied to attempt to determine hydride atom locations when we discovered the chemical formula was incorrect!

Quantum crystallography offers extremely powerful tools which if used indiscriminately have the potential to increase the noise in the chemical crystallography and chemistry literature. We must guard against fitting “interesting” models to data where data are not available for review. To this end I recommend that all images collected be deposited our community needs to be vigilant as our methods become more widely used.