Onwards to transition-metal complexes
Anisotropic thermal motion in transition-metal carbonyls from experiment and ab initio theory
V. L. Deringer, A. Wang, J. George, R. Dronskowski, U. Englert, Dalton Trans., 2016, 45, 13680.
In organometallic compounds, light atoms typically lie next to very heavy atoms. Routine X-ray diffraction measurements face problems in providing reliable ADPs for these lighter atoms (as their scattering strength is proportional to the number of electrons). In such cases, predicting ADPs from first-principles could be a meaningful complement to X-ray diffraction. As an initial test, we calculated the ADPs of three representative transition-metal carbonyls:
Calculated ADPs for 1 and 2 agree very well with experiment. In 3, the calculation underestimates the ADPs for the Os atoms: whether calculation or experiment is to ├ó┬?┬?blame├ó┬?┬Ł is yet unclear. Nonetheless, the ligand ADPs in 3 are faithfully recovered by theory: after all, it is these light ligand atoms for which experiments usually face most difficulties!
Experimental versus computed main-axis components of ADPs (see Theory section) in transition-metal carbonyl compounds; reprinted from Dalton Trans., 2016, 45, 13680.
A proof-of-concept for theory-aided structure refinement
Neutron powder diffraction and theory-aided structure refinement of rubidium and cesium ureate
K. B. Sterri, V. L. Deringer, A. Houben, P. Jacobs, C. M. N. Kumar, R. Dronskowski,
Z. Naturforsch. B, 2016, 71, 431.
Neutron diffraction is usually seen as the experimental method of choice for finding hydrogen atoms and their thermal ellipsoids. Unfortunately, there are limits to neutron experiments as well: for example, if the material under study does not form large single crystals but calls for powder diffraction instead.
Probably every molecular crystallographer (or theorist, for that matter) has studied urea at some point in their life. Our experimental colleagues have gone a step further and synthesised the first free alkali ureates, that is, compounds of de-protonated urea ( Z. anorg. allg. Chem., 2014, 640, 846-850, Z. Naturforsch. B, 2016, 71, 431). In fact, substituted ureates are well-known in chemistry, but the free anion requires specialised syntheses such as using liquid ammonia as a solvent. And even then, the compounds are so delicate that they refuse to form large enough single crystals (a fairly common problem, unfortunately).
In this work, we tried a different route: starting from an initial structure model, we calculated ADPs which then served as starting values for the refinement of the powder neutron data. Ultimately, this led to the first neutron-quality structural study of this peculiar class of materials!
last modified: 2016-09-26