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Enforced Proximity Interactions as Vehicle to Drive Quantum Crystallography

Applicant Professor Dr. Jens Beckmann, since 8/2019
Subject Area Inorganic Molecular Chemistry - Synthesis and Characterisation
Analytical Chemistry
Term from 2017 to 2021
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 383534171
 
Final Report Year 2024

Final Report Abstract

Single-crystal X-ray crystallography is arguably the most powerful technique for the structural characterization of small and large molecules. The impact of this technique on the developments of different branches of science including chemistry, physics, biology, medicine and material science can not be overstated. The structural characterization of proteins, DNA, vitamins, graphene, fullerene (just to name a few examples) are landmark achievements in science. To date, about 30 Nobel Prizes have been awarded that are closely associated with single-crystal X-ray crystallography. Unfortunately, crystallographic methods and software development did not develop at the same pace as the hardware development. The vast majority of structure refinements are still based on crucial simplifications introduced in the early days of crystallography when computational resources were limited. The Independent Atom Model (IAM) assumes atoms to be separate, non-interacting entities with spherical electron densities. This assumption neglects the fundamental basis of chemistry that interactions between electrons of different atoms are responsible for chemical bonds. The error introduced by this assumption is most pronounced for the light elements possessing only few or even no core electrons as in case of hydrogen. The field of quantum crystallography couples quantum chemical calculations with modern crystallographic methods to overcome the flaws of the IAM. The non-spherical structure refinement method called Hirshfeld Atom Refinement (HAR) utilizes calculated wavefunctions, which are the dissected into atomic electron density functions (e.g. Hirshfeld atoms) using Hirshfeld’s stockholder partitioning scheme. A Fourier-transformation of the electron density is carried out during the crystallographic refinement to produce atomic from factors until convergence is reached. To reduce the computational cost of this method, a database approach using extremely localized molecular orbitals (ELMOS) are used in combination with HAR. The validation of both HAR and HAR-ELMO were undertaken using proximity enforcing ligands (PELs) that create unusual bond situations between substituents in the peri-positions of acenaphthyl scaffolds. These unusual bond situations involves attractive and repulsive interactions between lone pairs, hydrogen/oxygen atoms and/or hydroxyl groups of main group elements, e.g. Si, P, S and late transition metal ions, e.g. Ni, Pd, Cu, Ag, Au. For the interpretation of these bond situations, complementary bond analyses were carried out.

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