Bonding in Polyiodides under High Pressure

Ernst, Michelle (1 June 2018). Bonding in Polyiodides under High Pressure (In Press). In: Quantum Crystallography School in Erice. Erice, Sizilien. 1-10. Juni.

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We are studying the bond formation in polyiodides induced by applying pressure on a crystal. Besides the analysis of the structure obtained from X-ray diffraction measurements we use various theoretical tools such as bond decomposition analyses and X-ray constrained wavefunctions (XCW ). While traditional methods for the refinement of X-ray diffraction data, like for example the multipolar refinement, provide only information about the diagonal elements of the one-electron reduced density matrix (the electron density), the XCW method enables calculation of the density matrices as well, although the
physical meaning is not fully understood. XCWs are obtained by calculating a wavefunction which simultaneously minimizes the energy (obtained as expectation value of the Hamilton operator) and an agreement statistics obtained by comparing the calculated structure factors to the measured structure factors.
Polyiodides form a very versatile class of compounds. I- and I2 are the building blocks which allow the formation of polyiodide chains with all kinds of different motives (branched and unbranched) and various lengths. The simplest combination gives rise to I3- which can be symmetric (two equal bonds) or asymmetric. We are for the first time studying the structure of polyiodides under high pressure, which allows the observation of structural changes as well as the process of bond formation within the very same crystals. Our model system is a tetraethylammonium-heptaiodide crystal where three units (I2, I3- and I2) can be distinguished that form a zigzag pattern.
X-ray diffraction measurements were done at different pressure points up to almost 12 GPa. At ambient pressure the distance of the two I2 units to the I3- unit is equivalent (and restricted by symmetry), but around 6 GPa the crystal undergoes a phase transition and one I2 is approaching I3- more than the other. This results in the formation of a pentaiodide unit (I5-) plus a separated I2, which in turn become a heptaiodide (I7-) upon further compression. However, there are no unique criteria for what is to be considered a bond. We used the energy decomposition analysis (EDA ) and the interacting quantum atom (IQA ) approach together with other criteria such as the delocalization index, the Laplacian of the electron density, or the energy density to clarify the bonding situation. Additionally, orbitals obtained from XCW calculations were analysed in order to better understand the interactions that lead to the bond formation.
In future we plan to use X-ray constrained ELMOs to localize the orbitals on the interacting fragments.

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