Tamara Husch

Alternative Single-Solvent Electrolytes Based on Cyanoesters for Safer Lithium-Ion Batteries.

To identify alternative single-solvent-based electrolytes for application in lithium-ion batteries (LIBs), adequate computational methods were applied to screen specified physicochemical and electrochemical properties of new cyanoester-based compounds. Out of 2747 possible target compounds, two promising candidates and two structurally equivalent components were chosen. A constructive selection process including evaluation of basic physicochemical properties as well assessing the compatibility towards graphitic anodes was initiated to identify the most promising candidates. With addition of a film-forming additive in a low concentration, the most promising candidate showed an adequate long-term cycling stability with LiNi1/3 Mn1/3 Co1/3 O2 [NMC(111)] in a full-cell setup using graphite as anode material. The main advantages of the new electrolyte formulation are related to its good thermal behavior, especially with regard to safety in combination with satisfying electrochemical performance.

Calculation of Ligand Dissociation Energies in Large Transition-Metal Complexes

The accurate calculation of ligand dissociation (or equivalently, ligand binding) energies is crucial for computational coordination chemistry. Despite its importance, obtaining accurate ab initio reference data is difficult, and density-functional methods of uncertain reliability are chosen for feasibility reasons. Here, we consider advanced coupled-cluster and multiconfigurational approaches to reinvestigate our WCCR10 set of 10 gas-phase ligand dissociation energies [ J. Chem. Theory Comput. 2014, 10, 3092]. We assess the potential multiconfigurational character of all molecules involved in these reactions with a multireference diagnostic [ Mol. Phys. 2017, 115, 2110] in order to determine where single-reference coupled-cluster approaches can be applied. For some reactions of the WCCR10 set, large deviations of density-functional results including semiclassical dispersion corrections from experimental reference data had been observed. This puzzling observation deserves special attention here, and we tackle the issue (i) by comparing to ab initio data that comprise dispersion effects on a rigorous first-principles footing and (ii) by a comparison of density-functional approaches that model dispersion interactions in various ways. For two reactions, species exhibiting nonnegligible static electron correlation were identified. These two reactions represent hard problems for electronic structure methods and also for multireference perturbation theories. However, most of the ligand dissociation reactions in WCCR10 do not exhibit static electron correlation effects, and hence, we may choose standard single-reference coupled-cluster approaches to compare with density-functional methods. For WCCR10, the Minnesota M06-L functional yielded the smallest mean absolute deviation of 13.2 kJ mol-1 out of all density functionals considered (PBE, BP86, BLYP, TPSS, M06-L, PBE0, B3LYP, TPSSh, and M06-2X) without additional dispersion corrections in comparison to the coupled-cluster results, and the PBE0-D3 functional produced the overall smallest mean absolute deviation of 4.3 kJ mol-1. The agreement of density-functional results with coupled-cluster data increases significantly upon inclusion of any type of dispersion correction. It is important to emphasize that different density-functional schemes available for this purpose perform equally well. The coupled-cluster dissociation energies, however, deviate from experimental results on average by 30.3 kJ mol-1. Possible reasons for these deviations are discussed.

Dispersion and Halogen-Bonding Interactions: Binding of the Axial Conformers of Monohalo- and (±)-trans-1,2-Dihalocyclohexanes in Enantiopure Alleno-Acetylenic Cages

Enantiopure alleno-acetylenic cage (AAC) receptors with a resorcin[4]arene scaffold, from which four homochiral alleno-acetylenes converge to shape a cavity closed by a four-fold OH-hydrogen-bonding array, form a highly ordered porous network in the solid state. They enable the complexation and co-crystallization of otherwise non-crystalline small molecules. This paper analyzes the axial conformers of monohalo- and (±)-trans-1,2-dihalocyclohexanes, bound in the interior cavity of the AACs, on the atomic level in the solid state and in solution, accompanied by accurate calculations. The dihedral angles ϑa,a (X-C(1)-C(2)-X/H) of the axial/diaxial conformers deviate substantially from 180°, down to 144°, accompanied by strong flattening of the ring dihedral angles. Structure optimization of the isolated guest molecules demonstrates that the non-covalent interactions with the host hardly affect the dihedral angles, validating that the host is an ideal means to study the elusive axial/diaxial conformers. X-ray co-crystal structures of AACs further allowed for a detailed investigation, both experimentally and theoretically, on the interplay between space occupancy, guest conformation, and chiral recognition based purely on dispersion forces and weak C-X···π (X = Cl, Br, I) and C-X···||| (acetylene) contacts (X = Cl, Br). The theoretical analysis of the non-covalent interactions between host and guest confirmed the high shape complementarity with fully enveloping dispersive interactions between the binding partners, rationalizing the high degree of enantioselectivity in the previously communicated complexation of (±)-trans-1,2-dimethylcyclohexane. This study also showed that (±)-trans-1,2-dihalocyclohexanes (X = Cl, Br) engage in significant halogen bonding (XB) interactions C-X···||| with the hosts. Slow host-guest exchange on the NMR time scale enabled the characterization of the encapsulated guests in solution, demonstrating that the complexes have identical geometries to those seen in the solid state, with the guests bound in axial/diaxial conformations.

Comprehensive Analysis of the Neglect of Diatomic Differential Overlap Approximation

Many modern semiempirical molecular orbital models are built on the neglect of diatomic differential overlap (NDDO) approximation. An in-depth understanding of this approximation is therefore indispensable to rationalize the success of these semiempirical molecular orbital models and to develop further improvements on them. The NDDO approximation provides a recipe to approximate electron-electron repulsion integrals (ERIs) in a symmetrically orthogonalized basis based on a far smaller number of ERIs in a locally orthogonalized basis. We first analyze the NDDO approximation by comparing ERIs in both bases for a selection of molecules and for a selection of basis sets. We find that the errors in Hartree-Fock and second-order Møller-Plesset perturbation theory energies grow roughly linearly with the number of basis functions. We then examine different approaches to correct for the errors caused by the NDDO approximation and propose a strategy to directly correct for them in the two-electron matrices that enter the Fock operator.

Semiempirical molecular orbital models based on the neglect of diatomic differential overlap approximation

Semiempirical molecular orbital (SEMO) models based on the neglect of diatomic differential overlap (NDDO) approximation efficiently solve the self-consistent field equations by rather drastic approximations. The computational efficiency comes at the cost of an error in the electron-electron repulsion integrals. The error may be compensated by the introduction of parametric expressions to evaluate the electron-electron repulsion integrals, the one-electron integrals, and the core-core repulsion. We review the resulting formalisms of popular NDDO-SEMO models (such as the MNDO(/d), AM1, PMx, and OMx models) in a concise and self-contained manner. We discuss the approaches to implicitly and explicitly describe electron correlation effects within NDDO-SEMO models and we dissect strengths and weaknesses of the different approaches in a detailed analysis. For this purpose, we consider the results of recent benchmark studies. Furthermore, we apply bootstrapping to perform a sensitivity analysis for a selection of parameters in the MNDO model. We also identify systematic limitations of NDDO-SEMO models by drawing on an analogy to Kohn--Sham density functional theory.