Quantum Chemical Microsolvation by Automated Water Placement

We developed a quantitative approach to quantum chemical microsolvation. Key in our methodology is the automatic placement of individual solvent molecules based on the free energy solvation thermodynamics derived from molecular dynamics (MD) simulations and grid inhomogeneous solvation theory (GIST). This protocol enabled us to rigorously define the number, position, and orientation of individual solvent molecules and to determine their interaction with the solute based on physical quantities. The generated solute–solvent clusters served as an input for subsequent quantum chemical investigations. We showcased the applicability, scope, and limitations of this computational approach for a number of small molecules, including urea, 2-aminobenzothiazole, (+)-syn-benzotriborneol, benzoic acid, and helicene. Our results show excellent agreement with the available ab initio molecular dynamics data and experimental results.

Co-Solvent Exfoliation of Hexagonal Boron Nitride: Effect of Raw Bulk Boron Nitride Size and Co-Solvent Composition

Exfoliation of two-dimensional boron nitride nanosheets (BNNSs) from parent bulk material has been receiving intensive attention because of its fascinating physical properties. Liquid exfoliation is a simple, scalable approach to produce single-layer or few-layer BNNS. In this paper, water/propanol co-solvent exfoliation of bulk boron nitride under the assistance of sonication was investigated in detail. Special attention was paid on the effect of raw bulk boron nitride size and co-solvent composition. The results show that sonication of small-size hexagonal boron nitride tends to generate large nanosheets, due to a predominant solvent wedge effect. In addition, it is found that the composition of water/propanol co-solvent has an important effect on exfoliation efficiency. Interestingly, although two isomers of 1-propanol (NPA) and 2-propanol (IPA) have the same molecular weight and similar surface tension, their aqueous solutions allow the formation of boron nitride nanosheets dispersion with markedly different concentrations. It is proposed that due to their spatial configuration difference, NPA with its longer molecular chain and fewer hydrophobic methyl group tends to form dynamic water-NPA clusters with larger size than water-IPA clusters. The hydrodynamic radius of the co-solvent “clusters” was calculated to be 0.72 nm for water/NPA system and 0.44 nm for water/IPA system at their maximum, respectively. Their size changes, represented by two curves, indicate a strong “cluster size” effect on exfoliation efficiency. Our work provides an insight into co-solvent exfoliation of hexagonal boron nitride and emphasizes the importance of co-solvent cluster size in exfoliation efficiency.

Proton dynamics in molecular solvent clusters as an indicator for hydrogen bond network strength in confined geometries

The size-dependence of proton-dynamics occurring upon core-ionisation is linked to a weakening of the hydrogen-bond network in molecular clusters.

Harmonic light scattering study reveals structured clusters upon the supramolecular aggregation of regioregular poly(3-alkylthiophene)

Solubilized poly(3-alkylthiophene)s are known to self-assemble into well-ordered supramolecular aggregates upon lowering the solvent quality. This supramolecular organization largely determines the optical and electronic properties of these polymers. However, despite numerous studies the exact mechanism and kinetics of the aggregation process and the role of external stimuli are still poorly understood. Classical characterization techniques such as electronic spectroscopy, dynamic light scattering, and diffraction-based techniques have not been able to provide a full understanding. Here we use second-harmonic scattering (SHS) and third-harmonic scattering (THS) techniques to investigate this supramolecular aggregation mechanism. Our results indicate that the actual supramolecular aggregation is preceded by the formation of structured polymer-solvent clusters consistent with a nonclassical crystallization pathway.

Solute-solvent electronic interaction is responsible for initial charge separation in ruthenium complexes [Ru(bpy)3]2+ and [Ru(phen)3]2+

Origin of the initial charge separation in optically-excited Ruthenium(II) tris(bidentate) complexes of intrinsic D3 symmetry has remained a disputed issue for decades. Here we measure the femtosecond two-photon absorption (2PA) cross section spectra of [Ru(2,2′-bipyridine)3]2 and [Ru(1,10-phenanthroline)3]2 in a series of solvents with varying polarity and show that for vertical transitions to the lower-energy 1MLCT excited state, the permanent electric dipole moment change is nearly solvent-independent, Δμ = 5.1–6.3 D and 5.3–5.9 D, respectively. Comparison of experimental results with quantum-chemical calculations of complexes in the gas phase, in a polarizable dielectric continuum and in solute-solvent clusters containing up to 18 explicit solvent molecules indicate that the non-vanishing permanent dipole moment change in the nominally double-degenerate E-symmetry state is caused by the solute-solvent interaction twisting the two constituent dipoles out of their original opposite orientation, with average angles matching the experimental two-photon polarization ratio.

Effects of Solvent-Salt Charge-Transfer Complexes on Oxidative Stability of Li-Ion Battery Electrolytes

Electrochemical stability windows of electrolytes largely determine the limitations of operating regimes and energy density of Li-ion batteries but the controlling degradation mechanisms are difficult to characterize and remain poorly understood. We investigate the oxidative decomposition mechanisms governing high voltage stability of multi-component organic electrolytes using computational techniques of quantum chemistry. The intrinsic oxidation potential is modeled using vertical ionization potentials (IP) of ensembles of anion-solvent clusters generated using molecular dynamics. In some cases, the IP of the solvent-anion complex is significantly lower than that of each individual component. This effect is found to originate from the oxidation-driven charge transfer complex formation between the anion and the solvent. We propose a simple model to quantitatively understand this phenomenon and validate it for 16 combinations of common anions (4,5-dicyano-2-(trifluoromethyl)imidazolium, bis-(trifluoromethane solfonimmide), tetrafluroborate, hexafluorophosphate) and solvents (dimethyl sulfoxide, dimethoxyethane, propylene carbonate, acetonitrile). This new understanding of the microscopic details of oxidation allows us to interpret trends in published experimental and computational results and to formulate design rules for rapidly assessing stability of electrolyte compositions.

Effects of Solvent-Salt Charge-Transfer Complexes on Oxidative Stability of Li-Ion Battery Electrolytes

Electrochemical stability windows of electrolytes largely determine the limitations of operating regimes and energy density of Li-ion batteries but the controlling degradation mechanisms are difficult to characterize and remain poorly understood. We investigate the oxidative decomposition mechanisms governing high voltage stability of multi-component organic electrolytes using computational techniques of quantum chemistry. The intrinsic oxidation potential is modeled using vertical ionization potentials (IP) of ensembles of anion-solvent clusters generated using molecular dynamics. In some cases, the IP of the solvent-anion complex is significantly lower than that of each individual component. This effect is found to originate from the oxidation-driven charge transfer complex formation between the anion and the solvent. We propose a simple model to quantitatively understand this phenomenon and validate it for 16 combinations of common anions (4,5-dicyano-2-(trifluoromethyl)imidazolium, bis-(trifluoromethane solfonimmide), tetrafluroborate, hexafluorophosphate) and solvents (dimethyl sulfoxide, dimethoxyethane, propylene carbonate, acetonitrile). This new understanding of the microscopic details of oxidation allows us to interpret trends in published experimental and computational results and to formulate design rules for rapidly assessing stability of electrolyte compositions.