Expanding the scope of integral equation-based solvation theory

Implizite Lösungsmittelmodelle werden verwendet, um die Eigenschaften von Lösungen mit realisierbarem Rechenaufwand vorauszusagen und zu verstehen. Viele dieser impliziten Lösungsmittelmodelle ignorieren jedoch die Granularität des Lösungsmittels und scheitern an der Beschreibung wichtiger gerichteter Wechselwirkungen, wie z. B. Wasserstoffbrücken. Einen Weg zur Beschreibung verschiedenster Lösungsmitteleigenschaften, der die Merkmale der Lösungsmittelmoleküle nicht vernachlässigt, ermöglicht das Reference Interaction Site Model (RISM). Hierbei wird das Lösungsmittel über seine Paarverteilungsfunktion charakterisiert. Diese stellt ein Maß für die Nahordnung der Lösungsmittelteilchen dar und ist im Rahmen des RISM-Ansatzes verbunden mit der freien Solvatationsenergie. Eine Kombination des RISM-Modells mit quantenmechanischer Präzision für die gelöste Substanz führt zu dem so genannten Embedded Cluster Reference Interaction Site Model, EC-RISM. Diese Kombination wird durch die Anwendung eines selbstkonsistenten Ansatzes, bei dem die Wechselwirkung zwischen dem Solvens und der gelösten Substanz durch eingebettete Punktladungen beschrieben wird, erreicht. Bisher wurde die EC-RISM-Methodik nur für Studien von freien Energien und verwandten Größen von dipolaren, hauptsächlich wässrigen Lösungen, verwendet. Die Anwendbarkeit von EC-RISM auf andere Eigenschaften von Lösungen sowie für nicht dipolare Lösungsmittel wurde bisher nicht sichergestellt. Daher stehen im Folgenden zwei Aspekte im Fokus der Arbeit. Einerseits werden chemische NMR-Verschiebungen berechnet, mit denen getestet wird, ob der Einfluss des Solvens auf die Wellenfunktion des gelösten Teilchens geeignet beschrieben wird und um die Wellenfunktion selbst zu überprüfen. Andererseits werden Benzol- und Hexafluorbenzolmodelle entwickelt um zu erforschen, ob diese komplexen, aber ähnlichen Lösungsmittel mit Hilfe von Integralgleichungsmethoden wie RISM unterscheidbar sind. Anschließend werden diese Modelle verwendet, um eine chemische Reaktion zu untersuchen, bei der eine Steigerung der Stereoselektivität durch den Einsatz von Hexafluorbenzol im Vergleich mit Benzol ausgelöst wird. Die Ergebnisse der Untersuchungen zeigen, dass die Anwendung von EC-RISM die chemische Verschiebung in wässriger Lösung systematisch verbessert und somit auch eine plausible Wellenfunktion wiedergibt. Weiterhin wird bestätigt, dass sich Lösungen in Benzol und Hexafluorbenzol adäquat beschreiben lassen und sogar Stereoselektivitätsvoraussagen in Übereinstimmung mit Experimenten möglich sind. Hierbei können verschiedenste Lösungsmitteleinflüsse separiert und analysiert sowie betrachtet und diskutiert werden, wobei die Bedeutung sowohl dispersiver als auch multipolarer Wechselwirkungsanteile verdeutlicht wird.Implicit solvation models are used to predict and to understand various properties of solutions within feasible time scales. But common implicit approaches ignore the granularity of the solvent and fail to describe substantial directional interactions like hydrogen bonds. A different way to calculate various properties of solvents without that retains this features is presented in this work, the reference interaction site model (RISM). Thereby the solvent is characterized by its pair distribution function, which is a measure for the near-order of the solvent particles. Furthermore the pair distribution functions are connected within the RISM approach with the free energy of solvation. Combining the RISM approach with quantummechanical precision for the solute leads to the so-called embedded cluster reference interaction site model, EC-RISM. This is achieved by application of a self-consistent approach for both parts that are connected with a cluster of embedded point charges, which describes the solvent-solute interaction. By now the EC-RISM methodology was only applied for studies of free energies and related subjects of dipolar, mostly aqueous solutions. The applicability of EC-RISM for different properties of solutions as well as for nondipolar solvents was so far not ensured in the past. Here, two important aspects are in the focus; on the one hand, NMR chemical shifts are calculated to test whether the EC-RISM approach properly describes the solvent influence on the wave function of the solute and to test the wave function itself. On the other hand benzene and hexafluorobenzene models are developed to research if these complex and very similar solvents are distinguishable with integral equation theories like RISM. Furthermore these models are applied for the investigation of a chemical reaction that shows a stereoselectivity enhancement that is caused by the application of hexafluorobenzene compared to benzene. These investigations show that EC-RISM systematically improves the chemical shifts in aqueous solutions and therefore displays the adequacy of the corresponding wave function. Additionally it is confirmed that benzene and hexafluorobenzene are properly described and that even the stereoselectivity of the previously mentioned chemical reaction is correctly predicted. Thereby the different solvent influences are separated, analyzed, considered and discussed, whereas the relevance of dispersive and multipolar parts is elucidated

Solvation Effects on Chemical Shifts by Embedded Cluster Integral Equation Theory

The accurate computational prediction of nuclear magnetic resonance (NMR) parameters like chemical shifts represents a challenge if the species studied is immersed in strongly polarizing environments such as water. Common approaches to treating a solvent in the form of, e.g., the polarizable continuum model (PCM) ignore strong directional interactions such as H-bonds to the solvent which can have substantial impact on magnetic shieldings. We here present a computational methodology that accounts for atomic-level solvent effects on NMR parameters by extending the embedded cluster reference interaction site model (EC-RISM) integral equation theory to the prediction of chemical shifts of <i>N</i>-methylacetamide (NMA) in aqueous solution. We examine the influence of various so-called closure approximations of the underlying three-dimensional RISM theory as well as the impact of basis set size and different treatment of electrostatic solute–solvent interactions. We find considerable and systematic improvement over reference PCM and gas phase calculations. A smaller basis set in combination with a simple point charge model already yields good performance which can be further improved by employing exact electrostatic quantum-mechanical solute–solvent interaction energies. A larger basis set benefits more significantly from exact over point charge electrostatics, which can be related to differences of the solvent’s charge distribution

Structure and thermodynamics of nondipolar molecular liquids and solutions from integral equation theory

<p>Solvent-induced solute polarisation of nondipolar solvents originates mainly from specific directional interactions and higher electrostatic multipole moments. Popular continuum solvation models such as the polarisable continuum models ignore such interactions and, therefore, cannot adequately model solvation effects on electronic structure in these environments. Important examples of nondipolar solvents that are indistinguishable by continuum methods are benzene and hexafluorobenzene. Both substances have very similar macroscopic properties, while solutes dissolved in either benzene or hexafluorobenzene behave differently due to their inverted electrostatic quadrupole moments and slightly different size. As a first step towards a proper and computationally feasible description of nondipolar molecular solvents, we present here integral equation theory results based on various forms of the reference interaction site model coupled to quantum-chemical calculations for benzene and hexafluorobenzene solutions of small molecules. We analyse solvation structures, also in comparison with molecular dynamics simulations, and show that predictions of transfer Gibbs energies, which define partition constants, benefit substantially from considering the exact, wave function-derived electrostatic field distribution beyond a simple point charge solute model in comparison with experimental data. Moreover, by constructing artificial uncharged and charge-inverted toy models of the solvents, it is possible to dissect the relative importance of dispersion and quadrupolar electrostatic effects on the partitioning equilibria. Such insight can help to design specifically optimised solvents to control solubility and selectivity for a wide range of applications.</p> <p></p

Design principles for high–pressure force fields: Aqueous TMAO solutions from ambient to kilobar pressures

Accurate force fields are one of the major pillars on which successful molecular dynamics simulations of complex biomolecular processes rest. They have been optimized for ambient conditions, whereas high-pressure simulations become increasingly important in pressure perturbation studies, using pressure as an independent thermodynamic variable. Here, we explore the design of non-polarizable force fields tailored to work well in the realm of kilobar pressures - while avoiding complete reparameterization. Our key is to first compute the pressure-induced electronic and structural response of a solute by combining an integral equation approach to include pressure effects on solvent structure with a quantum-chemical treatment of the solute within the embedded cluster reference interaction site model (EC-RISM) framework. Next, the solute's response to compression is taken into account by introducing pressure-dependence into selected parameters of a well-established force field. In our proof-of-principle study, the full machinery is applied to N,N,N-trimethylamine-N-oxide (TMAO) in water being a potent osmolyte that counteracts pressure denaturation. EC-RISM theory is shown to describe well the charge redistribution upon compression of TMAO(aq) to 10 kbar, which is then embodied in force field molecular dynamics by pressure-dependent partial charges. The performance of the high pressure force field is assessed by comparing to experimental and ab initio molecular dynamics data. Beyond its broad usefulness for designing non-polarizable force fields for extreme thermodynamic conditions, a good description of the pressure-response of solutions is highly recommended when constructing and validating polarizable force fields. (C) 2016 AIP Publishing LLC

The Chemical Shift Baseline for High-Pressure NMR Spectra of Proteins

High-pressure (HP) NMR spectroscopy is an important method for detecting rare functional states of proteins by analyzing the pressure response of chemical shifts. However, for the analysis of the shifts it is mandatory to understand the origin of the observed pressure dependence. Here we present experimental HP NMR data on the N-15-enriched peptide bond model, N-methylacetamide (NMA), in water, combined with quantum-chemical computations of the magnetic parameters using a pressure-sensitive solvation model. Theoretical analysis of NMA and the experimentally used internal reference standard 4,4-dimethyl-4-silapentane-1-sulfonic (DSS) reveal that a substantial part of observed shifts can be attributed to purely solvent-induced electronic polarization of the backbone. DSS is only marginally responsive to pressure changes and is therefore a reliable sensor for variations in the local magnetic field caused by pressure-induced changes of the magnetic susceptibility of the solvent

Time-sequential working wavelength-selective filter for flat autostereoscopic displays

A time-sequential working, spatially-multiplexed autostereoscopic 3D display design consisting of a fast switchable RGB-color filter array and a fast color display is presented. The newly-introduced 3D display design is usable as a multi-user display, as well as a single-user system. The wavelength-selective filter barrier emits the light from a larger aperture than common autostereoscopic barrier displays with similar barrier pitch and ascent. Measurements on a demonstrator with commercial display components, simulations and computational evaluations have been carried out to describe the proposed wavelength-selective display design in static states and to show the weak spots of display filters in commercial displays. An optical modelling of wavelength-selective barriers has been used for instance to calculate the light ray distribution properties of that arrangement. In the time-sequential implementation, it is important to avoid that quick eye or eyelid movement leads to visible color artifacts. Therefore, color filter cells, switching faster than conventional LC display cells, must distribute directed light from different primaries at the same time, to create a 3D presentation. For that, electric tunable liquid crystal Fabry–Pérot color filters are presented. They switch on-off the colors red, green and blue in the millisecond regime. Their active areas consist of a sub-micrometer-thick nematic layer sandwiched between dielectric mirrors and indium tin oxide (ITO)-electrodes. These cells shall switch narrowband light of red, green or blue. A barrier filter array for a high resolution, glasses-free 3D display has to be equipped with several thousand switchable filter elements having different color apertures