Grand challenges in quantum‐classical modeling of molecule–surface interactions

A detailed understanding of the adsorption of small molecules or macromolecules to a materials surface is of importance, for example, in the context of material and biomaterial research. Classical atomistic simulations in principle provide microscopic insight in the complex entropic and enthalpic interplay at the interface. However, an application of classical atomistic simulation techniques to such interface systems is a nontrivial problem, mostly because commonly used force fields cannot be straightforwardly applied, as they are usually developed to reproduce bulk properties of either solids or liquids but not the interfacial region between two phases. Therefore, a dual‐scale modeling approach has often been the method of choice in the past, in which the classical force field is parameterized such that quantum chemical information on near‐surface conformations and adsorption energies is reproduced by the classical force field. We will discuss in this review the current state‐of‐the‐art of quantum‐classical modeling of molecule–surface interactions and outline the major challenges in this field. In this context, we will, among other things, lay emphasis on discussing ways to obtain representable force fields and propose systematic and system‐independent strategies to optimize the quantum‐classical fitting procedure. © 2013 Wiley Periodicals, Inc.     

Second-order superintegrable quantum systems

A classical (or quantum) superintegrable system on an n-dimensional Riemannian manifold is an integrable Hamiltonian system with potential that admits 2n ? 1 functionally independent constants of the motion that are polynomial in the momenta, the maximum number possible. If these constants of the motion are all quadratic, then the system is second-order superintegrable, the most tractable case and the one we study here. Such systems have remarkable properties: multi-integrability and separability, a quadratic algebra of symmetries whose representation theory yields spectral information about the Schrödinger operator, and deep connections with expansion formulas relating classes of special functions. For n = 2 and for conformally flat spaces when n = 3, we have worked out the structure of the classical systems and shown that the quadratic algebra always closes at order 6. Here, we describe the quantum analogs of these results. We show that, for nondegenerate potentials, each classical system has a unique quantum extension.     

Tris(pentafluoroethyl)silanol Derivatives and the Lewis Amphoteric Tris(pentafluoroethyl)silanolate Anion, [Si(C2F5)3O]−

While alkyl-substituted siloxanes are widely known, virtually nothing is known about perfluoroalkyl siloxanes and their congener species, the silanols and silanolates. We recently reported on the tris(pentafluoroethyl)silanide ion, [Si(C₂F₅)₃]⁻, which features Lewis amphoteric character deriving from the pentafluoroethyl substituents and their strong electron-withdrawing properties. Transferring this knowledge, we investigated the Lewis amphoteric behavior of the tris(pentafluoroethyl)silanolate, [Si(C₂F₅)₃O]⁻. In order to examine such Lewis amphoteric behavior, we first developed a strategy for the synthesis of the corresponding silanol Si(C₂F₅)₃OH, which readily condenses at room temperature to the hexakis(pentafluoroethyl)disiloxane, (C₂F₅)₃SiOSi(C₂F₅)₃. Deprotonation of Si(C₂F₅)₃OH employing a sterically demanding phosphazene base allows the characterization of the first example of a dimeric triorganosilanolate: the dianionic hexakis(pentafluoroethyl)disilanolate, [{Si(C₂F₅)₃O}₂]²⁻, implies Lewis amphoteric character of the monomeric [Si(C₂F₅)₃O]⁻ anion.     

SLIM: A Short‐Linked,Highly Redox‐Stable Trityl Label for High‐Sensitivity In‐Cell EPR Distance Measurements

The understanding of biomolecular function is coupled to knowledge about the structure and dynamics of these biomolecules, preferably acquired under native conditions. In this regard, pulsed dipolar EPR spectroscopy (PDS) in conjunction with site‐directed spin labeling (SDSL) is an important method in the toolbox of biophysical chemistry. However, the currently available spin labels have diverse deficiencies for in‐cell applications, for example, low radical stability or long bioconjugation linkers. In this work, a synthesis strategy is introduced for the derivatization of trityl radicals with a maleimide‐functionalized methylene group. The resulting trityl spin label, called SLIM, yields narrow distance distributions, enables highly sensitive distance measurements down to concentrations of 90 nm , and shows high stability against reduction. Using this label, the guanine‐nucleotide dissociation inhibitor (GDI) domain of Yersinia outer protein O (YopO) is shown to change its conformation within eukaryotic cells.     

Assessing internal structure of polymer assemblies from 2D to 3D CryoTEM: Bicontinuous micelles

The self-assembly behaviour of block copolymers in solution has been of significant interest over the past two decades for a number of applications — for example, as delivery vectors and micro-reactors. More recently, attention has turned to the formation of aggregates with complex internal structure, such as multi-compartment micelles and the so-called “Janus” particles (biphasic aggregates) for their promising application as vectors for the simultaneous inclusion of chemically-different encapsulates and their possible catalytic and templating properties. The challenge has been to observe these complex aggregates in such a way as to be able to characterise their internal morphology whilst preserving their intricate structure. To this end, cryogenic transmission electron microscopy (cryoTEM) has become a powerful and indeed a necessary tool for the elucidation and observation of self-assembled polymer systems. Through its use, a new class of complex micelles has been discovered: amphiphilic block copolymer nanospheres with internal bicontinuous structure. These structures have been observed from a variety of block copolymer amphiphiles, although rarely. Intriguingly, there is no seemingly obvious unifying blueprint for their formation. This review will present the importance of cryoTEM in the elucidation and characterisation of internally-structured polymeric aggregates in recent years and highlight its significance in the definition of bicontinuous dispersions.     

Computing Complex Airy Functions by Numerical Quadrature

Integral representations are considered of solutions of the Airy differential equation w –zw=0 for computing Airy functions for complex values of z. In a first method contour integral representations of the Airy functions are written as non-oscillating integrals for obtaining stable representations, which are evaluated by the trapezoidal rule. In a second method an integral representation is evaluated by using generalized Gauss–Laguerre quadrature; this approach provides a fast method for computing Airy functions to a predetermined accuracy. Comparisons are made with well-known algorithms of Amos, designed for computing Bessel functions of complex argument. Several discrepancies with Amos' code are detected, and it is pointed out for which regions of the complex plane Amos' code is less accurate than the quadrature algorithms. Hints are given in order to build reliable software for complex Airy functions.     

Evolution of ultracold neutral plasmas

We present the first large-scale simulations of an ultracold neutral plasma, produced by photoionization of laser-cooled xenon atoms, from creation to initial expansion, using classical molecular-dynamics methods with open boundary conditions. We reproduce many of the experimental findings such as the trapping efficiency of electrons with increased ion number, a minimum electron temperature achieved on approach to the photoionization threshold, and recombination into Rydberg states of an anomalously low principal quantum number. In addition, many of these effects establish themselves very early in the plasma evolution ( similar ns) before the present experimental observations begin.     

Improved contrast to tissue ratio at higher harmonics

The challenge in ultrasound contrast imaging is a better discrimination between the perfused tissue and the contrast bubbles, which is usually expressed by contrast to tissue ratio (CTR). Imaging based on the second harmonic frequency showed a higher CTR than imaging at the fundamental frequency. However, because of nonlinear propagation of ultrasound waves, harmonic frequencies are generated. These harmonic frequencies will be linearly reflected by the tissue and therefore limit the CTR at the second harmonic frequency. In order to reduce the scattering of tissue at harmonic frequencies and by that increase the CTR, nonlinear distortion has to be reduced. We demonstrate in this study that the CTR increases with the harmonic number. The increase is substantial when transmitting at lower frequencies. To take advantage of the higher harmonics (third, fourth, fifth and the ultraharmonics and termed here super harmonics), we have developed a new phased array transducer with a wide frequency band. In-vitro measurements using the new probe show an increase of 40 dB of the CTR for super harmonic components over the conventional second harmonic system. The increase in CTR is in agreement with the calculations using existing models for the response of encapsulated bubbles and known theory of nonlinear propagation.     

Potentialtheoretische Randwertprobleme bei Tensorfeldern beliebiger Dimension und beliebigen Ranges

Ohne ZusammenfassungHerrn Prof. Dr. Ludwig Biermann zum 65. Geburtstag gewidmet Vorgelegt von H. Görtler Diese Arbeit entstand am Max-Planck-Institut für Physik und Astrophysik, München.