Alfred Schönhofer

Bio: Alfred Schönhofer is an academic researcher from Technical University of Berlin. The author has contributed to research in topics: Liquid crystal & Dipole. The author has an hindex of 13, co-authored 26 publications receiving 605 citations.

Theorie der ChiralitÄtsfunktionen

Abstract: Die pseudoskalaren Eigenschaften chiraler Molekule sind bei geeigneter Klassifizierung der Objekte Gegenstand einer algebraischen Theorie. Die Analyse des ChiralitAtsphAnomens an diesen Klassen fuhrt auf darstellungstheoretische Eigenschaften von ChiralitAtsfunktionen, auf eine Verbandsstruktur von Partitionen, auf damit verbundene Aussagen uber Permutationsgruppen und schlie\lich auf eine Strukturanalyse von NAherungsansAtzen fur ChiralitAtsfunktionen. Insofern enthAlt die vorliegende Arbeit rein mathematische Aspekte. Mathematische Theoreme, die ohne wesentliche Bezugnahme auf die physikalische Fragestellung formuliert und bewiesen werden, finden sich im Anhang. Im Text steht das physikalische PhAnomen im Vordergrund, und Begriffe wie ChiralitAtsordnung, ChiralitAtsindex, ChiralitAtszahlen, qualitative VollstAndigkeit, verkurzte AnsAtze, aktive und inaktive Ligandenpartitionen u. a., die sich aus dem mathematischen Formalismus anbieten, dienen der Systematik, der Einsicht und der Beantwortung spezieller Fragen im Bereich der Me\methoden des ChiralitAtsphAnomens bei Molekulen (Zirkulardichroismus, Rotationsdispersion usw.). So liefern beispielweise der ko- und der kontravariante Standpunkt beim Transformationsverhalten von Funktionen zwei Interpretationsmoglichkeiten bei ChiralitAtsfunktionen. Wir konnen nAmlich zu irreduziblen Darstellungen der $$\mathfrak{S}_n $$ gehorige Komponenten einer ChiralitAtsfunktion fur Molekule als Chiralfunktionen fur zugeordnete Isomerengemische verstehen. Dabei gewinnen Projektionsoperatoren fur die Komponenten als Ensembleoperatoren fur nichtrazemische Isomerengemische einen physikalischen Inhalt von praktischem Nutzen. Kapitel 12 gibt eine rohe Skizze von Anwendungsmoglichkeiten der vorliegenden Theorie. Erste uberzeugende Vergleiche theoretischer Werte nach den NAherungsmethoden aus den Kapiteln 8–11 mit experimentellen Daten fur die optische AktivitAt von Allenderivaten liegen vor; ihre Publikation befindet sich in Vorbereitung [4]. Ebenso werden mathematische Konsequenzen der in Kapitel 6 gegebenen Definition des Partitionenverbands, insoweit sie hier nicht ausgefuhrt sind, weiter verfolgt.

Näherungsformeln für spiegelungsantimetrische Moleküleigenschaften

Abstract: Eine Einteilung von Molekulen in Klassen mit dem Merkmal eines nichtchiralen Molekulgerusts bestimmter Bauart gestattet, spiegelungsantimetrische Molekuleigenschaften nach zwei verschiedenen theoretischen Gesichtspunkten naherungsweise zu beschreiben. Im folgenden werden die Methoden zur Ableitung von Formeln nach diesen beiden Approximationsstandpunkten und allgemeine Eigenschaften der so gewonnenen Naherungsansatze aufgezeigt. Dann werden fur sechzehn reprasentative Molekulklassen in einer Tabelle Formeln nach beiden Verfahren zusammengestellt und typische Zuge an diesen Beispielen studiert. Ein Theorem, das als Verallgemeinerung des Van't Hoffschen Superpositionsprinzips angesehen werden kann, ergibt sich bei der Anwendung beider Verfahren auf Molekule mit chiralen Liganden.

Computer simulation of chiral liquid crystal phases. I. The polymorphism of the chiral Gay-Berne fluid

Abstract: We report the results of computer simulation studies for a bulk system composed of chiral particles interacting via the Gay-Berne potential and an additive chiral potential. Using Monte Carlo (MC) simulations in the NVT ensemble, the chirality-temperature plane of the phase diagram was studied at different points by a variation of the chirality parameter c describing the strength of the chiral potential. Additionally to the well-known isotropic, nematic and smectic phases of the Gay-Berne fluid, we localized regions of cholesteric phase. For large values of the chirality parameter we also observed blue phases. Furthermore, when starting from a cholesteric phase and decreasing the temperature at constant c, we obtained a phase region showing characteristics of the recently discovered helical smectic A∗ phase. All phases have been characterized by correlation functions, order parameters, and visual representations of selected configurations. All results of the simulation are limited by the small sy...

Helical twisting power of chiral mono- and bis-aminoanthraquinones. Intramolecular and intermolecular chirality transfer in liquid-crystal phases

Abstract: The helical twisting power (HTP) of mono- and bis-aminoanthraquinones with chiral substituents depends strongly on the position of substitution. It is shown that the Intramolecular chirality transfer yields, for each compound, several conformers which are different for the 1-cyclohexylethyl, 1-phenylethyl and 1-naphthalen-1-ylethyl substituents. Each conformer contributes differently to the HIP of a compound. Furthermore, the HIP of a bis-substituted compound is approximately the sum of the HIP values of the corresponding mono-substituted compounds if the orientation of the substituents with respect to the orientation axis (principal axis of the order tensor belonging to the largest eigenvalue) is the same in each case. This means that the contribution of a ‘chiral area’, induced around an asymmetric carbon atom by Intramolecular chirality transfer, to the HTP depends on its orientation with respect to the orientation axis. A variation of this orientation can cause even a change of sign of the HIP in spite of the fact that the dependence of the HIP on the order parameters, S* and D*, is small.

Molecular Light Scattering and Optical Activity

Abstract: Ranging from the physics of elementary particles to the structure of viruses, the subject matter of this book stresses the importance of optical activity and chirality in modern science and will be of interest to a wide range of scientists. Using classical and quantum methods with a strong emphasis on symmetry principles, the volume develops the theory of varied optical activity and related phenomena from the perspective of molecular scattering of polarized light. First Edition Hb (1983): 0-521-24602-4

Aromaticity of polycyclic conjugated hydrocarbons.

Abstract: G. Clar 6n Rule versus Hückel 4n + 2 Rule 3464 H. Hydrocarbons versus Heteroatomic Systems 3465 IV. Hidden Treasures of Kekulé Valence Structures 3466 A. Conjugated Circuits 3467 B. Innate Degree of Freedom 3470 C. Clar Structures 3472 V. Graph Theoretical Approach to Chemical Structure 3473 A. Metric 3473 B. Chemical Graphs 3473 C. Isospectral Graphs 3473 D. Embedded Graphs 3475 E. Partial Ordering 3476 VI. On Enumeration of Benzenoid Hydrocarbons 3477 VII. Kekulé Valence Structures Count 3479 A. Non-branched Cata-condensed Benzenoids 3481 B. Branched Cata-condensed Benzenoids 3482 C. Benzenoid Lattices 3482 D. Peri-condensed Benzenoids 3483 E. Miscellaneous Benzenoids 3484 F. The Approach of Platt 3485 G. Computer Programs for Calculating K 3485 H. Transfer-Matrix Method 3486 I. Use of Recursion Relations 3486 J. Use of Signed Matrices 3487 VIII. Enumeration of Conjugated Circuits 3488 IX. Approximate Approaches versus Ambitious Computations 3490

TADDOLs, Their Derivatives, and TADDOL Analogues: Versatile Chiral Auxiliaries

Abstract: TADDOLs, which contain two adjacent diarylhydroxymethyl groups in a trans relationship on a 1,3-dioxolane ring, can be prepared from acetals or ketals of tartrate esters by reaction of the latter with aromatic Grignard reagents. They are extraordinarily versatile chiral auxiliaries. Here, a historical review of the subject is followed by discussion of the preparation of TADDOLs and analogous systems, including TADDOLs with N-, P-, O-, and S-heteroatom ligands appropriate for metals. Crystal structure analysis reveals that the heteroatoms on the diarylmethyl groups are almost always in close proximity to each other, joined together by H-bonds, and predisposed to form chelate complexes in which the metallic centers reside in propeller-like chiral environments. Applications of TADDOL derivatives in enantioselective synthesis extend from utilization as stoichiometric chiral reagents or in Lewis acid mediated reactions, to roles in catalytic hydrogenation and stereoregular metathesis polymerization. Derivatives and complexes based on the following metals have so far been investigated: Li, B, Mg, Al, Si, Cu, Zn, Ce, Ti, Zr, Mo, Rh, Ir, Pd, Pt. The number of stereoselective reactions already accomplished with TADDOLs is correspondingly large. It is also easy to prepare TADDOL derivatives that are readily polymerizable and graftable, and to transform them into immobilized solid-phase catalysts. The result is catalysts, simply or dendritically immobilized in polystyrene or on silica gel and characterized by unexpected stability even after multiple use in titanium TADDOLate mediated reactions. TADDOLs show further unusual characteristics that make them useful for applications in material science and supramolecular chemistry: they are the most effective doping agents known for phase transformations of achiral (nematic) into chiral (cholesteric) liquid crystals. The TADDOL OH group that is not involved in intramolecular H-bonding shows a strong tendency to associate intermolecularly with H-bond acceptors. In the process of crystallization this leads, enantioselectively, to the formation of inclusion compounds that lend themselves to the separation of racemic mixtures not otherwise suited to the classical method of crystallization through diastereomeric salts. The high melting points of TADDOLs even make possible the resolution of racemates by distillation! Host-guest compounds formed between TADDOLs and achiral partners can serve as platforms for enantioselective photoreactions. It seems safe to predict that many more applications will be discovered for the TADDOLs and their derivatives. Supporting information for this article is available on the WWW under http://www.angewandte.com or from the author.