A group additivity methodology for predicting the thermochemistry of oxygen-containing organosilanes

A combinatorial approach was applied to devise a set of reference Si–C–O–H species that is used to derive group-additivity values (GAVs) for this class of molecules. The reference species include 62 stable single-bonded, 19 cyclic, and nine double-bonded Si–C–O–H species. The thermochemistry of these reference species, that is, the standard enthalpy of formation, entropy, and heat capacities covering the temperature range from 298 to 2000 K was obtained from quantum chemical calculations using several composite methods, including G4, G4MP2, and CBSQB3, and the isodesmic reaction approach. To calculate the GAVs from the ab initio based thermochemistry of the compounds in the training set, a multivariable linear regression analysis is performed. The sensitivity of GAVs to the different composite methods is discussed, and thermodynamics properties calculated via group additivity are compared with available ab initio calculated values from the literature.     

Combined Photoredox and Carbene Catalysis for the Synthesis of Ketones from Carboxylic Acids

As a key element in the construction of complex organic scaffolds, the formation of C?C bonds remains a challenge in the field of synthetic organic chemistry. Recent advancements in single‐electron chemistry have enabled new methods for the formation of various C?C bonds. Disclosed herein is the development of a novel single‐electron reduction of acyl azoliums for the formation of ketones from carboxylic acids. Facile construction of the acyl azolium in situ followed by a radical–radical coupling was made possible merging N‐heterocyclic carbene (NHC) and photoredox catalysis. The utility of this protocol in synthesis was showcased in the late‐stage functionalization of a variety of pharmaceutical compounds. Preliminary investigations using chiral NHCs demonstrate that enantioselectivity can be achieved, showcasing the advantages of this protocol over alternative methodologies.     

B‐MWW Zeolite: The Case Against Single‐Site Catalysis

Boron‐containing materials have recently been identified as highly selective catalysts for the oxidative dehydrogenation (ODH) of alkanes to olefins. It has previously been demonstrated by several spectroscopic characterization techniques that the surface of these boron‐containing ODH catalysts oxidize and hydrolyze under reaction conditions, forming an amorphous B₂(OH)_xO_(3?x/2) (x=0–6) layer. Yet, the precise nature of the active site(s) remains elusive. In this Communication, we provide a detailed characterization of zeolite MCM‐22 isomorphously substituted with boron (B‐MWW). Using ¹¹B solid‐state NMR spectroscopy, we show that the majority of boron species in B‐MWW exist as isolated BO₃ units, fully incorporated into the zeolite framework. However, this material shows no catalytic activity for ODH of propane to propene. The catalytic inactivity of B‐MWW for ODH of propane falsifies the hypothesis that site‐isolated BO₃ units are the active site in boron‐based catalysts. This observation is at odds with other traditionally studied catalysts like vanadium‐based catalysts and provides an important piece of the mechanistic puzzle.     

Photochemical Transformations with Iodine Azide after Release from an Ion‐Exchange Resin

This report discloses the photochemical homolytic cleavage of iodine azide after its formation following release from polymer‐bound bisazido iodate(I) anions. A series of radical reactions are reported including the 1,2‐functionlization of alkenes and the unprecedented chemoselective oxidation of secondary alcohols in the presence of primary alcohols.     

Toward high-performance computational chemistry: I. Scalable Fock matrix construction algorithms

Several parallel algorithms for Fock matrix construction are described. The algorithms calculate only the unique integrals, distribute the Fock and density matrices over the processors of a massively parallel computer, use blocking techniques to construct the distributed data structures, and use clustering techniques on each processor to maximize data reuse. Algorithms based on both square and row-blocked distributions of the Fock and density matrices are described and evaluated. Variants of the algorithms are discussed that use either triple-sort or canonical ordering of integrals, and dynamic or static task clustering schemes. The algorithms are shown to adapt to screening, with communication volume scaling down with computation costs. Modeling techniques are used to characterize algorithm performance. Given the characteristics of existing massively parallel computers, all the algorithms are shown to be highly efficient for problems of moderate size. The algorithms using the row-blocked data distribution are the most efficient. © 1996 by John Wiley & Sons, Inc.     

Comparative study of the thermal properties of related aromatic polyhydrazides and poly(1,3,4-oxadiazole)s

Studies were undertaken to ascertain the thermal behavior of several new types of aromatic polyhydrazides and poly(1,3,4-oxadiazole)s containing different functional groups. Results of thermal analysis investigations indicate that all the polyoxadiazoles are remarkably heat-resistant when heated in nitrogen at elevated temperature but somewhat less heat-resistant than fully aromatic polyoxadiazoles. Most of the new polyoxadiazoles decompose when heated to about 450°C. The incorporation of tetraphenyl silane, hexafluoroisopropylidene, phthalido or phenoxytherephthalic groups into the main chain decrease the glass transition temperature of aromatic poly(1,3,4-oxadiazole)s. In the case of the silicon-containing polymers the glass transition temperature is independent of the other groups incorporated in the same macromolecule. The cyclization process of all investigated polyhydrazides takes place in the range between 320 and 390°C.     

Organoborane-mediated synthesis of functional polymers: A review

This article reviews recent advances in the synthesis of polar functional polymers via the mediation of organoborane monomers and polymers. Both direct polymerization and post-polymerization functionalization processes are described. Organoborane-assisted Ziegler–Natta, metathesis, and hydroboration (haloboration) polymerizations are presented. Moreover, the special problems and opportunities of polymer modification of olefin containing homo and copolymers by organoboranes are discussed.     

Polymerforschung und -technologie im Iran

Im Laufe der letzten Jahre wurden - von den meisten westlichen Fachkollegen unbemerkt - an mehreren iranischen Universitäten und Forschungsinstituten Abteilungen eingerichtet, die sich in Forschung und Lehre mit Chemie, Physik, Anwendungs- und Verfahrenstechnik der Polymere beschäftigen. Das 1986 gegründete Polymer Research Center of Iran organisierte im Mai dieses Jahres das 1. Internationale Seminar.