Relative activity of La2O3, LaOCl, and LaCl3 in reaction with CCl4 studied with infrared spectroscopy and density functional theory calculations

Relative activity of La2O3, LaOCl, and LaCl3 in the destructive adsorption of CCl4 to CO2 was studied with density-functional theory calculations and temperature-programmed reaction experiments monitored with IR spectroscopy. Integral absorbance of the IR peak for phosgene, which is a reaction intermediate, was obtained as a function of temperature, and initial reaction temperatures were compared for different sample amounts of La2O3 and LaOCl. The initial reaction temperatures of about 390 K for La2O3 and 365 K for LaOCl were practically independent of the tested sample weights, and the lower temperature for LaOCl was attributed to a higher activity of surface sites on this material. Calculations suggest that CCl4 decomposition proceeds through a stepwise Cl donation from CCl4 to the surface and that the overall rate is controlled by the first step: CCl4 splitting into a Cl anion and CCl3 cation over an acid-base pair of surface sites. A lanthanum acid site in the pair initiates the split by interacting with one of the chlorine atoms in CCl4, and an oxygen base site stabilizes the remaining CCl3 fragment. Transition state estimates suggest that the relative activity of surface sites can be ranked in the following order: LaOCl > LaCl3 with a partially dechlorinated surface > La2O3. Surface Lewis acidity and basicity of these materials are summarized in terms of the vibrational frequency for adsorbed CO, energy of the lowest unoccupied molecular orbital, and proton affinity. Higher activity of LaOCl is attributed to the higher acidity of the lanthanum site, the higher basicity of the oxygen site, and the geometry of the acid-base pair of sites that allows them to interact with CCl4 simultaneously.     

Selenolate gold complexes with aurophilic Au(I)-Au(I) and Au(I)-Au(III) interactions

The gold(I) selenolate compound [Au(2)(SePh)(2)(mu-dppf)] (dppf = 1,1'-bis(diphenylphosphino)ferrocene) has been prepared by reaction of [Au(2)Cl(2)(mu-dppf)] with PhSeSiMe(3) in a molar ratio 1:2. This complex reacts with gold(I) or gold(III) derivatives to give polynuclear gold(I)-gold(I) or gold(I)-gold(III) complexes of the type [Au(4)(mu-SePh)(2)(PPh(3))(2)(mu-dppf)](OTf)(2), [Au(3)(C(6)F(5))(3)(mu-SePh)(2)(mu-dppf)], or [Au(4)(C(6)F(5))(6)(mu-SePh)(2)(mu-dppf)], with bridging selenolate ligands. The reaction of [Au(2)(SePh)(2)(mu-dppf)] with 1 equiv of AgOTf leads to the formation of the insoluble Ag(SePh) and the compound [Au(2)(mu-SePh)(mu-dppf)]OTf. The complexes [Au(4)(C(6)F(5))(6)(mu-SePh)(2)(mu-dppf)] and [Au(2)(mu-SePh)(mu-dppf)]OTf (two different solvates) have been characterized by X-ray diffraction studies and show the presence of weak gold(I)-gold(III) interactions in the former and intra- and intermolecular gold(I)-gold(I) inter-actions in the later.     

Some remarks concerning matrices of the form A–A′, A−1A′

Matrices and operators of the formA ^–1 A ^* have received a certain amount of attention in recent years. Here some of the literature is surveyed and the caseA ^–1 A is studied for complex matrices withA denoting the transpose ofA. A generalization ofA ^–1 A is introduced.

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Zusammenfassung Matrizen und Operatoren, die in der FormA ^–1 A ^* ausgedrückt werden können, sind in den letzten Jahren häufig studiert worden. Hier wird ein Ueberblick über Teil der relevanten Literatur gegeben und auch der FallA ^–1 A für komplexe Matrizen studiert, wobeiA die Transponierte vonA ist. Es wird auch eine Verallgemeinerung vonA ^–1 A eingeführt.

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Dedicated to Eduard Stiefel by a long time admirer     

Efficient algorithms for the recognition of topologically conjugate gradient-like diffeomorhisms

It is well known that the topological classification of structurally stable flows on surfaces as well as the topological classification of some multidimensional gradient-like systems can be reduced to a combinatorial problem of distinguishing graphs up to isomorphism. The isomorphism problem of general graphs obviously can be solved by a standard enumeration algorithm. However, an efficient algorithm (i. e., polynomial in the number of vertices) has not yet been developed for it, and the problem has not been proved to be intractable (i. e., NPcomplete). We give polynomial-time algorithms for recognition of the corresponding graphs for two gradient-like systems. Moreover, we present efficient algorithms for determining the orientability and the genus of the ambient surface. This result, in particular, sheds light on the classification of configurations that arise from simple, point-source potential-field models in efforts to determine the nature of the quiet-Sun magnetic field.     

New Bio-Based Furanic Materials Effectively Absorb Metals from Water and Exert Antimicrobial Activity

Development of sustainable bio-based materials for removal of toxic contaminants from water is a high priority goal. Novel bio-based binary and ternary copolymers with enhanced ion-exchange, adsorption and antibacterial properties were obtained by using plant biomass-derived diallyl esters of furandicarboxylic acid (FDCA) as crosslinking agents and easily available vinyl monomers. The synthesized copolymer materials showed higher sorption capacities for Ni^II, Co^II and Cu^II compared to the commercial ion-exchange resins, and they maintained their high metal adsorption capacities for over 10 cycles of regeneration. The synthesized copolymer gels containing 1–5 wt % of the crosslinker showed excellent water absorption capacities. The synthesized copolymers with 1 % crosslinker content showed swelling ratios high enough to also act as moisture absorbents. Synthesized copolymers with crosslinker content of 10 wt % performed as contact-active antibacterials by inhibiting the growth of Gram-positive (S. aureus) and Gram-negative bacteria (E. coli, K. pneumonia) in suspension tests.     

The highly stereoselective decarboxylation of (+)-1-bromo-1-chloro-2,2,2-trifluoropropanoic acid to give (+)-1-bromo-1-chloro-2,2,2-trifluoroethane [(+)-halothane] with retention of configuration

The absolute configuration of the title acid 2 has been determined to be S by X-ray crystallography. Thus, decarboxylation of 2 produces (S)-(+)-halothane with 99% retention of configuration. This behavior is compared to other stereoselective decarboxylation reactions of α-haloacids from the literature that also gave high degrees of retention of configuration when in the form of their quaternary ammonium salts, which contain one proton. The proton of the ammonium salt is necessary in order to protonate the anionic intermediate formed from decarboxylation. In the absence of this relatively acidic proton, we had previously found that using triethylene glycol (TEG) as both the solvent and proton source for the decarboxylation reaction of acid 2 caused poor stereoselectivity. This was in contrast to 1,2,2,2-tetrafluoro-1-methoxypropionic acid 6, which showed a high degree of retention of configuration in TEG. In order to rationalize this differing behavior, we report DFT studies at PCM-B3LYP/6-31++G⁷ level of theory (the results were additionally confirmed with 6-311++G⁷ and aug-cc-pVDZ basis sets). The energy barrier to inversion of configuration of the anionic reaction intermediate 11 of acid 2 is 10.23 kcal/mol. However, we find that the anionic intermediate 10 from acid 6 would rather undergo β-elimination instead of inversion of configuration. Thus the planar transition state required for inversion of configuration is never reached, regardless of the rate of proton transfer to the anion.     

From data to knowledge—Use of the Cambridge Structural Database for studying molecular interactions

Abstract

The ordered arrangement of molecules and ions in crystal structures is the result of non-bonded interactions, albeit modulated in the crystalline state by random crystal packing forces. Each individual crystal structure gives but a snapshot of these interactions. It is only by examining a large number of structures containing similar molecules or functional groups that we can gain any generalized knowledge about the rules governing molecular interactions. Indeed if we knew these rules it should be possible to predict crystal structures likely to be formed by specific molecules, whereas at present each new crystal structure requires a de novo experimental solution. We would also be able to evaluate the relative contribution of non-bonded and crystal packing forces and extrapolate to the interactive behaviour of molecules in solution and, most importantly, in the environments encountered in biological systems. A key to these studies is the Cambridge Structural Database (CSD) system where experimental results from over 100,000 individual structure determinations are stored in computer readable form. The system also provides the computational tools needed to locate the relevant structures and analyse the numerical data using a variety of statistical techniques. This paper describes the latest Version of the CSD system (Version 5) released Autumn 1992. The use of the system is illustrated by several examples, such as the preferential interaction of certain functional groups, the study of the C—H…O bond and drug-DNA interactions.     

The Thermodynamics of Translesion DNA Synthesis Past Major Adducts of Enantiomeric Analogues of Antitumor Cisplatin

The Pt^II-coordination complex [PtCl₂(DAB)] (DAB=2,3-diaminobutane) belongs to a class of cytotoxic cisplatin analogues that contain chiral diamine ligands. Enantiomeric pairs of these compounds have attracted particular interest because they have different effects on different DNA conformations, which, in turn, influences the binding of damaged-DNA-processing enzymes that control downstream effects of the adducts, and thus exhibit different biological activities of the enantiomers. Herein, we studied the translesion synthesis across the major 1,2-d(GG) intrastrand cross-link formed by the R,R and S,S enantiomers of [Pt(DAB)]²⁺ in the TGGT sequence by using the enzyme that catalyzes the polymerization of deoxyribonucleotides into a DNA strand. We also employed differential scanning calorimetry (DSC) to measure the thermodynamic changes associated with replication-bypass past 1,2-d(GG) adducts of the [Pt(DAB)]²⁺ enantiomers. In the sequence TGGT, the 1,2-d(GG) intrastrand cross-links that were formed by the enantiomeric pairs of [Pt(DAB)]²⁺ inhibited DNA polymerization in a chirality-dependent manner. The thermodynamic data helped to understand the effect of the alterations in thermodynamic stability of DNA caused by the Pt-d(GG) adducts upon DNA polymerization across these lesions. Moreover, these data can possibly explain the influence of these alterations on the ability of many DNA polymerases to bypass adducts of antitumor platinum drugs. These results also highlighted the usefulness of DSC in evaluating the impact of DNA adducts of platinum-coordinated compounds on the processing of these lesions by damaged-DNA processing-enzymes.