Structure and Bonding in CE5− (E=Al–Tl) Clusters: Planar Tetracoordinate Carbon versus Pentacoordinate Carbon

The structure, bonding, and stability of clusters with the empirical formula CE₅^? (E=Al–Tl) have been analyzed by means of high‐level computations. The results indicate that, whereas aluminum and gallium clusters have C_2v structures with a planar tetracoordinate carbon (ptC), their heavier homologues prefer three‐dimensional C_4v forms with a pentacoordinate carbon center over the ptC one. The reason for such a preference is a delicate balance between the interaction energy of the fifth E atom with CE₄ and the distortion energy. Moreover, bonding analysis shows that the ptC systems can be better described as CE₄^?, with 17‐valence electrons interacting with E. The ptC core in these systems exhibits double aromatic (both σ and π) behavior, but the σ contribution is dominating.     

Ion beam sputtering deposition of silver nanoparticles and TiOx/ZnO nanocomposites for use in surface enhanced vibrational spectroscopy (SERS and SEIRAS)

Hybrids consisting of silver nanoparticles (in varying fractions) and of TiOx/ZnO were prepared via top-down ion beam sputtering (IBS) deposition on silicon substrates. The deposited nanomaterials were characterized by scanning electron microscopy and X-ray photoelectron spectroscopy. It is shown that such composites represent a viable substrate for use in both surface enhanced Raman spectroscopy (SERS) and surface enhanced infrared absorption spectroscopy (SEIRAS), as exemplarily shown for crystal violet as the model analyte. The C-H bending mode at about 1181 cm^?1 and the C-N vibration at 1361 cm^?1 observed in the SERS and SEIRAS spectra, respectively, have been used as analytical signal. The substrate consisting of TiOx NPs with 33% fraction of silver provides the strongest enhancement in SERS (up to 10,000-fold), while TiOx/AgNPs with thickness of 2 and 1 nm in ion beam sputtering, respectively, provides the best sensitivity in SEIRAS. The substrates also display photocatalytic activity as shown by the degradation of adsorbed crystal violet under ultraviolet irradiation.

Open image in new window

Graphical abstract Schematic of the preparation of hybrid substrates consisting of Ag and TiOx/ZnO nanoparticles via ion beam sputtering deposition. They were applied in both surface enhanced Raman and surface enhanced infrared absorption spectroscopies using crystal violet as model analyte, showing enhancements up to >10,000-fold in Raman.

     

Oxidative Cyclization of Iodo Ketones. Synthesis of 6-Oxa-5alpha-pregnane-3,20-dione

The steroidal delta- and gamma-iodo ketones 1 and 9 were converted to the cyclic hemiketals 3 and 10, by oxidation to the iodoso derivatives with m-CPBA. Spontaneous cyclization of the latter intermediates to the corresponding oxocarbenium ions, followed by stereoselective addition of water, rendered the hemiketals. Depending on the reaction conditions, the five-membered oxocarbenium ion derived from the gamma-iodo ketone 9 may add H(2)O or m-CPBA to give either the hemiketal or a Baeyer-Villiger type product 12, while the oxocarbenium derived from 1 gives exclusively the hemiketal. When the reaction was carried out in dry methanol, methyl ketals were formed. Use of this methodology allowed us to synthesize 6-oxa-5alpha-pregnanes with and without functionalization at C-19.     

Chirality and Crop Protection

The modern agrochemical industry is searching, more intensively than ever, for new substances to combat pests (weeds, deleterious insects, plant pathogens, etc.). In the complex and costly selection and optimization process, state-of-the-art scientific methods are always needed. The aims of the interdisciplinary optimization are mainly the reduction of the rate of application of the new substance, an increase in the selectivity against the target organism, and the optimal ecological profile. If a promising crop protection compound is a racemate or a diastereoisomeric mixture, the chemist has a unique opportunity to contribute to this optimization process through the synthesis of enantiomerically pure isomers for testing purposes. If the single isomer proves to be biologically superior to the racemate, the development of an economical and ecologically sound process for the production of the single isomer presents an even greater challenge. The average price of a crop protection compound is much lower than that for a pharmaceutical product, and this fact imposes a severe limitation upon the flexibility of the chemist who is concerned with the synthesis and production of a stereochemically pure agrochemical. This forces the crop protection chemist to make full use of both his scientific and creative capabilities. Fortunately, parallel to the development of the above optimization aims of a modern and ecologically sound crop protection research, there has been a continuous and worldwide advance in the area of asymmetric synthesis. Due to the interplay of these two parallel efforts there has been a great accumulation of chemical, biological, and agronomical knowledge in recent years, which should have implications beyond merely the synthesis of enantiomerically pure agrochemicals.     

X-ray reflectivity investigations of glass surfaces produced by float and draw techniques

Surfaces of soda-lime glass and borosilicate glass have been investigated by grazing incidence X-ray reflectivity (GIXR). Characteristic differences are obtained in dependence on the fabrication procedure, the composition and the cleaning procedure. Strong variation is recorded between the two soda-lime float glass surfaces while minor differences are analysed between the top and bottom side of borosilicate float glass. This is attributed to the reduced amount of tin diffused into the bottom side of the borosilicate glass surface. Different cleaning procedures generate characteristic changes on the glass surfaces which can be verified by GIXR. The results indicate that borosilicate float glass combines the merits of the good surface quality of float glass with the high chemical resistivity of borosilicate glass.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Photophysical Study of Pyridoxal 5'-Phosphate and Its Schiff Base with n-Hexylamine

Abstract— Phosphopyridoxal enzymes exhibit a wide variety of reactions. The activation mechanism proceeds with the formation of Schiff bases that undergo loss or transfer of one or more protons. Changes in the fluorescent properties of the pyridoxal 5'-phosphate and its Schiff base may signal changes in the protonation state and/or in the microenvironment of the cofactor. In this paper the average fluorescence lifetime of pyridoxal 5'-phosphate (PLP) as a function of pH was studied in a medium of 0.1 M ionic strength at 25°C. The ionic species of PLP with an unprotonated hydrogen atom were found to exhibit fluorescence lifetimes in the region of 100 ps on excitation at λ= 390 nm. The average fluorescence lifetimes for PLP in its Schiff bases with n-hexylamine in media of low polarity are reported.     

Ab initio calculations, potential representation and vibrational dynamics of He2Br2 van der Waals complex

An intermolecular potential energy surface for He(2)Br(2) complex in the ground state is calculated at the levels of fourth-order (MP4) Moller-Plesset and coupled-cluster [CCSD(T)] approximations, using large-core pseudopotential for Br atoms and the aug-cc-pV5Z basis set for He. The surface is characterized by three minima and the minimum energy pathways through them. The global minimum corresponds to a linear He-Br(2)-He configuration, while the two other ones to "police-nightstick" and tetrahedral structures. The corresponding well depths are -90.39/-89.18, -81.23/-80.78 and -74.40/-74.02 cm(-1), respectively, at MP4/CCSD(T) levels of theory. It is found that results obtained by summing three-body parametrized HeBr(2) interactions and the He-He interaction are in very good accord with the corresponding MP4/CSSD(T) configuration energies of the He(2)Br(2). Variational calculations using a sum of three-body interactions are presented to study the bound states of the vdW He(2)Br(2) complex. The binding energy D(0) and the corresponding vibrationally averaged structure are determined for different isomers of the cluster and their comparison with the available experimental data is discussed.     

New C2-symmetric bis(sulfonamide)-cyclohexane-1,2-diamine-RhCp complex and its application in the asymmetric transfer hydrogenation (ATH) of ketones in water

C₂-symmetric bis(sulfonamide) ligands derived from trans-(1R,2R)-cyclohexane-1,2-diamine were synthesized and complexed with [Cp^∗RhCl₂]₂ in situ and used in the ATH of aromatic ketones with aqueous sodium formate as the hydrogen source. The chiral secondary alcohols were obtained with 90-99% enantioselectivity and in 50-100% yield. Reductions in water were faster than those in isopropanol/KOH.     

Molecular crystals with moving parts: synthesis, characterization, and crystal packing of molecular gyroscopes with methyl-substituted triptycyl frames

We report a highly convergent synthesis for the preparation of molecular gyroscopes consisting of para-phenylene rotors linked by triple bonds to methyl-substituted triptycenes acting as pivots and encapsulating frames. The desired 1,4-bis[2-(2,3,6,7,12,13-hexamethyl-10-alkyl-9-triptycyl)ethynyl]benzenes were prepared from 2,3-dimethyl-1,3-butadiene using Diels-Alder cycloadditions and Pd(0)-catalyzed coupling as the key reactions. The main challenge in the synthesis came about in the preparation of 9-alkynyl-triptycenes by Diels-Alder reaction of benzynes and 9-alkynyl-2,3,6,7-tetramethylanthracenes. These reactions occurred with chemical yields and regioselectivities that were strongly influenced by steric and electronic effects of substituents at C10 of the anthracene core. Anthracenes with methyl, propyl, and phenyl substituents were utilized to complete the synthesis of their corresponding molecular gyroscopes, and their solid-state structures were determined by single-crystal X-ray diffraction analysis. Examination of these results indicated that, as expected, the bulky triptycyl groups encourage crystallization motifs that create more free volume around the phenylene rotor, as needed to facilitate fast gyroscopic motion in the solid state.