Quantum chemical study of mechanisms for oxidative dehydrogenation of propane on vanadium oxide

We have carried out a hybrid density functional study of mechanisms for oxidative dehydrogenation of propane on the (010) surface of V2O5. The surface was modeled using both vanadium oxide clusters and a periodic slab. We have investigated a Mars-van Krevelen mechanism that involves stepwise adsorption of the propane at an oxygen site followed by desorption of a water molecule and propene, and subsequent adsorption of an oxygen molecule to complete the catalytic cycle. The potential energy surface is found to have large barriers, which are lowered somewhat when the possibility of a triplet state is considered. The barriers for propane adsorption and propene elimination are 45-60 kcal/mol. The highest energy on the potential energy surface at the B3LYP/6-31G* level of theory is about 80 kcal/mol above the energy of the reactants and corresponds to formation of an oxygen vacancy after water elimination. Subsequent addition of an oxygen molecule to fill the vacancy is predicted to be energetically downhill. The reactions of propane at a bridging oxygen site and at a vanadyl site have similar energetics. The key results of the cluster calculations are confirmed by periodic calculations. Factors that may lower the barriers on the potential energy surface, including the interaction of vanadium oxide clusters with a support material and a concerted reaction with O2, are discussed.     

Contents of the proceedings

Table of Contents

Contents of the proceedings     

Distributions of fluorescence decay times for synthetic melittin in water-methanol mixtures and complexed with calmodulin,troponin C,and phospholipids

Frequency-domain measurements of the intensity decays of melittin were used to recover the distribution of decay times displayed by its single tryptophan residue. Melittin was examined in the monomeric random coil state (water), in the monomeric -helical state (water-methanol), in the tetrameric state, and with 6M guanidine hydrochloride. In the presence of denaturant, where melittin is expected to be devoid of secondary structure, we observed a narrow distribution of lifetimes, similar to a double-exponential decay. In water the intensity decay of melittin was found to be described better by the distribution of decay times, which became progressively wider as the amount of -helix was increased by the methanol cosolvent or upon formation of the -helical tetrameric state. We also examined the intensity decays of melittin when complexed with calmodulin, troponin C, or lipid vesicles of 1-palmitoyl-2-oleyl-l--phosphatidylcholine (POPC). The lifetime distributions of the complexes with lipid were comparable to those observed in methanol-water, suggesting a similarity of the structure and/or dynamics of the environment surrounding the tryptophan residue. A broad lifetime distribution was observed for the melittin-calmodulin complex, suggesting a rigid structure and/or heterogeneity in the form of the complex. The lifetime distribution of the melittin-troponin C complex was more narrow, suggesting a more uniform structure, at least in the region surrounding the tryptophan residue. These results demonstrate that the lifetime distributions of a single tryptophan protein can be a sensitive indicator of the conformational heterogeneity and dynamics of proteins.     

Structure of YAG crystals doped/substituted with erbium and ytterbium

Two series of yttrium aluminum garnets doped with erbium and ytterbium ions have been studied by single-crystal X-ray diffraction. The single crystals were obtained by the Czochralski method. The concentration of doping ions was established by the plasma emission spectroscopy method. For the Er series, it is equal to 23.4, 46.7, 72.5, and 100%, whereas, for Yb-doped YAGs, it equals 26.5, 48.2, 75.9, and 100%. The results are supplemented by the data obtained for pure YAG. The X-ray data for all samples were collected at two temperatures: 295(2) and 102(2) K. Additionally, for YAGs doped with ca. 50% of doping ions, some more detailed temperature measurements were performed. Pure single crystals of ErAG and YbAG were also studied as a function of diffraction angles. Careful single-crystal measurements utilizing a CCD diffractometer revealed the unit cell parameters, which slightly-but significantly-deviate from cubic symmetry. The average unit cell parameters change linearly with the amount of substituting Er and Yb cations, with two different slopes related to the ionic radii. Both of the dodecahedral distances depend linearly on the concentration of the substituting ions, but the longer dodecahedral distance also changes with temperature contrary to the shorter one. There is no correlation between the octahedral interatomic distance [Al(VI)...O] and the unit cell parameter or temperature; however, weak trends appear for the tetrahedral [Al(IV)...O] distance. Such weak trends are just the secondary consequences of the ion substitution taking place at the dodecahedral site. The dependences between the unit cell parameters and the concentration of doping ions allow us to establish concentrations of particular doping ions in mixed (Er, Yb) doped YAGs just on the basis of the measured unit cell parameters and knowledge of the amount of Y.     

Mechanisms of solvolysis of N,N-bis(halogenomethyldimethylsllyl)acetamides

The previously reported unusual course of the reactions of the compounds MeCON(SiMe₂CH₂X)₂ (X = Cl or Br) with methanol or water is suggested to involve neighbouring group participation.     

Preparation of samples of plant material for chromatographic analysis

The selection, collection, and preliminary treatment of plant material samples are discussed. Stages of sample preparation and extraction techniques presently used are described, and the most recent examples of sample preparation for chromatographic analysis are reviewed. Recent applications of gas chromatography for the assessment of emission of volatile organic compounds by plants are also described.     

Increased intensities of YOYO-1-labeled DNA oligomers near silver particles

DNA detection is usually performed using fluorescence probes. Using a DNA oligomer stained with the widely used dye 1,1'-[1,3-propanediylbis[(dimethylimino)-3,1-propanediyl]]bis[4-[(3-methyl-2(3H)-benzoxazolylidene)methyl]]-quinolinum tetraiodide (YOYO-1), we show that a substrate containing silver particles can lead to a greater than 10-fold increase in the fluorescence intensity. Proximity to silver particles also increases the photostability of YOYO-1-DNA. These results suggest that substrates or gels containing silver particles may be used for increased sensitivity in DNA detection.     

Structure and tautomerism of cyclic silylamides: I. Disiloxane derivatives of acetamide and benzamides

The reactions of 1,3-dichlorotetramethyldisiloxane (DCTDS) with acetamide and a series of ring-substituted benzamides give disiladioxazines. The acetamide derivative exists in a tautomeric equilibrium with N-acetyltetramethylcyclosilazoxane, whose structure represents a novel type of four-membered ring. The equilibrium concentration of the latter compound is increased by lowering the temperature and increasing the polarity of the solvent. The disiladioxazines produced from benzamides and DCTDS do not undergo tautomerization. Their structures were studied by ¹H, ¹³C and ²⁹Si NMR spectroscopy; the spectra do not change over a wide range of temperature.     

Thermal analysis and spectroscopic characteristics of tetrabutylammonium tetrachloroferrate(III)

The IR, far-IR, Raman and Mössbauer spectra have been utilized to identify a new compound consisting of a tetrachloroferrate(III) anion and a tetrabutylammonium cation [(C₄H₉)₄N][FeCl₄]. Its degradation has been studied by thermal analysis using TG, TG-MS, DTG and DTA, as well as DSC techniques. The measurements were run in static air and in argon atmosphere. Solid residues were identified by elemental analysis, far-IR and Mössbauer spectroscopy. The discussion was focused on processes proceeding during the first step of the thermal decomposition.     

Inspecting the structure-activity relationship of protein kinase CK2 inhibitors derived from tetrabromo-benzimidazole

CK2 is a very pleiotropic protein kinase whose high constitutive activity is suspected to cooperate to neoplasia. Here, the crystal structure of the complexes between CK2 and three selective tetrabromo-benzimidazole derivatives inhibiting CK2 with Ki values between 40 and 400 nM are presented. The ligands bind to the CK2 active site in a different way with respect to the parent compound TBB. They enter more deeply into the cavity, establishing halogen bonds with the backbone of Glu114 and Val116 in the hinge region. A detailed analysis of the interactions highlights a major role of the hydrophobic effect in establishing the rank of potency within this class of inhibitors and shows that polar interactions are responsible for the different orientation of the molecules in the active site.