Effect of the structure of the <Emphasis Type="Italic">ortho</Emphasis>, <Emphasis Type="Italic">meta</Emphasis>, and <Emphasis Type="Italic">para</Emphasis> isomers of perhydroterphenyl on their reactivity in heterogeneous catalytic dehydrogenation

The kinetics of the dehydrogenation of the individual ortho, meta, and para isomers of perhydroterphenyl and their mixtures over a (3 wt % Pt)/C catalyst has been investigated in a flow reactor at 280–340°C. The rate of the isomerization of the stereoisomers of the initial substrate (perhydroterphenyl) and terphenyl dehydrogenation products has an effect on the hydrogen release kinetics. The highest reactivity in isomerization is shown by the ortho isomer. The largest amount of hydrogen (7.0 wt %) is released in the dehy-drogenation of perhydro-meta-terphenyl and perhydro-para-terphenyl, whose conversion at 320°C is 96%.     

Efficient peptide coupling involving sterically hindered amino acids

Hindered amino acids have been introduced into peptide chains by coupling N-(Cbz- and Fmoc-alpha-aminoacyl)benzotriazoles with amino acids, wherein at least one of the components was sterically hindered, to provide compounds 3a-e, (3c +3 c'), 5a-d, (5a + 5a'), 6a-c, (6b + 6b'), 8a-c, 9a-e, 10a-d, and (10a + 10a') in isolated yields of 41-95% with complete retention of chirality as evidenced by NMR and HPLC analysis. The benzotriazole activation methodology is a new route for the synthesis of sterically hindered peptides. (Note: compound numbers written within brackets represent diastereomeric mixtures or racemates; compound numbers without brackets represent enantiomers.).     

Quantum chemical investigation of formation of polychlorodibenzo-p-dioxins and dibenzofurans from oxidation and pyrolysis of 2-chlorophenol

Density functional theory (DFT) calculations have been used to obtain thermochemical parameters for formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/PCDF) from the oxidation of 2-chlorophenol. Formation mechanisms of PCDD through radical-radical coupling have been investigated in detail. The sequence of 2-chlorophenoxy radical coupling has been studied. The formation of chlorinated bis keto dimers which results from cross coupling of 2-chlorophenoxy at the ortho carbon bearing hydrogen (a known direct route for PCDF formation) passes through a tight transition structure whose barrier is 9.4 kcal/mol (0 K). Three routes for the formation of the most abundant PCDD/PCDF species (viz., 4,6-dichlorodibenzofuran, 4,6-DCDF, and 1-monochlorodibenzo-p-dioxin, 1-MCDD) in oxidation and pyrolysis of 2-chlorophenol are discussed. In the case of 4,6-DCDF, formation through H or HO + keto-keto <==> H2 or H2O + keto-keto* <==> H2 or H2O + enol-keto* <==> H2 or H2O + 4,6-DCDF + HO is shown to be the preferred route. The other two routes proceed via closed shell processes (keto-keto <==> enol-keto <==> enol-enol <==> H2O + 4,6-DCDF) and (keto-keto <==> enol-keto <==> (H-,OH-) 4,6-DCDF <==> H2O + 4,6-DCDF). Results indicate that 1-MCDD should be the favored product in 2-chlorophenol pyrolysis in agreement with experimental findings. According to our results, tautomerization (inter-ring hydrogen transfer) and intra-annular displacement of HCl would not be competitive with paths deriving from H abstraction from the phenolic oxygen and the benzene ring followed by displacement of Cl in the formation of dibenzo-p-dioxin (DD) and 1-MCDD. The results presented here will assist in construction of detailed kinetic models to account for the formation of PCDD/PCDF from chlorophenols.     

Flow-injection determination of total organic fluorine with off-line defluorination reaction on a solid sorbent bed

Considering recent reports on widespread occurrence and concerns about perfluoroalkyl substances (PFAS) in environmental and biological systems, analysis of these compounds have gained much attention in recent years. Majority of analyte-specific methods are based on a LC/MS/MS or a GC/MS detection, however many environmental or biological studies would benefit from a total organic fluorine (TOF) determination. Presented work was aimed at developing a method for TOF determination. TOF is determined as an amount of inorganic fluoride obtained after defluorination reaction conducted off-line using sodium biphenyl reagent directly on the sorbent without elution of retained analytes. Recovered fluoride was analyzed using flow-injection system with either fluorimetric or potentiometric detection. The TOF method was tested using perfluorocarboxylic acids (PFCA), including perfluorooctanoic acid (PFOA), as model compounds. Considering low concentrations of PFAS in natural samples, solid-phase extraction as a preconcentration procedure was evaluated. Several carbon-based sorbents were tested, namely multi-wall carbon nanotubes, carbon nanofibres and activated carbon. Good sorption of all analytes was achieved and defluorination reaction was possible to carry out directly on a sorbent bed. Recoveries obtained for PFCAs, adsorbed on an activated carbon sorbent, and measured as TOF, were 99.5 ± 1.7, 110 ± 9.4, 95 ± 26, 120 ± 32, 110 ± 12 for C4, C6, C8, C10 and C12-PFCA, respectively. Two flow systems that would enable the defluorination reaction and fluoride determination in a single system were designed and tested.     

Sequence-specific fluorescence detection of DNA by polyamide-thiazole orange conjugates

Fluorescent methods to detect specific double-stranded DNA sequences without the need for denaturation may be useful in the field of genetics. Three hairpin pyrrole-imidazole polyamides 2-4 that target their respective sequences 5'-WGGGWW-3', 5'-WGGCCW-3', and 5'-WGWWCW-3' (W = A or T) were conjugated to thiazole orange dye at the C-termini to examine their fluorescence properties in the presence and absence of match duplex DNA. The conjugates fluoresce weakly in the absence of DNA but showed significant enhancement (>1000-fold) upon the addition of 1 equiv of match DNA and only slight enhancement with the addition of mismatch DNA. The polyamide-dye conjugates bound specific DNA sequences with high affinity (Ka > 10(8) M(-1)) and unwound the DNA duplex through intercalation (unwinding angle, phi, approximately 8 degrees). This new class of polyamides provides a method to specifically detect DNA sequences without denaturation.     

Master equation simulations of bistable and excitable dynamics in a model of a thermochemical system

The effect of fluctuations on the dynamics of a model of a bistable thermochemical system is studied by means of the master equation. The system has three stationary states and exhibits two types of bistability: the coexistence of two stable focuses and the coexistence of a stable focus with a stable limit cycle separated by a saddle point. Stochastic effects are important when the system is close to the bifurcation, in which the stable limit cycle disappears through a homoclinic orbit. In this case the distribution of the first passage time from the stable limit cycle to the stable focus has a multipeak form. The dependence of this distribution on the number of particles is presented. Near the homoclinic orbit bifurcation, the system also exhibits excitability due to a particular shape of the basin of attraction of the stable focus.     

{(Mo)Mo5O21(H2O)3(SO4)}12(VO)30(H2O)20]36-: a molecular quantum spin icosidodecahedron

Self-assembly of aqueous solutions of molybdate and vanadate under reducing, mildly acidic conditions results in a polyoxomolybdate-based {Mo72V30} cluster compound Na8K16(VO)(H2O)5[K10 subset{(Mo)Mo5O21(H2O)3(SO4)}12(VO)30(H2O)20].150H2O, 1, a quantum spin-based Keplerate structure.     

A facile synthesis of 1,1-bis(silyl)ethenes

Symmetrical 1,1-bis(silyl)ethenes have been easily prepared via ruthenium complex-catalyzed silylative coupling cyclization of 1,2-bis(dimethylvinylsiloxy)ethane to give 2,2,4,4-tetramethyl-3-methylene-1,5-dioxa-2,4-disilacycloheptane with excellent selectivity and good yield, followed by its reaction with Grignard reagents. The cyclic product can also be effectively transformed into cyclic carbosiloxane, 2,2,4,4,6,6,8,8-octamethyl-3,7-dimethylene-1,5-dioxa-2,4,6,8-tetrasilacyclooctane.     

Crystallization and second harmonic generation in thermally poled niobium borophosphate glasses

Crystallization of glasses with compositions (1−x)(0.95 NaPO₃+0.05 Na₂B₄O₇)+xNb₂O₅, x=0.4, 0.43, 0.45, 0.48 was investigated by differential scanning calorimetry and X-ray powder diffraction. Crystallization of two phases was observed in the glasses with x=0.43-0.48. First phase is a sodium niobate with the structure of tetragonal tungsten bronze () and second phase is Na₄Nb₈P₄O₃₂ (). The crystallization of sodium niobate is correlated with increasing of nonlinear optical efficiency reported for thermally poled glasses with x>0.4. The results of Raman spectroscopy show the formation of three-dimensional (3D) niobium oxide framework in the glasses with increase of niobium concentration. This framework is supposed to have tetragonal tungsten bronze structure and to be responsible for nonlinear optical properties of the glass. Second harmonic generation signals of as prepared and crystallized glass after thermal poling are compared. The nucleation and crystallization do not improve the NLO properties of the glasses under study.     

A new class of stereoregular vinylene‐arylene copolymers with double‐decker silsesquioxane in the main chain

A synthesis of a new macromolecular class of vinylene‐arylene copolymers with double‐decker silsesquioxane in the main chain is presented. Two transition‐metal‐catalyzed processes, which is silylative‐coupling copolycondensation (SCC) and ADMET copolymerization of divinyl‐substituted double‐decker silsesquioxanes (DDSQ‐2SiVi) with selected diolefins, are reported to be highly efficient tools for the formation of stereoregular copolymers containing DDSQ‐silylene‐vinylene‐arylene units. The copolymeric products are studied in terms of their structural, thermal, and mechanical properties. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2016 , 54, 1044–1055