Initiating radical polymerization of vinyl monomers by silicon peroxides

The initiating ability of organosilicon peroxides, Me_4-mSi(OOCMe₃)_mwherem = 1−3, for polymerization of vinyl monomers has been quantitatively characterized. Complex formation between monomers and Lewis acids has been found to activate the polymerization process. The activation effect is observed for a ratio [Lewis acid]/[monomer] of 0.003 to 0.02. The complex formation parameters (equilibrium constant and complex composition) have been determined by PMR, thermochemical and cryoscopic methods. Organosilicon peroxides have been found to influence the composition of copolymers; polyfunctional peroxides can be used to prepare block copolymers.     

EXAFS study of ceria-lanthana-based TWC promoters prepared by sol-gel routes

Extended X-ray absorption fine structure (EXAFS) experiments at the Ce K- and La K-edges were performed on ceria-lanthana-alumina three-way catalysts promoters prepared by sol-gel routes, in order to investigate the effect of lanthanum doping on the ceria structure. The formation of Ce_1−xLa_xO_2−x/2 solid solution, already observed by X-ray diffraction, was confirmed by EXAFS analysis, while no experimental evidence of a Ce-Al interaction was found. In presence of cerium and aluminum, lanthanum is involved in the formation of solid solution with CeO₂ and of La-Al compounds. When the La:Al molar ratio is sufficiently high, the growth of a tridimensionally ordered LaAlO₃ perovskite compound is observed. For increasing values of x/1−x in the solid solution Ce_1−xLa_xO_2−x/2, the Ce-O distance decreases, while La-O distance remains nearly constant.     

Quantum chemical model of dehydration of 2-butanol catalyzed by zeolite X

The CNDO/2 method was used for quantum chemical calculations of the 2-butanol interaction with zeolite X modelled by the cluster Si₃AlO₁₂H₉. Two-site adsorption on a pair consisting of an acidic and a basic catalytic site promotes dehydration of the alcohol. The activation energy of trans-2-butene formation was estimated to be much higher than those of cis-2-butene and 1-butene formation, in agreement with experimental findings.     

Carbonylations mediated by dicyclopentadienyl samarium

Reaction of t-BuBr with SmCP₂ and CO leads to the formation of different carbonylation products according to experimental conditions. t-BuCHOHCHOHt-Bu is obtained if the reaction is followed by hydrolysis. t-BuCOCHOHC₆H₁₃ is produced if the carbonylation of t-BuBr is followed by addition of heptanal. The product t-BuCOCOCHOHt-Bu which has incorporated three CO units, has also been isolated in a low yield.     

Effective absolute rate constant for the formation of negatively-charged CO2 cluster ions by electron transfer from state-selected Rydberg atoms

We have studied electron transfer from state-selected Ar** (nd) Rydberg atoms to (CO₂) _m clusters for 12≦n≦48. The relative rate constant for the total negative ion formation reaches its maximum forn=21 and decreases towards lower and highern. From an accurate study of Penning ionization of nozzle beams with and without cluster formation we were able to derive the condensed fraction in our (CO₂) _m cluster beam and from a statistical analysis of cluster distributions we obtain an estimate for the absolute value of the effective rate constant for the total negative ion formation, which amounts to (4.5 ± 3) · 10^?8 cm³/s for Ar** (21d).     

Crystal growth and thermodynamic properties of lithium tungstate doped by 4% molybdenum

For the first time, single crystal of Li₂W_0.96Mo_0.04O₄ has been grown by low-temperature-gradient Czochralski technique. The thermodynamic characteristics (standard formation enthalpy and lattice enthalpy) that are necessary to improve the growth technology have been studied by solution calorimetry. For Li₂W_1–xMo_xO₄ single crystals, correlations of lattice enthalpies and standard formation enthalpies with tolerance factor were found.     

The association in mixtures of dimethylsulphoxide and water as studied by Raman spectroscopy

In mixtures of dimethylsulphoxide (DMSO) and water, formation of a complex (DMSO) · (H₂O)₂ is indicated by wavenumber shifts and linewidth variations of the v SO Raman band. Higher temperatures interfere with this complex formation.     

Electron capture by tetra- and di-chlorobenzene molecules. Comparative studies by position annihilation and ODESR methods

Experiments on inhibition and anti-inhibition of Ps formation and also on optical detection of ESR spectra in squalene solutions with 1,2,4,5-C₆H₂Cl₄ and p-C₆H₄Cl₂ have demonstrated the presence of short-lived molecular radical-anions (products of the spur electron capture by additives), their lifetimes being longer than roughly 10 ps and shorter than 10–30 ns. The cross section of electron capture by p-C₆H₄Cl₂ in squalane is nearly one-tenth of that by 1,2,4,5-C₆H₂Cl₄ and CCl₄ at concentration ? 0.02 M, while 10% of the electrons cannot be trapped even at 0.25 M p-C₆H₄Cl₂. Probably the electrons in the high-electric-field (0.3–3 MV/cm) regions of the spurs cannot be effectively captured by p-C₆H₄Cl₂, which is a shallow electron trap. Similar effects of the high electric fields of the charged spur series seem to influence the Ps formation appreciably, along with the other effects discussed in previous papers. Several new results can be predicted, by use of this interpretation. Both methods employed are emphasized to be selective with respect to geminate spur particles.     

Formation of WF−6 and its dissociative products by collisional ionization

The formation of negative ions in electron transfer reactions between hyperthermal alkali atoms (Na, K) and WF₆ has been studied in the energy range 0–30 eV c.m. Relative cross sections and translational energy thresholds for ion pair formation have been measured, from which the following electron affinities (EA) and bond dissociation energies (D) have been derived: EA(WF₆) = 3.7 eV, EA(WF₅) = 1.25 eV, D(WF₅—F) = 5.1 eV, D)WF₅—F^?) = 5.4 eV, D(WF^?₅—F) = 7.6 eV. Several ion molecule reactions are discussed which result in formation of secondary fragmentation ions and WF^?₇.     

Carbonylation of nitrobenzene to phenyl isocyanate and methyl carbamate catalyzed by palladium and rhodium activated by chelating nitrogen donor ligands

During our studies on the reductive carbonylation of nitrobenzene, we have found that palladium metal supported on alumina is an effective catalyst for the formation of phenyl isocyanate, PhNCO, when activated by ortho-phenanthroline or its derivatives, and by 2,4,6-trimethylbenzoic acid (TMBA) (T = 180 °C, P(CO) = 40 atm, in dry benzene). In similar conditions (T = 200 °C, P(CO) = 100 atm in toluene) rhodium metal showed very poor activity. However, when methanol was also present in the reaction medium, N-phenyl methyl carbamate was obtained in good yields and selectivities. The presence of TMBA was not required in the latter case. In both cases, at the end of the reaction, the catalytic solution showed the presence of metal complexes, suggesting the formation in situ of homogeneous catalytic systems. In fact, preformed Pd(O₂CC₆H₂-2,4,6-Me₃)₂ and Rh₄(CO)₁₂ gave even better results when used as catalysts in the same experimental conditions.     

Access to lincomycin N-oxide isomers controlled by reaction conditions

Oxidation of lincomycin with H₂O₂ in alkaline media leads to N-oxides, besides the conversion of thiomethyl group into sulfoxides and sulfones. NH₄OH favors formation of the S-isomer; both R- and S-isomers of the N-oxide are formed in the presence of NaOH. Addition of acetonitrile markedly accelerates the reaction.     

Short synthesis of methylenecyclopentenones by intermolecular Pauson-Khand reaction of allyl thiourea

N,N,N′-Trimethylallylthiourea promotes the intermolecular Pauson-Khand reaction with alkynes in the presence of Co₂(CO)₈ and moderate pressure of CO followed by thiourea elimination allowing the formation of methylenecyclopentenone derivatives.     

Generation and stabilization of free selenium-containing radicals by complex formation

The reactions of complex formation of some p-XC₆H₄SeSeC₆H₄p-X-MX₃-benzene (or cyclohexane) systems, where X = H, CH₃, F, Cl, Br and MX₃ = GaCl₃ AlBr₃, have been studied by calorimetry, cryoscopy, dielectric measurements, GLC, and ESR spectroscopy. Diaryldiselenides react with gallium trichloride in solution forming 1:1 complexes. The enthalpies of formation and the dipole moments of the complexes have been determined. Formation of diaryldiselenides complexes with aluminium bromide, which is a stronger acceptor, involves cleavage of the SeSe and CSe bonds in the diselenide molecule and yields stable selenium-centred radical complexes of the type ArSeSe^? · AlBr₃ (g-value 2.073) and ArSe^? · AlBr₃ (g value 2.033). In addition, the ESR spectrum reveals a singlet (g value 2.0025) assigned to a hydrocarbon radical. Corresponding di- and mono-selenides as well as biphenyls have been identified by GLC in the products of interaction of free radicals displaced from the mixture of radical complexes by an electron donor (diethyl ether, water) stronger than the radical.     

Oxidation of arylamidoximes by various chemical and biomimetic systems: comparison with their oxidations by hemeproteins

Oxidation of 4-chlorobenzamidoxime by various chemical systems leads to the corresponding amide and nitrile, and to three dimeric compounds in greatly variable amounts as a function of the oxidizing agent. Monoelectronic oxidants, such as Pb(OAc)₄ and Ag₂CO₃, selectively led to 4-chlorobenzonitrile, whereas hydroperoxides such as H₂O₂ and tBuOOH only led to 4-chlorobenzamide. Other oxidants like m-chloroperbenzoic acid and Br₂ gave more complex mixtures. Introduction of an ironporphyrin catalyst in the oxidation performed by tBuOOH and m-chloroperbenzoic acid resulted in a spectacular increase of the formation of 4-chlorobenzonitrile. These results suggest that high-valent porphyrin ironoxo species preferentially oxidize arylamidoximes in a manner similar to monoelectronic oxidants.     

Selective synthesis of (E)-triethyl(2-arylethenyl)silane derivatives by reaction of aryl bromides with triethyl vinylsilane catalysed by a palladium-tetraphosphine complex

Cis, cis, cis-1,2,3,4-tetrakis(diphenylphosphinomethyl)cyclopentane / 0.5 [PdCl(C₃H₅)]₂ system catalyses the Heck reaction of vinylsilane derivatives with a range of aryl bromides with high ratio substrate/catalyst in good yields. The formation of mixtures of styrene, (E)-triethyl(2-arylethenyl)silane and triethyl(1-arylethenyl)silane derivatives was observed in some cases. Very high selectivities (up to 100%) in favour of the formation of (E)-triethyl(2-arylethenyl)silane derivatives were obtained in the presence of sodium acetate as base. With other bases such as potassium carbonate, the formation of large amounts of styrene derivatives was observed. The reaction tolerates several functions such as fluoro, trifluoromethyl, methoxy, dimethylamino, acetyl, formyl, benzoyl, carboxylate, nitro or nitrile. Moreover, turnover numbers up to 10,000 can be obtained for this reaction.     

Oxysulfides formed by a rare earth and a second metal: II. Shear structures

Many of the oxysulfides containing two metals have layered structures of (RO) sheets and (M_xS_y) sheets (R = rare earth, M = Cu, Ag, Ga, In, Ge, Sn, As, Sb, Bi). In the new compounds (La₄O₃)(AsS₃)₂ and (La₅O₃)In₆S₁₇, parallel ribbons of stoichiometry (La₄O₃) and (La₅O₃) substitute for the (LaO) sheets. The formation of ribbons is explained by shear mechanisms involving two different orientations of the periodic shear planes in the tetragonal (LaO) sheet. In a third compound La₃GaS₅O or (La₂O)LaGaS₅, the formation of (La₂O) ribbons involves similar shear mechanism. All these shear structures preserve the main characters of the layered oxysulfides: oxygen is only bound to lanthanum, and the second metal is only bound to sulfur.     

522—Catalytic oxydation of biological components on platinum electrodes modified by adsorbed metals: Anodic oxidation of glucose

The catalytic oxidation of glucose on Pt electrodes modified by adsorbed metals was studied in 1 M HClO₄ by linear sweep voltammetry. The adsorbed metals (denoted as M_ad, such as Bi_ad and Pb_ad) formed on Pt in the potential region more positive than the reversible potential of an M⁼⁺/M^o couple, lead to a marked increase in the anodic c?urrent of glucose by about one order of magnitude. The catalytic activity depends on the surface coverage by the M_ad. The strongly adsorbed species of lactone type, which are responsible for blocking the successive oxidation, are formed on the electrode surface in the anodic processes of glucose on a bare Pt electrode. The formation of such poisonous species is accelerated in the presence of adsorbed hydrogen on Pt. The effects of M_ad were discussed on the basis that M_ad plays its major role on the Pt electrode surface in removal of the adsorbed hydrogen which initiates the formation of the poisonous species.     

Modification of palladium-based catalysts by chalcogenes for direct methanol fuel cells

Palladium-based catalysts, such as PdS_x/C and PdSe_x/C, have been developed as oxygen reduction catalysts for direct methanol fuel cells. Pd/C catalysts containing chalcogens have been synthesized and tested for oxygen reduction and the results have been analyzed based on changes in the palladium phase. Selenium addition to the catalyst promotes the oxygen reduction due to the modification of the palladium surface. However, sulfur reduces the oxygen reduction activity to a great extent as a result of semi-amorphous palladium phase formation. Both PdS_x/C and PdSe_x/C are highly methanol tolerant.     

Stabilization of highly reactive cyclic polyolefins by coordination to iron carbonyls : Methylpentalenediiron pentacarbonyl

Thermolysis of (cis-bicyclo[6.1.0]nonatriene)diiron hexacarbonyl (FeFe) involves rearrangement of the starting organic moiety with formation of four iron carboyl complexes. The major product is the known cis-8,9-dihydroindeneiron tricarbonyl complex (VI). Two complexes have the same formula, C₉H₈Fe₂(CO)₅ (VII and VIII); VII can be also obtained by reaction of VI with Fe₂(CO)₉, while VIII is a methylpentalenediiron pentacarbonyl complex and represents a further example of stabilization of this reactive organic molecule by coordination; IX is probably a polycyclic cyclopentadienyl derivative [C₉H₉Fe(CO)₂]₂.Possible mechanisms for the formation of the four compounds are discussed.