Conversion of 2-propanol over chromium orthophosphates

The conversion (dehydration/dehydrogenation) of 2-propanol on a series of CrPO₄ (Cr/P=1) catalysts, differently prepared and thermally treated at 773–1073 K, has been studied by microcatalytic pulse reactor technique at different temperatures (473–573 K). Catalytic performance is strongly affected by the precipitation agent. Catalysts obtained in propylene oxide-aqueous ammonia showed the highest activity towards propene compared to other catalysts. Calcination at increasing temperatures caused a decrease in activity due to the decrease in surface acid character. Dehydrogenation to 2-propanone occurs to a small extent at all reaction temperatures. Propene selectivity strongly increases with increasing reaction temperature.     

Thermal and optical control of electronic states in a single layer of switchable paramagnetic molecules

The control of the charge distribution in a monolayer of switchable cobalt–dioxolene complexes undergoing Valence Tautomerism (VT) has been achieved by means of thermal and optical stimuli. Thiol-derivatised VT molecules have been grafted on polycrystalline gold surface as monolayers from solution. X-ray photoelectron spectroscopy and time-of-flight secondary ions mass spectrometry evidenced the formation of a covalent bond between intact VT molecules and the surface and excluded the presence of physisorbed molecules. X-ray absorption spectroscopy revealed that the temperature- and light-induced conversion profiles of the monolayer closely reproduce the ones found for the crystalline phase. This study demonstrates that a wet chemistry based approach allows to transfer switchable paramagnetic molecules at the nanoscale, widening the playground to develop new hybrid molecular based architectures for novel technologies.     

Quantum chemical investigation by the AM1 method of the catalytic cyclodehydration of 1,4-butanediol

1,4-Butanediol is converted into tetrahydrofuran in the presence of silica gel at temperatures of 245–340°C. The mechanism of the conversion of 1,4-butanediol has been studied by the quantum chemical AM1 method with full optimization of the geometry. It was established that two parallel reactions are possible depending on the conformation of the molecule. The cyclodehydration reaction is effected by conversion of a semicyclic conformer of the protonated diol molecule at a basic center of the catalyst by a concerted mechanism. The heat of the reaction forming tetrahydrofuran is —161.768 kcal/mole.Latvian Institute of Organic Synthesis, Riga LV-1006. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 608–613, May, 1998.     

Characterization of the thermally induced topochemical solid-state transformation of NH4[N(CN)2] into NCN[double bond]C(NH2)2 by means of X-ray and neutron diffraction as well as Raman and solid-state NMR spectroscopy

The mechanism of the solid-solid transformation of NH(4)[N(CN)(2)] into NCN[double bond]C(NH(2))(2), which represents the isolobal analogue of W?hler's historic conversion of ammonium cyanate into urea, has been investigated by temperature-dependent single-crystal and powder X-ray diffraction, neutron powder diffraction, and Raman and solid-state NMR spectroscopy as well as thermoanalytical measurements. The transformation of the ionic dicyanamide into its molecular isomer upon controlled thermal treatment was found to proceed topochemically in the solid state with little molecular motion, giving rise to a single-crystal to single-crystal transformation which manifests itself by a defined metric relation between the unit cells of the two isomers. The exothermic phase transition is thermally activated and was observed to commence at temperatures > or =80 degrees C. The pronounced temperature dependence of the onset of the transformation may be assessed as an indication for the metastability of ammonium dicyanamide at elevated temperatures. Thermal analyses reveal a decrease in the reaction enthalpy (56-13 kJ mol(-1)) at higher heating rates and an average mass loss of 10% gaseous ammonia. Evidence was found for crucial mechanistic steps of the transformation, which is likely to proceed via proton transfer from the ammonium ion to one of the terminal nitrogen atoms of the anion. The protonation is followed by nucleophilic attack of the in situ generated ammonia at the electrophilic nitrile carbon. The proposed mechanistic pathway is based on the results of combined Raman and solid-state NMR spectroscopic as well as neutron powder diffraction measurements.     

Laser-induced fluorescence monitoring of higher alkanes production from pure methane using non-oxidative processes

A novel method for the study of non-oxidative methane conversion process into higher value hydrocarbon and hydrogen has been invented. The method involves the multiphoton dissociation of methane under the influence of the high power pulsed ultraviolet laser radiation at 355 nm wavelength at room temperature (293 K) and standard pressure (1 atm). The products generated as a result of methane conversion like ethane, ethylene, propane, propylene and isobutane are analyzed using an online gas chromatograph while the other species such as CH, CH₂ and C₂H₂, atomic and molecular hydrogen are characterized by real-time laser-induced fluorescence technique for the first time. A typical 7% conversion of methane into ethane has been achieved using 80 mJ of laser irradiation at 355 nm. The important features of this method are that it is non-oxidative, does not require any catalyst, high temperatures or pressures, which is normally the case in conventional techniques for methane conversion.     

Vinyl-polymerization of norbornene with novel anilido–imino nickel complexes/methylaluminoxane: Abnormal influence of polymerization temperature on molecular weight of polynorbornenes

Herein reported are investigations of norbornene polymerization by novel anilido–imino nickel complexes [(Ar¹NCHC₆H₄NAr²)NiBr]₂ (Ar¹ = Ar² = 2,6-dimethylphenyl, 1; Ar¹ = 2,6-dimethylphenyl, Ar² = 2,6-diisopropylphenyl, 2; Ar¹ = Ar² = 2,6-diisopropylphenyl, 3; Ar¹ = 2,6-diisopropylphenyl, Ar² = 2,6-dimethylphenyl, 4) activated with methylaluminoxane (MAO). It was found that at polymerization temperatures below 50 °C, the average molecular weights of the obtained polynorbornenes catalyzed by these four catalytic systems increase with raising temperature, displaying bimodal distribution in GPC curves. The abnormal influence of polymerization temperature could be attributed to the existence of two kinds of catalytic species: heterobimetallic species LNi(II)(μ-Me)₂AlMe₂ (I) and monometallic species LNi(II)Me (II) (L = anilido–imino ligand) at lower temperature. The former affords a lower molecular weight polymer and the latter higher molecular weight one. With raising polymerization temperature above 50 °C, the species I disappears and only species II exists in polymerization systems, resulting in a normal relation of molecular weight to polymerization temperature. From a kinetic study of the norbornene polymerization catalyzed by 1/MAO catalyst at 70 °C, the polymerization rate (R_p) can be expressed by the formulation: R_p = k[NBE]^1.93[Ni]^0.88. Moreover, the mechanism of the norbornene polymerization using the anilido–imino nickel complexes activated with MAO is also presented and discussed.     

Synthesis and properties of amphoteric copolymer of 5‐vinyltetrazole and vinylbenzyl phosphonic acid

Amphoteric polymers have been studied for various applications such as separation of low molecular weight organic molecules from inorganic salt mixtures, selective ion transport, drug delivery through membranes of biological interest, separation of ionic drugs and proteins, and separation of alcohol and water. Typical amphoteric polymers consist of weak base and weak acid groups. In present study, the copolymerization of 5‐vinyltetrazole (VT) and diisopropyl‐p‐vinylbenzyl phosphate (DIPVBP) via free radical polymerization is studied. The reactivity ratio of VT and DIPVBP, which is calculated from Kelen‐Tudos plot, is 0.251 and 0.345, respectively. The amphoteric copolymer of VT and diisopropyl‐p‐vinylbenzyl phosphonic acid (poly(VT‐co‐VBPA)) is obtained from hydrolysis of the copolymer of VT and DIPVBP (poly(VT‐co‐DIPVBP)). Poly(VT‐co‐VBPA) is thermally stable under 190 °C. The anhydrous proton conductivity of amphoteric poly(VT‐co‐VBPA) can reach 1.54 × 10^‐4 S cm^?1 at 170 °C with an activation energy of 114.7 kJ mol^?1. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013 , 51, 3486–3493     

Molecule-based valence tautomeric bistability synchronized with a macroscopic crystal-melt phase transition

Here, we show a synchronic bistability of valence tautomeric (VT) molecular interconversion and a macroscopic crystal-melt phase transition in long alkoxy-functionalized cobalt-dioxolene complexes. Studies have been carried out for a novel series of complexes, [Co(CnOpy) 2(3,6-DTBQ)2] (3,5-dialkoxy(C(n)H2(n+1)O-; n = 9, 12, and 17)pyridine (CnOpy) and 3,6-di-tert-butyl semiquinonate or catecholate ligands (3,6-DTBQ)). All complexes show the molecular VT interconversion with thermal hysteresis attributed to the synchronous crystal-melt phase transition. Thermodynamic analysis has revealed that the molecular VT interconversion is restricted over 50 K and crystal-melt phase transition is accelerated about 50 K by the synchronicity. The synchronicity is attributed to enthalpic and entropic effects of the alkoxy chains in the crystalline phase of the one tautomer and the melt phase of the other, respectively. Our results show efficient chemical and thermodynamic strategies to combine molecule-based and macroscopic bistabilities.     

Highly enantioselective lipase-catalyzed reactions at high temperatures up to 120°C in organic solvent

Lipase-catalyzed kinetic resolutions of 1,1-diphenyl-2-propanol were performed at high temperatures up to 120°C. Burkholderia cepacia lipase immobilized on porous ceramic particles, lipase PS-C II (Amano Enzyme Inc.), gave an enantiopure product at 40–120°C, with the highest conversion (39%) at 80–90°C. The mechanism of high enantioselectivity retained even at 120°C is also described briefly.     

Photophysics of inter- and intra-molecularly hydrogen-bonded systems: Computational studies on the pyrrole–pyridine complex and 2(2′-pyridyl)pyrrole

The role of electron and proton transfer processes in the photophysics of hydrogen-bonded molecular systems has been investigated with ab initio electronic-structure calculations. We discuss generic mechanisms of the photophysics of a hydrogen-bonded aromatic pair (pyrrole–pyridine), as well as an intra-molecularly hydrogen-bonded π system composed of the same molecular sub-units (2(2′-pyridyl)pyrrole). The reaction mechanisms are discussed in terms of excited-state minimum-energy paths, conical intersections and the properties of frontier orbitals. A common feature of the photochemistry of these systems is the electron-driven proton transfer (EDPT) mechanism. In the hydrogen-bonded complex, a highly polar charge transfer state of ¹ππ^* character drives the proton transfer, which leads to a conical intersection of the S₁ and S₀ surfaces and thus ultrafast internal conversion. In 2(2′-pyridyl)pyrrole, out-of-plane torsion is additionally needed for barrierless access to the S₁–S₀ conical intersection. It is pointed out that the EDPT process plays an essential role in the fluorescence quenching in hydrogen-bonded aromatic complexes, the function of organic photostabilizers, and the photostability of biological molecules.     

Solvation effects on the valence tautomeric transition of a cobalt complex in the solid state

A detailed investigation of a valence tautomeric (VT) transition for the new complex [Co(III)(3,5-DBCat)(3,5-DBSQ)(py)?]/[Co(II)(3,5-DBSQ)?(py)?] (1) is reported, where 3,5-DBCatH? is 3,5-di-tert-butyl-catechol, 3,5-DBSQH is 3,5-di-tert-butyl-semiquinone and py is pyridine. Complex 1 exists as a mixture of the two valence tautomers, with the relative proportion of each depending on the external conditions. Three differently solvated forms of the complex have been synthesized and variable temperature structural and magnetic investigations of one of these, 1·0.5py, reveals that this compound undergoes a thermally-induced VT transition from the [Co(III)(3,5-DBCat)(3,5-DBSQ)(py)?] tautomer at temperatures below 150 K to a 1 : 1 mixture of the two tautomers at temperatures above 300 K. The VT transition may also be photo-induced at 9 K, affording a similar mixture of the two tautomers. In both cases the incomplete transition is attributed to the presence of π-π stacking interactions between the pyridine molecules of solvation and one of the two crystallographically independent complex molecules, which inhibits the expansion of this molecule that would accompany a VT transition. Studies on alternatively solvated forms 1·2MeCN and 1·1.67hexane also suggest a significant dependence of the VT transition on solvation-induced packing effects and/or intermolecular interactions.     

Methane conversion by various metal, metal oxide and metal carbide catalysts

The progress in the field of methane conversion into higher hydrocarbons including aromatics and oxygenated compounds in the recent five years will be reviewed shortly, together with a new type of the methane conversion reaction with carbon monoxide at lower temperatures (600–700 K) by supported group VIII metal catalysts. Benzene was formed selectively among hydrocarbons in the CH₄–CO reaction over silica-supported Rh, Ru, Pd and Os catalysts under atmospheric pressure. Both CH₄ and CO were required for benzene formation, and only ethane and ethylene were formed besides benzene. The amount of C₃–C₅ hydrocarbons was negligible, which suggests that a completely different mechanism from the CO–H₂ reaction may be operating over these catalysts despite of the similarity in the reaction conditions with the CO–H₂ reaction. The mechanism of benzene formation was studied deeply by means of kinetical investigation as well as infrared spectroscopy and isotopic tracer method in connection with that of CO hydrogenation.     

On the role of PIII ligands in the conjugate addition of diorganozinc derivatives to enones

A mechanistic study on the conjugate addition of diethylzinc to cyclohexenone catalyzed by various chiral PIII ligands, provides new insights into its mechanism. Complete in situ conversion of the catalytic amount of Cu(OTf)2 into CuI species by excess ZnEt2 is demonstrated by EPR spectroscopy. Experimental evidence is presented in favor of a critical ternary 1:1:1 complex between enone, Et2Zn and catalyst, supporting a rate-limiting reductive elimination or carbocupration from a preformed mixed Cu/Zn cluster carrying one- or two-ligand molecules. A crystal structure has been obtained for a CuIL2 complex, which shows catalytic turnover on addition of reagent and substrate. NMR spectroscopic analyses and VT experiments reveal that steric hindrance may prevent complexation of a second ligand molecule on the CuI cation, leading to extremely fast precatalysts based on CuX.L species.     

H NMR study of hydrogen bonding and molecular dynamics of 5CB confined to molecular sieves

It has been found that the main mechanism of ¹H protons spin–lattice relaxation of bulk 5CB at 200 MHz is its intramolecular motion, namely, the reorientation of CH₂ and CH₃ groups of its alkyl chain. Activation parameters of such motions have been estimated.

Drastic decrease in proton spin–lattice relaxation times at the nematic-to-isotropic phase transition can be explained by the activation of molecular translational diffusion and reorientations around long and short molecular axes of bulk 5CB.

Our NMR analysis revealed the slowing-down of molecular dynamics of confined 5CB molecules and their fragments. This can be explained by the interaction of some part of 5CB molecules with the surface active Si(Al)–OH centers of MCM matrix via hydrogen bonds of Si(Al)–OHN≡C-type.     

Influence of Intermolecular Interactions on Valence Tautomeric Behaviors in Two Polymorphic Dinuclear Cobalt Complexes

Two polymorphic structures have been well determined in a valence tautomeric (VT) dinuclear cobalt complex. These polymorphs showed distinct thermal‐ and photomagnetic behavior, and are thus ideal for studying the “pure” intermolecular factors to VT transitions. In polymorph 1A , the VT cations are arranged head‐to‐waist with their neighbors and exhibit weak π???π interactions, resulting in a gradual and incomplete thermal VT transition. In contrast, the cations in polymorph 1B are arranged head‐to‐tail and exhibit relatively strong π???π interactions, leading to an abrupt and complete thermal VT transition with adjustable hysteresis loop at around room temperature. The VT process for both polymorphs can be induced by light, but the light‐excited state of 1B? 2H₂O has a higher thermal relaxation temperature than that of 1A? 3H₂O.     

Thermodynamics and mechanism of oxidation of aqua(ethylenediaminetetraacetato)chromium(III) in ethanolwater solvents

Summary The kinetics of the oxidation of aqua(ethylenediaminetetraacetato)chromium(III) by periodate have been investigated in various ethanol: water mixtures in the 0–54 wt% ethanol range, at six different temperatures in the 15–40°C range. The effect of solvent on the rate and mechanism of the reaction has been studied and an innersphere mechanism for the reaction is proposed, consistent with the calculated activation parameters.     

Conversion of anisole in the presence of methanol over AlPO4?Al2O3 catalysts modified with fluoride and sulfate anions

The gas-phase microcatalytic conversion of anisole in the presence of methanol (methanol/anisole molar ratio=4) was studied at temperatures ranging from 523 to 673 K over anion treated (1–3 wt.% F or SO ₄ ^2– ) AlPO₄–Al₂O₃ (25 wt.% Al₂O₃) catalysts. Anisole conversion gave a mixture of dealkylated and C-alkylated products (C-alkylation preferentially in ortho-position) where dealkylation always predominates. The influences of the reaction temperature and both anion type and anion loading upon the conversion of anisole and the selectivities of the products were investigated. The higher increase in surface acidity by fluoride loading increases both the C-alkylation selectivity (mainly to 2-methylanisole) and dealkylation to phenol; so that a lower anisole conversion (smaller pseudokinetic constant) and higher methylanisoles selectivity is found for APAI-P-F catalysts related to unmodified one.     

Vibrational excitation and relaxation of five polyatomic molecules in an electrical discharge

Vibrational excitation and relaxation of five linear polyatomic molecules, OCS, OC3S, HC3N, HC5N, and SiC2S, have been studied by Fourier transform microwave spectroscopy in a supersonic expansion after the application of a low-current dc electric discharge. For each chain, the populations in bending and stretching modes have been characterized as a function of the applied discharge current; for stable OCS and HC3N, vibrational populations were studied as well in the absence of a discharge. With no discharge present the derived vibrational temperatures are slightly below T, the temperature of the gas before the supersonic expansion (i.e., 300 K). In the presence of the discharge, vibrational excitation occurs via inelastic collisions with the electrons and the vibrational temperatures rise as the applied current increases. Global vibrational relaxation is governed by rapid vibration-vibration (VV) energy transfer and slow vibration-translation (VT) energy transfer. The latter process is rate-determining and depends primarily on the wave number of the vibration. Vibrational modes with wave numbers near and below kT/hc (where T = 300 K and kT/hc-210 cm(-1)) are efficiently cooled by VT transfer because a sufficient number of collisions occur in the initial stages of the supersonic expansion. Vibrational modes with wave numbers around 450 cm(-l) appear to be inefficiently cooled in the molecular beam; at these energies VV and VT rates are probably comparable. For high-frequency vibrations, VV energy transfer dominates. For the longer chains OC3S and HC5N, higher-lying modes are generally not detectable and vibrational temperatures of most lower-lying modes were found to be lower than those of OCS and HC3N, suggesting that as the size of the molecules increases, intermode VV transfer becomes more efficient, plausibly due to the higher density of vibrational levels. New high resolution spectroscopic data have been obtained for several vibrationally excited states of OC3S, HC3N, and HC5N. Rotational lines of the 13C and 15N isotopic species of HC5N have been measured, yielding improved rotational and centrifugal distortion constants; 14N nitrogen quadrupole coupling constants for the isotopic species of HC5N with 13C have been determined for the first time.     

Less toxic acetylacetonates as initiators of trimethylene carbonate and 2,2‐dimethyltrimethylene carbonate ring opening polymerization

This article presents the results of TMC and DMC polymerization with the use of acetylacetonates of low‐toxic metals: iron, zinc, and zirconium. Zinc (II) acetylacetonate proves to be a very good initiator of homopolymerization. The reaction carried out with the use of this initiator at 110 °C is very rapid and of high yield. Using both zinc and iron (III) acetylacetonates, as well as the zirconium (IV) one, in high temperatures it is possible to obtain PTMC possessing high molecular mass, thus ensuring optimization of the relation between the duration of the polymerization and its yield. A strong influence of thermal degradation on the course of the reaction has been observed, particularly at 160 °C, with the use of Fe(acac)₃ as the initiator. DMC polymerization proceeds much more slowly when initiated by iron and zinc acetylacetonates. A high conversion of the monomer is obtained in this case as well. The relation between the molecular mass of the obtained PDMC and the conversion of the monomer is directly proportional; however, those masses, determined on the basis of polystyrene standards, are much lower than those estimated theoretically. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1913–1922, 2005     

Reactivity of oxide precursor states on Ru(0001)

Smooth and defect-rich Ru(0001) surfaces prepared under ultrahigh-vacuum (UHV) conditions have been loaded with oxygen under high-pressure (p 相似文献