Multipolar complexes–II The self association of -NO2 -CN and -COCN polar groups

Fluorine nuclear magnetic resonance (F-NMR) has been used to determine dissociation constants for multipolar complexes formed by several p-substituted fluorobenzenes in CCl₄ and cyclohexane at 25°. Data in CCl₄ were also obtained at ?20°. The results are interpreted in simplified terms as dipole-dipole interactions which lead to a complex of net zero dipole moment. Although the dissociation constants are not of high accuracy, the values obtained do indicate that both polarity and steric effects are important in the presumed multipolar complexes.     

Miscible blends prepared from two crystalline polymers

Differential scanning calorimetry was used to determine the miscibility behavior of several polyester/Saran blends, the two polymers forming these blends being semicrystalline. It was found that Saran is miscible with polycaprolactone (PCL), polyvalerolactone, poly(butylene adipate), and poly(hexamethylene sebacate) since a single glass transition temperature T_g was observed at each composition. However, immiscibility was found between Saran and poly(ethylene adipate), poly-(ethylene succinate), poly(β-propiolactone), and poly(α-methyl-α-n-propyl-β-propiolactone) since two T_g's were recorded at several compositions. Blends were then obtained containing, over a wide range of composition, a miscible amorphous phase and two different types of crystals. From melting-point depression data on PCL and Saran crystals, thermodynamic interaction parameters χ were calculated and found to be different for PCL-rich blends and for Saran-rich blends. This result suggests a variation of χ with composition. Saran is a polymer which does not contain α-hydrogens and its miscibility with polyesters may result from a β-hydrogen bonding interaction or a C?O/C? Cl dipole-dipole interaction.     

Theoretical investigation on cyclohexane dehydrogenation catalyzed by V<Subscript>2</Subscript><Superscript>+</Superscript> in gas-phase

The dehydrogenation reaction mechanism of cyclohexane catalyzed by dimer transition metal cluster V₂⁺ has been investigated at the B3LYP/6-31G (d, p) level of density functional theory. Density of states (DOS) graph is used to understand more deeply the roles of the front molecular orbital of the initial complexes. After the first molecular dehydrogenation, the reaction mainly consists of two competition mechanisms. First, the C-H bonds of cyclohexane can be effectively activated by the V₂⁺ cation, yielding the same-face dehydrogenation products. Second, the C-C bonds are activated, forming the different-face dehydrogenation products. Our calculations indicate that the reaction takes place more easily along the low-spin potential energy surface on the same-face and is a low-barrier or even barrier-free transformation. Carbon-carbon single bonds are nonpolar and generally far less reactive. A comparison of the reaction mechanism of V₂⁺ and congener Ti₂⁺ with cyclohexane has been presented. The bond dissociation energies (BDEs) of V₂⁺ are greater than that of Ti₂⁺, leading to difficulties in forming sandwich complexes in the different-face dehydrogenation of cyclohexane, and the same-face dehydrogenation is an important reaction channel.     

Preparation of activated carbon from date pits: Effect of the activation agent and liquid phase oxidation

Two series of activated carbons have been prepared from date pits; series C, using carbon dioxide as activating agent, and series S, prepared by activation with steam under the same experimental conditions. The obtained samples were oxidized with nitric acid in order to introduce more oxygen surface groups. The surface area and porosity of the parent and oxidized activated carbons were studied by N₂ adsorption at 77 K and CO₂ adsorption at 273 K. The oxygen surface complexes were characterized by temperature-programmed decomposition (TPD). The results show that carbon dioxide and steam activations produce microporous carbons with an increasing amount of CO evolving groups when increasing the burn-off. On the other hand, oxidation with nitric acid increases the amount of CO and CO₂ evolved by the decomposition of surface oxygen groups, this increase being related to the development of porosity in the carbon with the degree of activation and to the activating agent used (CO₂ versus steam).     

Synthesis and adsorption properties of colloid-imprinted mesoporous carbons using poly(vinylidene chloride-co-vinyl chloride) as a carbon precursor

A facile synthesis of micro- and mesoporous carbons has been proposed using colloidal silica nanoparticles with diameter of ∼24 nm and poly(vinylidene chloride-co-vinyl chloride) (Saran) as a carbon precursor. The resulting carbons possessed large specific surface area, ∼800 m²/g, and approximately the same volume of micro- and mesopores, each about 50% of the total pore volume. While the size of micropores was around 1 nm, the large and uniform spherical mesopores (about 24 nm) resemble the diameters of silica colloids used. Nitrogen adsorption measurements proved that these mesopores were interconnected and accessible. The well-developed microporosity was created mainly by decomposition of Saran copolymer during carbonization.     

IR,NMR, XPS study of 1-(d-3-mercapto-2-methylpropionyl)-l-proline and its zinc complexes

Two Zn(II) complexes of captopril were prepared and characterized with different analytical methods: IR, NMR and XPS. Captopril is a molecule which has been proven to be very effective in reducing blood pressure. The ligand chelates via the OCO group to a zinc ion and via S and amidic oxygen atoms to another zinc ion when the ZnL complex is obtained. The sulphur atom and the amidic CO group are the only atoms involved in the coordination when the sodium—zinc complex, Na₂ZnL₂, is synthesized.     

A modified oxidative microcoulometric method for determination of sulphur in hydrocarbons containing large amounts of chlorine

The oxidative coulometric method for trace sulphur determinations has been modified and a procedure is described which includes the elimination of the interferences caused by chlorine whilst retaining a high recovery of sulphur. The liquid hydrocarbon sample is combusted in an excess of oxygen at 1000 K followed by dilution with a proper flow of carbon monoxide at 1300 K. In this way the partial pressure of oxygen is kept small and the interfering chlorine compounds are effectively converted into hydrogen chloride which does not interfere with the coulometric titration. A recovery of sulphur of 96 +/- 1% was found for thiophene in mixtures of chlorobenzene (0-10%) and cyclohexane, thus indicating the absence of significant interference.     

Simultaneous determination of trace amounts of sulphur hexafluoride and cyclic perfluorocarbons in reservoir samples by gas chromatography

Summary Both sulphur hexafluoride and a group of compounds among the perfluorinated cyclic hydrocarbons: perfluorodimethyl cyclobutane, perfluoromethyl cyclopentane, perfluoromethyl cyclohexane and the 1,2- and 1,3-isomers of perfluordimethyl cyclohexane are well known to be good gas tracers, in various applications. Unfortunately different methods have been used for the analysis of SF₆ and the perfluorocarbons. Therefore, in many cases, the analytical costs may have hindered the use of both SF₆ and the perfluorocarbons in the same tracer experiments. The analytical method described in this paper enables the determination of both types of gas tracer in one single run. The new method has already replaced two former methods and was used to run the gas tracer analysis routinely, proving the reliability of the method.     

Excess volume of mixing of some polar and nonpolar liquids with propylene carbonate

The excess volume of mixing as a function of composition has been measured at 30°C and 40°C for mixtures of propylene carbonate with nitrobenzene, chlorobenzene, benzene, toluene, cyclohexane, dioxane, carbon tetrachloride, and chloroform. The highly polar nitrobenzene forms an ideal mixture with propylene carbonate. Chloroform, carbon tetrachloride, dioxane, chlorobenzene, benzene, and toluene give negative volume changes on mixing. In mixtures with cyclohexane,V _m ^E is positive at lower mole fractions of cyclohexane but becomes negative as the mole fraction of cyclohexane increases.     

Determination of the thermodynamic polymer-polymer interaction parameter of miscible blends prepared from two crystalline polymers

The melting points of miscible blends prepared from two crystalline polymers, poly(caprolactone) (PCL), and Saran a random copolymer of vinylidene chloride with vinyl chloride [P(VCl₂-VC)], vinyl acetate [P(VCl₂-VA)], or acrylonitrile [P(VCl₂-AN)], have been measured by differential scanning calorimetry as a function of the crystallization temperature. The equilibrium melting points of these blends have been determined by Hoffman-Weeks plots. From two series of data, one of which was obtained by measuring the melting points of PCL crystals and the other by measuring the melting points of Saran crystals, the thermodynamic polymer-polymer interaction parameters of PCL/Saran blends have been calculated over the full range of composition. The two series of data merge into a smooth curve, which is composition dependent, despite the fact that the melting points of PCL and P(VCl₂-VC) or P(VCl₂-VA) are very different at 58.1, 183.5, and 184.2°C, respectively. Calculations using the “equation of state” Prigogine-Flory thermodynamic theory indicate that the temperature dependence of the thermodynamic interaction parameter in typical polymer blends is small, which agrees with the experimental results.     

A comparative study of some oxygen and sulphur substituted alkyl radicals

Several sulphur containing alkyl radicals have been generated and their ESR spectra and hyperfine coupling constants compared with those of the oxygen analogues. Deviations from planarity are smaller when a tervalent carbon atom is bonded to sulphur than when bonded to oxygen. Moreover greater delocalization of the unpaired electron is apparent in the sulphur radicals and this leads to higher barriers to internal rotation in radicals of the form H₂SR compared with H₂OR.     

Mg/K2S2O8‐promoted direct carboxylation of saturated hydrocarbons with CO

Saturated hydrocarbons react with carbon monoxide in the presence of magnesium powder and potassium peroxodisulfate in trifluoroacetic acid (TFA) to afford the corresponding carboxylic acids as major products and alkyl trifluoroacetates as minor products in high yields. The use of equimolar amounts of magnesium (5 mmol) and K₂S₂O₈(5 mmol) is best for this reaction. Cyclohexane is carboxylated most effectively at 80 °C for 30 h using 50 atm CO, 3 ml of TFA and 1 mmol of cyclohexane, giving 80% conversion (95% conversion yield) based on cyclohexane. The reactivity of various saturated hydrocarbons has been determined by competitive reactions of equimolar amounts of cyclohexane (5 mmol) and another hydrocarbon (5 mmol). The order of decreasing reactivity obtained is cycloheptane > cyclohexane > cyclopentane > n‐propane > methane. Copyright ­© 1999 John Wiley & Sons, Ltd.     

吸附与润湿II.Triton X-100和Triton x-305在炭黑/水和炭黑/环己烷界面上的吸附层结构

测定了15℃和30℃时炭黑自水和环己烷中吸附非离子型表面活性剂TritonX-100和Triton X-305的等温线;计算了吸附过程的标准热力学函数;测定了石墨/水/环己烷和石墨/水/空气的接触角与表面活性剂浓度的关系, 分析所得结果,可得结论:在炭黑/水或石墨/水界面上,Triton型表面活性分子形成单分子吸附层,分子以憎水的iso-C8H17C6H4基团附着在表面,而以亲水的聚氧乙烯链伸入水相的方式取向;在炭黑/环已烷或石墨/环己烷界面上,分子是通过聚氧乙烯链吸附到表面上的,当浓度增加时分子在表面可能通过聚氧乙烯链间的相互作用而发生聚集,即可能形成表面反式胶团。     

A variable temperature H NMR study of stereochemical non-rigidity in group VI metal pentacarbonyl complexes of 1,3,5,7-Tetrathian; [M(CO)5( H2)] (M=Cr OR W)

Stereochemical non-rigidity in the complexes [M(CO)₅( H₂)] (M = Cr or W) has been studied by dynamic nuclear magnetic resonance spectroscopy. In the temperature range ca. −100 to 120°C two intramolecular processes were observed, namely pyramidal atomic inversion about sulphur atoms, and a commutation of the M(CO)₅ moiety between coordination sites on different sulphur atoms. Accurate energy barriers for both processes have been obtained by detailed computer simulations of the static and the dynamic spectra. The magnitudes of ΔG≠ (298.15 K) are compared with those reported for related complexes. The exact nature of the M(CO)₅ shift cannot be unambiguously assigned from the observed spectral line-shape changes. Two possible mechanisms are proposed and discussed.     

Atomic inversion barriers of sulphur and selenium in the dimethylsulphide and dimethylselenide complexes of the halogeno carbonyls of rhenium

We have synthesised four rhenium carbonyl complexes of general formula [ReX(CO)₃(Me₂E)₂] (X  Cl, Br, I, E  S, Se), and studied their temperature variable NMR spectra. All complexes were formed as the fac isomer, with the exception of [ReI(CO)₃(Me₂Se)₂], which was obtained as a mixture of mer and fac forms. In all of these fac complexes pyramidal inversion of sulphur or selenium atoms has been demonstrated, and energy barriers to inversion have been determined either by computer simulation of complete line shapes or by coalescence temperature methods. The value of ΔG^≠ for inversion in this class of complex has been found to be about 17 kJ mol^?1 higher for selenium than for sulphur, and variation of the cis halogen made no pronounced effect.     

Wetting and adsorption: II. Adsorption of Triton X-100 and Triton X-305 onto carbon black from water and cyclohexane solutions

The adsorption isotherms of nonionic surfactants Triton X-100 and Triton X-305 from water and cyclohexane on carbon black have been determined at 15 and 30°C. The Langmuir-type and BET-type isotherms are obtained for adsorption of Triton X-100 and Triton X-305 from water and cyclohexane respectively. Both the contact angles of water for graphite/water/air and graphite/water/cyclohexane decrease monotonously with increasing surfactant concentration. From these results, it is proposed that the adsorption of Triton X-100 and Triton X-305 on carbon black or graphite from water is monolayer. For the adsorption from cyclohexane solutions, the ethyleneoxide group of the surfactant molecules may be adsorbed onto the polar spot at the surface of carbon black, and the hydrophobic group of adsorbed molecules may direct toward the liquid phase or attaches to the nonpolar surface region around the polar spot. As the concentration increases, the ethylene oxide groups of the adsorbed molecules can be aggregated with each other via polar interactions to form hemi-reversed micelle.     

Selective polymeric effects on solvent NMR relaxations

Measurements of the spin-lattice relaxation times (T₁) of solvent protons have been performed on systems containing mixed solvents with and without polymer. It has been found that the motion of solvent is selectively affected by polymers present in the system. Polyisobutylene (10%) in mixed solvents of carbon tetrachloride (or cyclohexane) and dichloromethane at various proportions produces little effect on T₁ values of dichloromethane, but it affects significantly the T₁ values of cyclohexane; whereas poly(methyl methacrylate) (10%) in carbon tetrachloride and dichloromethane (or acetone) selectively associates with dichloromethane (or acetone), resulting in an approximate 50% reduction of the T₁ values for dichloromethane (or acetone). In systems of poly(methyl methacrylate) and three mixed solvents of carbon tetrachloride, dichloromethane, and cyclohexane, the polymer (10%) has a negligible effect on the T₁ values of cyclohexane, but brings about a 50% reduction of the T₁ values of dichloromethane. These phenomena are discussed in terms of local selective interactions between the solvent molecules and the polymeric chain segments.     

Synergistic extraction of radium using 2-thenoyltrifluoroacetone and tributyl phosphate or trioctylphosphine oxide

The extraction of radium into a mixture of 2-thenoyltrifluoroacetone (HTTA) and tributyl phosphate (TBP) or trioctylphosphine oxide (TOPO) in n-hexane or cyclohexane has been investigated with regard to the dependence on pH, and TBP and TOPO concentrations. It has been found that the formation of mixed complexes of the type Ra(TTA)₂ (TBP)₂ and Ra(TTA)₂(TOPO)₂ occurs and the overall extraction constants of both complexes were calculated. With the systems described, very high distribution ratios of radium have been attained; these may advantageously be used for the separation and concentration of traces of radium.     

A TPD-MS study of the adsorption of ethanol/cyclohexane mixture on activated carbons

The adsorption of ethanol/cyclohexane binary mixtures on different types of activated carbons was studied in this work by temperature programmed desorption coupled with mass spectroscopy (TPD-MS). The texture, morphology and surface chemistry of the carbons were evaluated by N₂ adsorption, scanning electron microscopy (SEM) and TPD-MS techniques. The ethanol and cyclohexane TPD-MS desorption profiles showed that specific interactions between the carbon material and the adsorbate are involved during the adsorption. Most of the activated carbons adsorb strongly ethanol on the surface, leading to desorption temperatures above 100 °C. Only one carbon exhibits an affinity for cyclohexane. These observations were correlated to the different surface chemistry of the materials.     

Observation and evaluation of the synchronous ligand atom pyramidal inversions in binuclear rhenium carbonyl halide complexes of disulphides and diselenides: an X-ray crystal structure of [Re2Br2(CO)6(C6H5CH2SeSeCH2C6H5)]

The dinuclear complexes [Re₂X₂(CO)₆(RCH₂EECH₂R)] (X = Cl or Br, R = Ph or Me₃Si, E = S or Se) have been prepared and characterized. A variable temperature ¹H NMR study on these complexes demonstrated the pyramidal atomic inversion process at the coordinated sulphur and selenium atoms. Total band-shape fittings were used to yield activation parameters for the rate process, in which the two sulphur or selenium atoms undergo synchronous or correlated inversion.