Influence de la tension de vapeur sur une transition de phases et un equilibre peritectoide

The compound As₄Se₃ exhibits two polymorphs, α andβ. The latter undergoes a peritectoid decomposition. The two phenomena require a vapour pressure action. They are both explained on the basis of aT- V- x diagram. Two cases are described. In the first hypothesis, no solid solution exists. The second hypothesis implies two solid solutions for the two forms α andβ. A comparative study of these two cases has been made. The existence of two invariant planes has been shown in the second case, which makes it rather different from the first one.     

Synthesis of structurally diverse benzimidazolyl benzimidazolones by application of soluble polymer support

Chemical stability and reactivity of a bifunctional polymer conjugate containing an ortho-amino arylamide linkage have been successfully exploited to achieve a parallel synthesis of methoxycarbonylated head-tail bis-benzimidazoles. Regioselective alkylation of the two nitrogens in the benzimidazolone moiety has been carried out by ipso-fluoro displacement, and N-alkylation to generate two diversities. Cleavage of the polymer support has resulted in two libraries of di- and tri-substituted benzimidazolyl benzimidazolones in high purity and high yield. All reaction steps have been monitored by ¹H NMR on the support directly.     

Resolution and chiroptical properties of the neurotoxin 1-trichloromethyl-1,2,3,4-tetrahydro-β-carboline (TaClo) and related compounds: quantum chemical CD calculations and X-ray diffraction analysis

Separation and stereochemical attribution of the two enantiomers of the neurotoxin 1-trichloromethyl-1,2,3,4-tetrahydro-β-carboline (TaClo) has been achieved by applying chromatography on a chiral phase HPLC column in hyphenation with circular dichroism (CD) spectroscopy (LC-CD coupling). Assignment of the absolute configuration of TaClo and its N-methyl analog has been achieved by quantum chemical CD calculations and has finally been confirmed by single-crystal X-ray diffraction analyses of the two enantiomers of N-formyl-TaClo as obtained in enantiomerically pure form by crystallization.     

Practical and theoretical aspects concerning the use of salt bridges in electrochemistry

The emf of electrochemical cells containing various sorts of salt bridges between various concentrations has been measured. The net diffusion potential over a salt bridge has been treated theoretically. An expression to calculate the net diffusion potential has been derived and tested on values estimated from the emf measurements. A merit factor has been calculated for a saturated KCl bridge separating two HCl solutions and compared with the merit factor estimated from emf measurements on the cell:Pt|Ag|AgCl, HCl(c₁)|KCl(sat)|HCl(c₂), AgCl|Ag|Pt The practical performance of two commonly used reference electrodes with salt bridges has been tested on emf measurements on KBr solutions.     

Cyclohexene-sulfur bichloride adducts.: Stereochemical considerations.

The reaction of sulfur dichloride with cyclohexene produces two isomeric trans-adducts of meso and dl configurations. Derivatives of both series have been obtained and configurations have been assigned by means of pseudoasymmetry. General principles for the conformational analysis of systems which contain two inverting 6-membered rings have been discussed and the positions of conformational equilibria have been evaluated. The new conformational effect-stabilization of “heteroconformer”—has been found in the case of dl-bis(2-chlorocyclohexyl) sulfoxide.     

Quest for inosito-inositols: synthesis of novel, annulated and conformationally locked inositols

A new family of annulated inositols (inosito-inositols) has been conceptualized. Naphthalene has been elaborated into novel cyclohexa-annulated neo- and chiro-inositols, with two additional hydroxyl functionalities, through a series of stereoselective oxyfunctionalization protocols. The trans-ring fusion present in these new annulated inositols ensures conformational locking.     

Nonlinear level crossing spectroscopy of highly excited levels in yttrium using stepwise laser excitation

A technique based on the detection of nonlinear level-crossing resonances in finite magnetic fields has been extended to the study of high-lying electronic levels excited by stepwise excitation with two cw dye lasers. The method has been applied to the determination of |A/g _J | values for four highly excited even-parity levels and two odd-parity levels in yttrium I. Using recently determined values of the magnetic hyperfine interaction constantA, Landég _J -factors have been deduced for these levels with an accuracy of about 1%. The results show significant differences from previous determinations.     

Frame-transformed R-matrix treatment for e−-CO scattering: A preliminary report

A modified R-matrix approach is here proposed to study e^?-molecule collisions for systems with permanent dipole moments. The scattering problem has been separately solved in two regions of space by using two different but physically equivalent formalisms in order to properly represent both strong short-range and weak long-range interactions.The R-matrix has been defined at the common boundary point to transform the solutions from the inner- to the outer-region frames of reference     

Transformation of aromatic compounds under the action of the basidiomycetesPhanerochaete sanguinea andCoriolus villosus

The pathways of the transformation of some aromatic compounds by rhe basidiomycetesPhanerochaete sanguinea andCoriolus villosus have been studied. It has been shown that the degradation of these compounds has an oxidative nature and depends on the type of substituents in the benzene ring and the propane chain. A difference has been found in the mechanisms of the reactions of the two fungi that is a consequence of the different compositions of their enzyme complexes.     

Infra-red and electronic absorption spectra of 2-bromopyrimidine

The electronic absorption spectrum of 2-bromopyrimidine in the u.v. region has been recorded in vapour phase and in solution phase in different solvents. Only one system has been observed in vapour phase and it has been identified as π* ← n transition. In solution phase two systems have been identified. The system on the longer wavelength side has been identified as corresponding to the one observed in vapour phase while the other one has been assigned to a π* ← π transition. The infrared spectrum of this molecule has also been recorded and analysed. Help of these i.r. data has been taken to analyse the u.v. spectrum of 2-bromopyrimidine, considering the molecule as belonging to C_2υ point group.     

Neutron diffraction study of the deuterides of the over-stoichiometric compounds LaNi5+x

The deuterides of three intermetallic compounds LaNi_5+x with x=0, 0.2 and 0.4 have been prepared and analyzed by neutron powder diffraction at two different deuterium concentrations. On one hand, the crystallographic properties of the α and the β phases have been studied with the two phases in equilibrium on the pressure plateau characteristic for the phase transition. On the other hand, the β phase has been studied as a single phase in the solid solution domain. Crystal structures were refined using the Rietveld method. Crystal symmetry, lattice parameters and deuterium sites and occupancy parameters are reported for all the different phases. Anisotropic line profile analysis has been used to characterize the strains induced by deuterium absorption in the various regions of the Pressure–Composition–Isotherm curves. Results are compared for the different values of x and related to the cycle life properties for each compound.     

Development of master curves for discriminating the mechanism of solid state reactions from experimental data obtained by using the cyclic and controlled rate thermal analysis

The cyclic and Controlled Rate Thermal Analysis method (CRTA) has been used. The two rates automatically selected in the cyclic curve are small enough to allow the two states of the sample to be compared have nearly the same reacted fraction. Thus, the activation energy can be calculated without previous knowledge of the actual reaction mechanism. Provided that the activation energy,E, is known, a procedure has been developed for determining the kinetic law obeyed by the reaction by means of master curves that represent the values of the reacted fraction, α, as a function of?E/R(1/T-1/T _0.5),T _0.5 being the temperature at which α=0.5. This procedure has been tested by studying the thermal decomposition reaction of BaCO₃.     

Vibrational spectra and conformational behavior of dicyclopropylmethane

Raman spectra of the condensed phases of dicyclopropylmethane have been recorded. In addition, i.r. spectra of this compound have been obtained for all three phases. From the appearance of spectral doublets in the liquid phase Raman (and i.r.) spectra, one member of each of which vanishes upon crystallization, it has been concluded that, in the liquid state, DCPM exists as an equilibrium mixture of at least two conformers. Based upon the spectral results for DCPM, and upon the conformational preferences of a number of related molecules, it has been concluded that these two conformers are the C₂ and C_sgauche/gauche rotational isomers, with the C₂ form being the one which remains in the solid phase. From a variable temperature study of liquid phase Raman peaks, it has been determined that the C₂ conformer of DCPM is more stable than the C_s conformer by 0.93 ± 0.10 kcal/mole. In addition, it appears that the C₂ rotamer of DCPM also predominates in the gaseous phase. Tentative assignments of the major spectral bands of DCPM have also been proposed.     

The oxidation of 2-alkoxy-3,6-di-tert-butylphenols. The reversible dimerization of 2-alkoxy-3,6-di-tert-butylphenoxy radicals

The oxidation of 2-alkoxy-3,6-di-tert-butylphenols has been studied. It was found that 2-RO-3,6-di-tert-butylphenoxy radicals (R = Et, Pr, Ph) undergo dimerization by C-O coupling. The X-ray structure of 2-ethoxy-3,6-di-tert-butylphenoxy dimer has been determined. Radical dimers dissociate reversibly in solution to give two phenoxy radicals at 200-350 K. By using EPR spectroscopy the equilibrium concentration of the phenoxy radicals, equilibrium constants and ΔH and ΔS of dissociation have been determined.     

Synthesis of the benzo-β-carboline isoneocryptolepine: the missing indoloquinoline isomer in the alkaloid series cryptolepine, neocryptolepine and isocryptolepine

7H-Indolo[2,3-c]quinoline has been synthesized in two steps via a new approach starting from commercially available 3-bromoquinoline and 2-bromoaniline. The new methodology consists of two consecutive palladium-catalyzed reactions: a selective Buchwald/Hartwig amination followed by an intramolecular Heck-type reaction. Alternatively, the same skeleton has also been prepared via the combination of a Suzuki arylation with an intramolecular nitrene insertion starting from 4-chloroquinoline and {2-[(2,2-dimethylpropanoyl)amino]phenyl}boronic acid. Selective methylation of 7H-indolo[2,3-c]quinoline yielded 5-methyl-5H-indolo[2,3-c]quinoline (Isoneocryptolepine) which is an interesting new lead compound in the search for new antiplasmodial drugs.     

Synthesis, crystal, and molecular structure of the solvated complex [MoO2(L)] · DMF (L2− = 2-[N-(2-hydroxynaphthylidene)amino]propane-1,2,3-triol anion)

A new dioxomolybdenum(VI) complex, [MoO₂(L)] · DMF, where L^2? = 2-[N-(2-hydroxynaphthylidene)amino]propane-1,2,3-triol, has been synthesized. Its structure has been determined by X-ray diffraction analysis. The Mo atom is octahedrally coordinated by two oxo ligands that are in cis-positions with respect to each other, two oxygen atoms, the nitrogen atom of the tridentate bis(chelate) ligand L, and the DMF oxygen atom.     

Solid phase synthesis of 3-(5-arylpyridin-2-yl)-4-hydroxycoumarins

A solid phase strategy has been developed for the synthesis of 3-(5-arylpyridin-2-yl)-4-hydroxycoumarins. The key transformation is an intramolecular ipso substitution reaction which forms the coumarin heterocycle and culminates with cleavage of product from the polymer support. The pyridine moiety at C3 can be further modified with Suzuki coupling. A sample library with two diversity elements has been synthesized to demonstrate this ipso substation-based cyclo-elimination strategy.     

Current transient study of the kinetics of nucleation and diffusion-controlled growth of bimetallic phases

A model to describe the kinetics of nucleation and diffusion-controlled growth of bimetallic phases has been developed, and analytical expressions have been obtained for the elucidation of nucleation kinetics through determination of the number density N ₀ of active sites and their nucleation frequency A, from experimental current transients obtained under potentiostatic control. The validity of the model has been demonstrated with the electrodeposition of Ag–Hg phases at two distinct compositions.     

Vibrational spectra,methyl torsional potential functions,internal rotational potential functions,and conformational properties of ethyl vinyl ether

The vibrational spectra of ethyl vinyl ether in both the fluid and solid states have been recorded from 20 to 3500 cm^?1. The 33 fundamental modes of vibration have been assigned. Three rotational isomers have been observed and their structures have been determined. The most stable conformer, s-cis/s-trans, is planar and of C_s symmetry. The two less stable rotamers, skew/s-trans and skew/gauche, are non-planar and of C_i molecular symmetry. The barrier to internal rotation of the methyl rotor has been determined for each conformation; these barriers are 3.43 kcal mol^?1 (s-cis/s-trans), 3.35 kcal mol^?1 (skew/s-trans) and 3.19 kcal mol^?1 (skew/gauche). A potential function for each of the two asymmetric internal rotations has been calculated and barriers to conformer interconversion have been determined. From the asymmetric potential function calculations, ΔH, the enthalpy difference between the conformers, has been determined. The s-cis/s-trans conformer is 1.87 kcal mol^?1 more stable than the skew/s-trans conformer; the skew/s-trans conformer is more stable than the skew/gauche conformer by 1.10 kcal mol^?1. The energetics of conformer interconversion and methyl internal rotation have been described in terms of molecular geometry and non-bonded interactions. These results are compared to those found in other alkyl vinyl and dialkyl ethers.