AB initio study of the regular polyhedral molecules N4, P4, As4, N8, P8 and As8

Ab initio pseudopotential SCF calculations were performed on tetrahedral X₄ molecules using double-zeta basis sets with and without d functions. The inclusion of d orbitals shortens the bond lengths, stabilizes the X₄ structures and intensifies the electron density inside the tetrahedron. The cubic X₈ molecules, calculated without d AOs, are not predicted to be more stable than 2X₄. Repulsions between parallel bonds in X₈ may compensate the lack of ring strain.     

Analyse de couches minces de verres chalcogenures par diffusion elastique de protons et mesure de gammas prompts

Résumé Une méthode d'étude de la composition stoéchiométrique de couches minces (10 à 500 μg/cm²) de verres chalcogénures est développée. L'étude des réactions nucléaire; induites par bombardement de protons de basse énergie (<4 MeV) sur les éléments constitutifs Ge, As, Te et S, conduit à choisir la diffusion élastique et la détection de gammas prompts pour le dosage simulatené. La précision et la sensibilité de la méthode sont discutées à partir des résultats obtenus. L'utilisation de la résonance fine sur le soufre (p, p'γ) est appliquée à l'étude de l'homogénéité des cibles.     

Preclinical evaluation of a novel water-soluble chlorin E6 derivative (BLC 1010) as photosensitizer for the closure of the neovessels

In the present study, photodynamic activity of a novel photosensitizer (PS), Chlorin e(6)-2.5 N-methyl-d-glucamine (BLC 1010), was evaluated using the chorioallantoic membrane (CAM) as an in vivo model. After intravenous (i.v.) injection of BLC 1010 into the CAM vasculature, the applicability of this drug for photodynamic therapy (PDT) was assessed in terms of fluorescence pharmacokinetics, i.e. leakage from the CAM vessels, and photothrombic activity. The influence of different PDT parameters including drug and light doses on the photodynamic activity of BLC 1010 has been investigated. It was found that, irrespective of drug dose, an identical continuous decrease in fluorescence contrast between the drug inside and outside the blood vessels was observed. The optimal treatment conditions leading to desired vascular damage were obtained by varying drug and light doses. Indeed, observable damage was achieved when irradiation was performed at light doses up to 5 J/cm(2) 1 min after i.v. injection of drug doses up to 0.5 mg/kg body weight(b.w.). However, when irradiation with light doses of more than 10 J/cm(2) was performed 1 min after injection of drug doses up to 2 mg/kg body weight, this led to occlusion of large blood vessels. It has been demonstrated that it is possible to obtain the desired vascular occlusion and stasis with BLC 1010 for different combinations of drug and/or light doses.     

Apparent molar volumes,heat capacities,and molecular excluded volumes of methylated,hydroxy and methoxy derivatives of 1-methylcytosine in aqueous solutions at 25°C

Densities and specific heat capacities of aqueous solutions: 1-methylcytosine; 1-methyl-N⁴-hydroxycytosine; 1,5-dimethylcytosine; I,N⁴-dimethylcytosine; 1,5-dimethyl-N⁴-hydroxycytosine; 1-methyl-N⁴-methoxycytosine; 1,N⁴,N⁴-trimethylcytosine, 1,5-dimethyl-N⁴-methoxycytosine were determined using flow calorimetry and flow densimetry at 25°C. Apparent molar volumes and heat capacities were then determined. Molecular excluded volumes were evaluated. A relationship was found between the values of the increments in partial molar values and the kind of groups substituted. Four types of contributions were distinguished: substitution of hydrogen on C, N, and O (in OH group on N4) atoms by CH₃ group and replacement of hydrogen on N4 atom by OH group. The correlation between the experimental partial molar volumes and calculated molecular excluded volumes was also elaborated.     

On the size consistence of a few approximate multireference CI schemes

Using the canonical problem of N separate electron pairs, theN dependence of approximate multireference CI schemes is analyzed. When the DCI is taken as multireference space, the second order Quasi Degenerate Many Body Perturbation Theory (QD(MB)PT) gives twice the expected correction, while the CIPSI algorithm gives 66% of it, and the MRDCI a vanishing part of it. A modified QDPT effective hamiltonian, and a combination of CIPSI and QDPT algorithms seem to give better trends.     

Phase transitions and band structure in metallic perovskites (carbides and nitrides)

Properties of Mn₃XN and Mn₃XC types of compounds are analyzed on the basis of a model for the electronic band structure, which consists of a large conduction band overlapping a narrow band that results from the strong hybridization between the p orbitals of the metalloid and some of the d orbitals of the manganese. The crystal field at the sites of manganese is assumed to be strong. The structure of the narrow band is calculated in the tight-binding model. The Fermi energy lies very close to a sharp singularity in the density of states. In the cubic and Pauli paramagnetic phase, such a singularity has a sixfold degeneracy. The magnetic and structural instabilities, which appear when the temperature is decreased, are explained by the removal of that degeneracy by a shear strain and the formation of small magnetic moments. The phase transitions can be studied in detail by expanding the variation of the free energy with respect to the shear strains and the magnetic moments. The coefficients of the expansion are calculated as functions of the temperature. The variation of the volume is explained by the existence of coupling terms to the shear strain and to the magnetic moments.     

Etude semiempirique de la configuration moléculaire stable des molécules ionisées ou excitées

Résumé Les déformations stabilisatrices des molécules ionisées ou excitées sont étudiées en supposant connues les coordonnées normales et les constantes de force de l'état fondamental et en calculant les variations de l'énergie d'ionisation ou d'excitation dans les déformations. On en déduit les coordonnées des minimas, les énergies de stabilisation, et les intersections des surfaces potentielles. Cette méthode est appliquée aux anions de la pyrazine calculés par les méthodes SCF et UHF dans les approximations de Pariser et Parr: les variations des longueurs de liaison sont de l'ordre de 0.15 å, celle de l'affinité électronique de 1 eV, et celle des densités de spin de 0.05 e. Ce phénomène peut jouer un rÔle dans l'eifet de charge. En méthode de Hückel des formules générales sont déduites, donnant l'énergie de stabilisation par déformation des ions et des états excités des polyènes.

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The stabilizing deformations of ionized or excited molecules are studied using experimental normal modes and force-constants, and calculating the variations of ionization or excitation energies in the deformations. We deduce new minimas, stabilizing deformations, crossing lines of potential surfaces. This method is applied to negative ions of pyrazine by SCF and UHF (Unstricted Hartree-Fock) methods in the Pariser-Parr approximations: the variations of bond lengths could reach 0.15 å, the electroaffinity could increase of 1 eV, and the spin densities change about 0.05 e. This phenomenon may play a role on the so-called charge-effect. In that scheme, general formulas giving the stabilizing deformation energies for ions and excited states of polyens are deduced from Hückel method.

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Zusammenfassung Die stabilisierenden Deformationen ionisierter oder angeregter Moleküle werden bei bekannten Normalkoordinaten und Kraftkonstanten des Grundzustandes studiert und die Änderungen der Ionisations- und Anregungsenergie bei den Deformationen berechnet. Hieraus werden die Minimumskoordinaten, Stabilisierungsenergien und Schnitte der PotentialflÄchen hergeleitet. Diese Methode wird auf die Anionen von Pyrazin mittels der SCF- und UHF-Methode im Rahmen der Pariser-Parr-NÄherung angewandt. Die Änderungen der BindungslÄngen betragen grö\enordnungsmÄ\ig 0,15 å, die der ElektronenaffinitÄten 1 eV und die der Spindichten 0,05 e. Dieses PhÄnomen spieltbeim sogenannten Ladungseffekt eine Rolle. Mit der Hückel-Methode werden allgemeine Formeln abgeleitet, die die Stabilisierungsenergie der Deformation der Ionen und angeregten ZustÄnde von Polyenen angeben.

     

Molar heat capacities and volumes of transfer of cytosine,thymine, caffeine and 1,3-diethylthymine to aqueous solutions of glycyl-glycine and L-α-alanyl-L-α-alanine at 25°C

Densities and specific heat capacities of ternary aqueous systems containing dipeptides (glycyl-glycine or L--alanyl-L--alanine) and nucleic acid bases (cytosine or thymine) or their alkyl derivatives (1,3-diethylthymine or caffeine) were determined at 25°C by flow calorimetry and flow densimetry. The partial molar volumes and heat capacities of transfer at infinite dilution of the different nucleic acid bases from water to water+dipeptide solutions were obtained therefrom. Except for the case of the transfer of cytosine to aqueous glycyl-glycine solutions where a small positive dependence of the transfer quantities was observed with the dipeptide concentration, the values of the heat capacities of transfer were in general low, positive or negative, depending on the compensation of hydrophobic-hydrophilic interactions between the dipeptide and the base. The volumes of transfer of most of the bases are very small, within the limit of the experimental error.     

Wetting transitions of ionic solutions

Cahn's phenomenological theory of wetting of a solid substrate by a saturated vapor is generalized to the case where the substrate is charged and the wetting film contains counterions, with or without added salt. The electrostatic contribution to the grand potential associated with these ions is calculated within a nonlinear Poisson-Boltzmann theory. In the salt-free case, when the wetting film includes only counterions released by the substrate, the wetting transition is always first order, regardless of its nature in a neutral system. When salt is present, other wetting scenarios may arise, depending on the salt concentration and substrate surface charge. Over a restricted range of salt concentrations, a wetting scenario similar to that of prewetting, is predicted to occur along the liquid-vapor coexistence line. This scenario includes a discontinuous wetting transition between microscopic and mesoscopic film thicknesses, followed by a continuous divergence of the film thickness at higher temperatures.