Calculs non empiriques sur une réaction SN2 typique

Résumé On a calculé la structure électronique de l'état initial et de l'état de transition de la réaction de substitution F^–+CH₃FFCH₃+F^– par la méthode des orbitales moléculaires SCF dans l'approximation gaussienne. On trouve que l'énergie de l'état de transition est inférieure à la somme des énergies des produits séparés CH₃F et F^–. Cependant, cette contribution négative à l'énergie d'activation est contrebalancée par la variation des énergies de solvatation.

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Ab initio calculations on a typical SN₂ ReactionElectronic structure of methyl fluoride and of the transition state (FCH₃F)^–

The electronic structure of the initial and transition states of the substitution reaction F^–+CH₃F FCH₃+F^– have been calculated by the SCF molecular orbital method in the Gaussian approximation. It is found that the energy of the transition state is lower than the sum of the energies of the separated systems CH₃F and C^–. However, this negative contribution to the activation energy is balanced by the variation of solvatation energies.

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Zusammenfassung Die Elektronenstruktur des Ausgangs- und des Übergangszustandes der Substitutionsreaktion F^–+CH₃FFCH₃+F^– wurden mit einer SCF-MO-Methode mit Gaußfunktionen berechnet. Die Energie des Übergangszustandes ist niedriger als die Summe der Energien der separierten Systeme CH₃F und F^–. Dieser negative Beitrag zu der Aktivierungsenergie wird allerdings ausgeglichen durch eine Änderung der Solvatationsenergie.

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Remerciements. Nous remercions bien sincèrement M. A. Rassat (CEN, Grenoble) de nous avoir suggéré ce problème et M. Ph. Millie (ENS, Paris) de nous avoir apporté des éléments de discussion constructifs.     

Generalized brillouin theorem for multiconfigurational SCF theories

The matrix elements of the total Hamiltonian between a multiconfigurational SCF wave function and some well-defined linear combinations of excited Slater determinants are equal to zero. By means of this generalized Brillouin theorem it is possible to estimate the improvements to be expected from a subsequent configuration-interaction treatment. The expression of the effective potential for the orbitals can be also derived in the frame of a given multiconfigurational theory. As an example, the case of the CMC-SCF method recently suggested [9] is examined.     

Chimie des sucres sans groupements protecteurs : synthese de carbamates,d'urees et de thiourees en position 1 du lactose

The anomeric hydroxyl of lactose was carbamoylated selectively by treating the free sugar with alkylisocyanates, thus without preliminary protection of the other hydroxyls of the lactose molecule. Moreover, the readily available 1-aminolactose and 1-octyl-aminolactose reacted with isocyanates and isothiocyanates to afford selectively $\text{N}$-lactosyl $\text{N′}$-alkylureas and $\text{N}$-lactosyl $\text{N′}$-alkylthioureas, respectively.     

Self-assembling mini cell-penetrating peptides enter by both direct translocation and glycosaminoglycan-dependent endocytosis

A small library of cell-penetrating peptides (CPPs) containing a minimized cationic domain and a lipophilic domain of different size was studied. CPPs that could self-assemble were found to enter cells more efficiently, triggering a glycosaminoglycan-dependent pathway.     

Application of conjoint liquid chromatography with monolithic disks for the simultaneous determination of immunoglobulin G and other proteins present in a cell culture medium

The aim of this study was to develop a chromatographic method, as a substitute for enzyme-linked immunosorbent assays, for the rapid and simultaneous detection of IgG, insulin, and transferrin present in a cell culture medium. Conjoint liquid chromatography (conjoint LC) using monolithic disks was applied for this purpose. An anion-exchange disk was combined with a Protein G affinity disk in a preparative HPLC system. IgG bound to the Protein G disk, whereas transferrin and insulin were captured on the quaternary ammonium (QA) disk. Using this method, it was possible to simultaneously determine the concentrations of IgG, transferrin, and insulin in the cell culture medium. Thus, conjoint LC could be used for the rapid and simultaneous detection of different proteins present in a cell culture medium.     

Emission-photoactivity cross-processing of mesoporous interfacial charge transfer in Eu3+ doped titania

Periodic mesoporous Eu(3+) doped titania materials were obtained through the EISA (Evaporation Induced Self Assembly) process. Eu(3+) ions, entrapped within the semi-crystalline walls of the highly porous framework, appear to be advantageous during the probing of surface photochemical reactions. Its emission intensity is very sensitive to the presence of physisorbed molecules, in gas or liquid phase, that reside within the pores. In particular, strong fluctuations in intensity of the (5)D(0)→(7)F(2) transition were observed under UV light exposure on the time scale of tens of seconds. The emission modulation dynamics show a strong correlation with the crystallinity of the titania matrix. Correlation of the emission with the photocatalytic activity of the semiconductor for photodegradation of an organic molecule is observed. A model is proposed to describe the involved mechanisms.     

Adiabatic versus nonadiabatic photoisomerization in photochromic ruthenium sulfoxide complexes: a mechanistic picture from density functional theory calculations

Polypyridine ruthenium sulfoxide complexes are intriguing compounds which can display both photochromic and electrochromic properties. These properties are based on the Ru-S → Ru-O linkage isomerization capability of the sulfoxide group. The photoisomerization mechanism is of particular importance in order to understand the photophysical properties of such molecules. Density functional theory calculations demonstrate that the main photoisomerization mechanism is nonadiabatic for the system under study in agreement with the experimental observations. Indeed, funnels for efficient radiationless decay back to the ground state are shown to be easily accessible compared to transition states on the adiabatic triplet potential energy surface. However, we highlight for the first time that triplet metal-centered states play a central role in the photoisomerization mechanism of these compounds.     

AlCl3-promoted reaction of cycloalkanones with hydrazones: a convenient direct synthesis of 4,5,6,7-tetrahydro-1H-indazoles and their analogues

The AlCl₃-promoted reactions of cycloalkanones with hydrazones are described. This approach represents a mild and operationally simple method to access 2,3-diaryl-4,5,6,7-tetrahydro-1H-indazoles and their analogues in good to moderate yields.