Une méthode simple de calcul des structures électroniques (σ + π) des borazines et des boroxines substitués de symétrie D3h: une estimation théorique de l'aromaticité comparée de ces molécules

Résumé Une utilisation conjointe des méthodes L.C.A.O.-C.U.V. et d'itération tournante permet d'atteindre des structures électroniques ( + ) cohérentes avec l'U.V.(et donc le gradient de population électronique de liaison qui conditionne l'aromaticité) de huit borazines et boroxines, substitués ou non, de symètrie D _3h . L'échelle d'aromaticité obtenue permet de rendre compte des propriétés magnétiques (RMN, RQ, effet Faraday, diamagnétisme) de ce type de molécules. On parvient en particulier aux deux conclusions essentielles suivantes: les borazines sont nettement plus aromatiques que les boroxines isologues et la greffe de groupements méthyles ou d'atomes de chlore, qu'elle se fasse sur le bore ou sur l'azote, confère toujours au dérivé substitué une aromaticité au moins égale à celle du borazine lui-même.

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A simple method to calculate the - and -electron structure of borazines and of substituted boroxines of symmetry D_3h: A theoretical estimation of the relative aromaticity of those molecules

( + ) electronic structures calculations are performed for eight D _3h borazines and boroxines, using in concert previously published U.V.-consistent H.M.O.-L.C.A.O. and bond by bond iteration methods. An estimation of bond electronic populations gradients is so obtained, allowing to set up a new aromaticity scale for such molecules. The main results are the following: Borazines are much more aromatic than isologous boroxines and B-or N-graft of alkyl groups or chlorine atoms never decreases (and generally increases) aromaticity.

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Zusammenfassung Es wird für acht Borazol- und Boroxolverbindungen mit D _3h -Symmetrie unter Verwendung einer am UV-Spektrum geeichten iterativen HMO-LCAO-Theorie die - und -Elektronenstruktur bestimmt. Hiermit kann eine Abschätzung des Gradienten der Bindungselektronendichten gewonnen werden, aus der eine Einteilung der Moleküle nach ihrem aromatischen Charakter hervorgeht. Es ergibt sich, daß die Borazole aromatischer sind als die isoelektronischen Boroxole und daß eine Substitution der H-Atome am Bor oder Stickstoff durch Chlor oder Methylgruppen die Aromatizität fast immer erhöht, jedenfalls aber nie erniedrigt.

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Recherche effectuée dans le cadre de la R.C.P. n du C.N.R.S.     

The electronic structure of sulphur chloride penta-fluoride

The electronic structure of SCIF₅ is computed within the framework of the CNDO/2 approximation on the basis of Kewley's microwave geometrical data and is compared with our previous results on SF₆. The charge distribution and the value of the dipole moment are discussed as a function of the F-S-F_ax angle and an inversion of the net charge on Cl is observed when passing through the 95° value for (F-S-F_ax) angle.     

Properties of polysaccharides grafted on nanoparticles investigated by EPR

The in vivo fate of nanoparticles developed as drug delivery systems is influenced by the surface characteristics of the colloidal particles. In the present work, surface characteristics of a series of poly(isobutylcyanoacrylate) nanoparticles prepared by redox radical emulsion polymerization with polysaccharides of different molecular weight and nature were characterized by EPR. To this aim, a spin label was grafted on the polysaccharide chains after synthesis of the nanoparticles. The percentage of label showing fast movements was evaluated from EPR spectra which were analyzed according to the Kivelson theory. The results showed that mobility depended on temperature, type, and molecular weight of the polysaccharides. Differences between nanoparticles appeared with low-molecular-weight polysaccharides, while over a defined molecular weight which depended on the nature of the polysaccharide, the spin label behaved almost the same way in the different types of nanoparticles. Paradoxically, the percentage of fast moving label was the highest when linked to the shortest chitosan, which was the most rigid polysaccharide tested in this study. Thus, it was concluded that the apparent mobility of the polysaccharide evaluated by the EPR method depended on the capacity of the polysaccharide chains to fold making possible hydrophobic interactions between the label and the nanoparticle core. The transition between the unfolded-folded regiment depended on the molecular weight and on the nature of the polysaccharide. Results of this study may be useful to improve the understanding of the nanoparticle interactions with blood proteins and complement which in turn influence the in vivo fate of nanoparticles used as drug delivery systems.     

Molecular orbital study of the bridge bonding in an electron deficient molecule [(CH3)2ALH]2

The electronic distribution in the AlH₂Al bridge of the dimethyl aluminium hybride dimer was computed through ab initio SCF calculation. Comparison with diborane shows an increased role of the ionic Al⁺H ₂ ^2– Al⁺ structures with respect to the usual covalent three-center bonds.     

Evaluation of the surface properties of dextran-coated poly(isobutylcyanoacrylate) nanoparticles by spin-labelling coupled with electron resonance spectroscopy

Poly(alkylcyanoacrylate) nanoparticles are developed as carrier for the in vivo delivery of drugs. In this area of research, one of the major challenges is to design nanoparticles able to carry a drug to a specific site in the body. This appears to be mainly governed by the surface properties of the carrier. Results from previous independent studies suggest that the way dextran chains are arranged at the nanoparticle surface can affect the in vivo fate of the carrier. Thus, the purpose of the present study was to investigate for the first time whether electronic paramagnetic resonance (EPR) could highlight a difference between the physico-chemical surface properties of dextran-coated nanoparticles obtained by two different emulsion polymerisation mechanisms of isobutylcyanoacrylate. Poly(isobutylcyanoacrylate) nanoparticles were prepared either by anionic or by radical polymerisation, initiated in both cases by dextran. The respective copolymers self-organised as nanoparticles. Dextran chains located at the nanoparticle surface could be labelled with a free nitroxide radical containing a probe and EPR analysis could be performed on freeze-dried nanoparticles, rehydrated nanoparticles and dispersed nanoparticles in water. The mobility of dextran chains appeared to differ according to the degree of hydration of the systems. More interestingly, EPR spectra clearly highlighted differences in dextran chain mobility comparing the nanoparticles obtained by radical and anionic polymerisation. Therefore, this technique opens an interesting prospect of investigating surface properties of polysaccharide-coated nanoparticles by a new physico-chemical approach to further correlate the mobility of the polysaccharide chains with the fate of the nanoparticles in biological systems.     

Analyse conformationnelle theorique des complexes acide-base de lewis: III. Une approche semi-empirique (méthode CNDO/2) des geométries et de l'analyse conformationnelle du borazane et de ses dérives perfluoré

The optimized geometries and theoretical data from conformational analysis cf borazane and its perfluoro derivatives are investigated within the framework of the CNDO/2 approximation. The effect that fluorination has on the geometrical and electronic parameters of the investigated series of compounds is pointed out and the authors offer an explanation for the anomalous behaviour of H₃N.BF₃. Furthermore, it is shown that the assumption of tetragonal hybridization for the N and B atoms of any borazane is incorrect if the F₃N molecule is used as the Lewis base.     

Cyclophosphazenes and Relatives as Anticancer Drugs

Abstract

Aziridinocyclophosphazenes N₃P₃Az₆ (code name MYKO 63), N₄P₄Az₈ (code name MYKO 83) and relatives constituted the first generation of anticancer drugs whose efficiency on several rodent neoplasms was made conspicuous in a quantitative manner from 1976 to 1978 both in our Laboratory and by EORTC Screening Pharmacology Groups¹. MYKO 63 appeared at that time as a promising drug for industrial development owing to its wide spectrum of activity and its very low mutagenicity². However, this hope failed as a consequence of a cumulative toxicity which occurs upon heavy polyinjections schedules. In other words, MYKO 63 exhibits an uncomfortable kinetics of action on the tumor - and, consequently, of excretion - presumably due (it was our assumption) to a too high chemical stability of the molecule.     

First-Order Frameworks for Continuous Newton-like Dynamics Governed by Maximally Monotone Operators

Set-Valued and Variational Analysis - In a Hilbert framework, we discuss a continuous Newton-like model that is well-adapted in view to numerical purposes for solving convex minimization and more...