Une extension de la méthode CNDO/2 á l'étude des complexes d'éléments de transition

An extension of the CNDO/2 formalism, similar to the method of Clack, Hush and Yandle but with a different parametrisation, is proposed for investigating the electronic structure of transition metal complexes. The results for Ni(CO)₄ and Ni(PF₃)₄ agree well with those ofab initio calculations and with some experimental data. Such a semi-empirical approach may be performed, usingab initio results as reference data, in order to interpret the physical and chemical behaviour of largeseries of complexes in their ground state.

     

An answer to the spiro versus ansa dilemma in cyclophosphazenes: Part I. N3P3Cl4[NH(CH2)3NH]

Structural investigation, mainly by mass spectrometry and X-ray crystallography, of the product obtained from the reaction of propylenediamine and N₃P₃Cl₆ shows it to have a monospiro structure. It is demonstrated that in this case data collected through indirect structural methods such as IR or NMR spectroscopy could not provide a priori the right answer.     

Influence d'un ou de plusieurs groupements tertiobutyle sur le comportement magnétooptique du doublet libre d'un atome de phosphore tricoordonné

A magnetooptical study of some halogenotertiobutylphosphines is performed in order to improve the so-called regular relationship which seemed to exist between the magnetic rotation, ?dP, of the lone pair and the bond angles in PX₃ compounds. It is clearly shown that, in any case, ?dP is largely lower than the extreme value which was observed in PX₃ molecules for which XPX=100°. Thus, the existence of the previously invoked relationship vanishes and the question remains as to why there exists a maximum of ?dP for XPX of 100°.     

Analyse conformationnelle théorique du diméthylsulfoxyde (CH3)2SO

Résumé La méthode SCF-LCAO-MO dans l'approximation CNDO/2 est utilisée pour l'étude de l'analyse conformationnelle du diméthylsulfoxyde (CH₃)₂SO. On montre ainsi que, parmi les déterminations géométriques de la littérature, celle (r _OIV) obtenue par Dreizler et Coll. en 1969 au moyen de la spectroscopie de microondes semble devoir être retenue. La carte de potentiel, décrivant la rotation simultanée des deux groupements méthyle autour des liaisons (S-C), est tracée: elle montre que la molécule (CH₃)₂SO appartient bien au groupe de symétrie C _s, non seulement au niveau des atomes lourds (C, S, O) mais aussi à celui des hydrogènes. On note toutefois que les deux hydrogènes qui, dans la conformation (60, 60), auraient été dans le plan CSC, sont alors décalés tous deux de quelque 3,64° hors de ce plan et du côté de l'atome d'oxygène. La barrière à la rotation d'un CH₃ est trouvée égale à 3,5 kcal/mole (expérimentalement: 2,8 kcal/mole [7]). Un découpage bicentrique de l'énergie totale montre enfin que 60% des variations de cette grandeur sont dûs à la seule somme, E (S...H) des interactions dans l'espace entre l'atome de soufre et les six atomes d'hydrogène.

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Theoretical conformational analysis of dimethylsulfoxide (CH₃)₂SO

The conformational analysis of (CH₃)₂SO is investigated within the framework of the CNDO/2 approximation. The preferred conformation of heavy atoms (C, S, O) is found to be the one (r _OIV) which was proposed by Dreizler et al. in 1969 on the basis of microwave measurements. The map of isoenergy curves was drawn, describing the variation of energy as a result of mutual rotation of the two methyl groups. This showed that the equilibrium symmetry for the complete molecule, including the hydrogens, was C _sHowever, it may be noticed that the two hydrogens which, for the (60, 60) conformation, would have been located into the CSC plane, are actually staggered of 3.64° out of this plane and nearer to the oxygen atom than to the sulphur lone pair. The rotational barrier for one CH₃ group is 3.5 kcal/mole, in agreement with the experimental value (2.8 kcal/mole [7]). A bicentric energy partitioning shows that about 60% of the variation in total energy is reflected by the single sum, E(S...H), of the spatial interaction terms between sulphur and the six hydrogens.

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Zusammenfassung Die Konformation von (CH₃)₂SO wird im Rahmen des CNDO/2-Verfahrens analysiert, wobei sich für C-, S- und O-Atome das gleiche Ergebnis wie bei Mikrowellenmessungen (Dreizler, 1969) ergibt. Ferner wird die Energiehyperfläche für Methyl-Rotationen angegeben, mit der Symmetrie C _sam tiefsten Punkt. Allerdings sind die zwei H-Atome, die bei der (60, 60)-Konformation in der CSC-Ebene liegen sollten, um 3,64° aus dieser Ebene herausgedreht und näher an den einsamen Elektronen des Sauerstoffs als an denen des Schwefels. Die Rotationsbarriere einer CH₃-Gruppe beträgt 3,5 kcal/ Mol (experimentell 2,8 kcal/Mol [7]). Eine Energieaufspaltung auf Zentrenpaare zeigt, daß etwa 60% der Änderung der Gesamtenergie in der Einfachsumme E(S...H) der räumlichen Wechselwirkungs-systeme zwischen den S- und den sechs H-Atomen enthalten ist.

     

E´tude the´orique de la conformation de la 1,1-dime´thylhydrazine et du complexe (CH3)2N-NH2 · BF3

CNDO/2 calculations are performed in order to determine the preferred conformations of 1,1-dimethylhydrazine and 1,1-dimethylhydrazine-boron trifluoride complex. In each case, the calculations reveal the existence of an equilibrium between gauche and trans forms (25 % of trans form in the free molecule, 65 % in the complex). In agreement with experiment, the results confirm that BF₃ is attached to the nitrogen atom bearing the methyl groups.     

Crystalline thiophene—I: Phase diagram and structures of two orientationally disordered crystalline phases. crystallographic evidence for a metastable low temperature phase

The first order phase diagram of thiophene has been determined from 217.5 to 298 K up to 4̃00 MPa. It shows that a high pressure phase which melts at room temperature is in fact Waddington et al.'s atmospheric pressure phase II and discloses a ～R ln2 entropy increment at the II→I phase transition. The structures of phases I and II have accordingly been investigated taking this relationship into account. For phase I, the best result is obtained for space group Cmca with 20 equiprobable molecular orientations. This leads us to assume that phase II is better described by space group Pnma with 10 molecular orientations. Finally, a metastable phase I′ can easily be obtained by cooling phase I at atmospheric pressure. Its unit cell is derived from that of I by multiplying parameter b by 2 and parameter c by 20: this can be considered as an a posteriori justification of the multiple-of-five number of molecular orientations in phases I and II.