Theoretical conformational analysis of lewis adducts,VI. A semi-empirical approach on dimethylsulphoxide-boron trifluoride: An example of rock and roll internal motion

The preferred conformations and the electronic structure of (CH₃)₂SO.BF₃ are investigated within the framework of the CNDO/2 approximation. The two skew equivalent preferred forms C and C? are symmetrical about the bisecting plane of the acute CSC angle. The rotational barrier between C and C? is very low (0.45 kcal mole^?1) and corresponds to a conformation S in which the BF₃ group is twisted through 50° or so relatively to its position in the C conformation. Thus, it appears that the rocking of the OBF₃ group around the (S.O) bond from C to C? is attended by a rolling of the BF₃ group around the (O.B) bond. For this reason, we suggest this peculiar conformational equilibrium be named the rock and roll internal motion.     

Analyse conformationnelle théorique de la diméthylsulfone, (CH3)2SO2

Résumé La méthode SCF-LCAO-MO dans l'approximation CNDO/2 est utilisée pour l'étude de l'analyse conformationnelle de la diméthylsulfone (CH₃)₂SO₂. On montre ainsi que, parmi les déterminations géométriques les plus récentes, celle obtenue par Jacob et Lide en 1971 au moyen de la spectroscopie de micro-ondes 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)₂SO₂ appartient bien au groupe de symétrie C_2v , non seulement au niveau des atomes lourds (C, S, O) mais aussi à celui des hydrogènes. La barrière à la rotation d'un CH₃ est ainsi trouvée égale à 3,65 kcal/mole (expérimentalement: 3,4 kcal/mole [10]). Un découpage bicentrique de l'énergie totale montre enfin que 70% des variations de cette grandeur sont dus à la seule somme, E(S-H), des interactions dans l'espace entre l'atome de soufre et les six atomes d'hydrogène. Ce résultat éclaire d'un jour nouveau le concept qualitatif d'hyperconjugaison.

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Theoretical conformational analysis of the dimethylsulfone, (CH₃)₂SO₂

The conformational analysis of (CH₃)₂SO₂ 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 which was proposed by Jacob and Lide in 1971 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_2v . The rotational barrier for one CH₃ group is 3.65 kcal/mole, in good agreement with the experimental value (3.4 kcal/mole [10]). A bicentric energy partitioning shows that at least 70 per cent 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. This result, which is consistent with previous work, strongly supports the classical concept of hyperconjugation.

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Zusammenfassung Die Konformationsanalyse von (CH₃)₂SO₂ wird im Rahmen der CNDO/2-Näherung untersucht. Die bevorzugte Konformation der schweren Atome (C, S, O) ergibt sich als diejenige, die von Jacob und Lide 1971 auf der Grundlage von Mikrowellenmessungen vorgeschlagen wurde. Die iso-energetischen Kurven wurden angegeben; sie beschreiben die Variation der Energie als ein Ergebnis der gleichzeitigen Rotation der beiden Methylgruppen. Dies zeigte, daß für das Gesamtmolekül — die Wasserstoffatome eingeschlossen — im Gleichgewicht C_2v -Symmetrie besteht. Die Rotationsschwelle für eine CH₃-Gruppe ist 3,65 kcal/mole, in guter Übereinstimmung mit dem experimentellen Wert (3,4 kcal/mole [10]). Eine bizentrische Energieaufspaltung zeigt, daß mindestens 70% der Variation der totalen Energie auf die eine Summe E(S-H) der räumlichen Wechselwirkungsterme zwischen Schwefel und den sechs Wasserstoffatomen entfallen. Dieses Ergebnis — in Übereinstimmung mit früheren Arbeiten — erhärtet von neuem das klassische Konzept der Hyperkonjugation.

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Remerciements. Les auteurs tiennent à remercier très vivement le Professeur C. A. Coulson pour l'intérêt qu'il a bien voulu porter à ce travail et pour les suggestions positives qu'il a faites aussi bien quant au fond qu'à la forme de ce Mémoire.     

An extension of the CNDO/2 formalism for the study of transition metal complexes

The electronic structure and the conformational analysis of some Ni(PX₃)₄ (X = F, OCH₃, Cl, CH₃, H) complexes are investigated within the framework of the previously described extended CNDO/2 method in order to interpret their magneto-optical behaviour and to propose a suitable scheme for the electronic content of a metal-ligand ( + ) dative bond. Chatt and Wilkinson's pattern, used up to now, is criticized and a new scheme is proposed which — taking accurately the role of electronegativity into account and abiding by Pauling's principle of electroneutrality — fits well the sets of available physico-chemical data and allows us to understand whyab initio bonds overlap populations as well as IR force constants are low for any metal-ligand ( + ) dative bond.     

On the scent of Spherical Dendrimers: Cyclophosphazenic Dandelion Dendrimers Up to the Eighth Generation

Abstract

Preparation of spherical dendrimers up to the eighth generation from D_3h cyclophosphazenic hexadangling cores (coded as sexapus) involves two repetitive steps: aminolysis of hexachlorocyclotriphosphazene, N₃P₃Cl₆ 0, by long-chain aliphatic diamines (such as 1,6-Diaminohexane and higher cousins) leading to sexapus cores with dangling diamino groups followed by a grafting of N₃P₃Cl₅ flagstones as 5-fold growing multipliers on these amino endings. Dendrimers of the first (compounds 1a, b) to the eighth (compounds 8a, b) are described. Dendrimer of the eighth generation, 8b, possesses 2,343,750 terminal (P-Cl) functions (molecular weight 228,977,179).     

THE B.A.S.I.C. (BINO-ANSA-SPIRO-IN-CYCLOPHOSPHAZENES) SYSTEM. PART I: NEW DERIVATIVES OF THE BIS-(TETRACHLORO-CYCLOTRIPHOSPHAZENYL)-SPERMINE

Abstract

The synthesis of new derivatives of bis-(tetrachlorocyclotriphosphazenyl)-spermine was achieved upon reaction of some MONOSPIRO–N₃P₃Cl₄[HN–(CH₂) _n –NH] (n = 2,3,4) derivatives and of the gem–N₃P₃Az₂Cl₄ with spermine. All compounds were obtained in the monomeric state. ³¹P and ¹³C high-resolution NMR were used, together with IR spectroscopy and mass spectrometry, to assign molecular structures and to reveal conformational equilibria, if any.     

The Gem-Tetraziridinocyclotriphosphazene Diazide,A Synthon to Monophosphazenylmonoazido Derivatives of Biological Interest

Abstract

The gem-tetraziridinocyclotriphosphazenes, N₃P₃Az₄XY, are both antitumor agents (1, 2) and immuno-modulators (3, 4) whatever X and Y are, but their selectivity for malignant cells is commonly poor. In an attempt at the production of more selective drugs, we prepared monoazido derivatives (X=N₃, Y=something else) for covalent binding to antibodies.     

On the conformational analysis of (CH3)3M (M = N,P, CH,SiH) molecules: CNDO/2 calculations versus experimental data. LEM or calder conformation?

CNDO/2 calculations are performed for the homogeneous series of (CH₃)₃M compounds (M = N, P, CH, SiH) in order to determine the molecular equilibrium conformation. In agreement with available experimental data for the amine, the phosphine and the silane, the theoretical energy minimum is found for the (60, 60, 60) - so-called LEM - conformation in which each methyl group is staggered with respect to the two opposite (MC) bonds. The same LEM conformation is predicted to be theoretically preferred for the isobutane molecule, this result being very sensitive to the “C_3V or not” quality of the methyl groups. The values of optimized CMC angles, rotational barriers and dipole moments are well reproduced.