Ring current theories in nuclear magnetic resonance

The molecular structure of methane sulfonyl chloride in the vapors was studied by electron diffraction. Three models were found in excellent agreement with experiment differing largely in the values of the O–S–O angle and the mean amplitudes of vibration for some of the nonbonded distances. The favored model has the following geometrical parameters (with their errors parenthesized): C–H 1.101 (0.013) A, S–O 1.424 (0.003) A, S–C 1.763 (0.005) A, Cl–S 2.046 (0.004) A, ∠ Cl–S–C 101.0° (1.5°), ∠ O–S–O 120.8° (2.4°), ∠ Cl–S–O 107.1° (0.7°), ∠ H–C–H 112.0° (2.0°). The structure of SO2Cl2, CH3SO2Cl, and (CH3)2SO2 are compared.

On the molecular structure of methane sulfonyl chloride as studied by electron diffraction

In chemical research attention is increasingly being focussed on composite “complex” systems with special properties and functions. As components of such systems 1,4-diazines are steadily gaining in popularity. Here, 1,4-diazines means pyrazine and its derivatives as well as compounds with partial pyrazine structure; examples are quinoxaline, phenazine, pteridine, flavin and their derivatives. All these compounds are characterized by a low lying unoccupied π-molecular orbital and by the ability to act as bridging ligand. Due to these two properties 1,4-diazines, and especially their parent compound pyrazine, possess a characteristic reactivity. 1,4-Diazines may be employed to study inter- and intramolecular electron transfer in organic, inorganic and biochemical reactions. In the redox system of 1,4-diazines the paramagnetic 7π-electron intermediate exhibits exceptional stability, whereas the 1,4-dihydro-1,4-diazines with 8π-electrons in a six-membered ring are not generally accessible due to their potential antiaromaticity and their large excess of π electrons. Its inherent bifunctionality and the low lying unoccupied molecular orbital permit pyrazine to form coordination polymers having unusual electrical and magnetic properties. Finally, the phenomena observed for pyrazines may be used in the interpretation of the reactivity of naturally occurring 1,4-diazines, such as flavins and bioluminescent natural products.

The Versatile Chemistry of 1,4‐Diazines: Organic, Inorganic and Biochemical Aspects

An electron diffraction study of the molecular geometry of dimethyl sulphone

Das Interesse der chemischen Forschung richtet sich zunehmend auf zusammengesetzte, “komplexe” Systeme mit speziellen Eigenschaften und Funktionen. Als Komponenten solcher Systeme finden 1,4-Diazine wachsende Beachtung. Unter 1,4-Diazinen sollen hier Pyrazin und seine Derivate sowie Verbindungen mit Pyrazin-Teilstruktur verstanden werden; Beispiele sind Chinoxalin, Phenazin, Pteridin, Flavin und deren Derivate. Alle diese Verbindungen zeichnen sich durch ein niedrig liegendes unbesetztes π-Molekulorbital und durch die Fahigkeit zur Brucken-Koordination aus. Die Kombination dieser beiden Eigenschaften bedingt eine spezifische Reaktivitat der 1,4-Diazine und insbesondere der Stammverbindung Pyrazin. 1,4-Diazine eignen sich ausgezeichnet fur Untersuchungen zum inter und intramolekularen Elektron-Transfer bei organischen, anorganischen und biochemischen Reaktionen. Im Redoxsystem der 1,4-Diazine ist die paramagnetische 7π-Elektronen-Zwischenstufe auserordentlich stabil, wahrend die 1,4-Dihydro-1,4-diazine mit 8 π-Elektronen im sechsgliedrigen Ring aufgrund der potentiellen Antiaromatizitat und des hohen Elektronenuberschusses nicht generell zuganglich sind. Die inharente Difunktionalitat und das niedrig liegende unbesetzte Molekulorbital des Pyrazins ermoglichen die Bildung von Koordinationspolymeren mit ungewohnlichen elektrischen und magnetischen Eigenschaften. Die an Pyrazinen beobachteten Phanomene lassen sich schlieslich zur Interpretation der Reaktivitat naturlich vorkommender 1,4-Diazine heranziehen; diskutiert werden Flavine und biolumineszierende Naturstoffe.

Die vielseitige Chemie der 1,4-Diazine: Organische, anorganische und biochemische Aspekte

La methode SCF-LCAO-MO dans l'approximation CNDO/2 est utilisee pour l'etude de l'analyse conformationnelle de la dimethylsulfone (CH3)2SO2. On montre ainsi que, parmi les determinations geometriques les plus recentes, celle obtenue par Jacob et Lide en 1971 au moyen de la spectroscopie de micro-ondes semble devoir etre retenue. La carte de potentiel, decrivant la rotation simultanee des deux groupements methyle autour des liaisons (S-C), est tracee: elle montre que la molecule (CH3)2SO2 appartient bien au groupe de symetrie C2v, non seulement au niveau des atomes lourds (C, S, O) mais aussi a celui des hydrogenes. La barriere a la rotation d'un CH3 est ainsi trouvee egale a 3,65 kcal/mole (experimentalement: 3,4 kcal/mole [10]). Un decoupage bicentrique de l'energie totale montre enfin que 70% des variations de cette grandeur sont dus a la seule somme, Σ E(S-H), des interactions dans l'espace entre l'atome de soufre et les six atomes d'hydrogene. Ce resultat eclaire d'un jour nouveau le concept qualitatif d'hyperconjugaison.

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

La methode SCF-LCAO-MO dans l'approximation CNDO/2 est utilisee pour l'etude de l'analyse conformationnelle du dimethylsulfoxyde (CH3)2SO. On montre ainsi que, parmi les determinations geometriques de la litterature, celle (rOIV) obtenue par Dreizler et Coll. en 1969 au moyen de la spectroscopie de microondes semble devoir etre retenue. La carte de potentiel, decrivant la rotation simultanee des deux groupements methyle autour des liaisons (S-C), est tracee: elle montre que la molecule (CH3)2SO appartient bien au groupe de symetrie Cs, non seulement au niveau des atomes lourds (C, S, O) mais aussi a celui des hydrogenes. On note toutefois que les deux hydrogenes qui, dans la conformation (60, 60), auraient ete dans le plan CSC, sont alors decales tous deux de quelque 3,64° hors de ce plan et du cote de l'atome d'oxygene. La barriere a la rotation d'un CH3 est trouvee egale a 3,5 kcal/mole (experimentalement: 2,8 kcal/mole [7]). Un decoupage bicentrique de l'energie totale montre enfin que 60% des variations de cette grandeur sont dus a la seule somme, ΣE (S...H) des interactions dans l'espace entre l'atome de soufre et les six atomes d'hydrogene.

Analyse conformationnelle théorique du diméthylsulfoxyde (CH3)2SO

The problem of Aromaticity* has always been one of the most difficult and fascinating problems in chemistry. It is usually stated in the literature that aromaticity has constituted a challenge to both the theoretican and the chemist since 1865, when August Kekule introduced his intuitive idea on the structure of the benzene molecule i). But the trouble is that the right question has never been asked. Which questian have we to answer: "Is aromaticity an out-of-date or an up-to-date concept?" o r (that most tedious question) "Is aromaticity a myth or reality? ". These questions must be discussed before we can venture upon defining and measuring aromaticity. It is commonly asserted that the words "aromatic compounds" were introduced at the end of the eighteenth century to distinguish such molecules from their aliphatic homologues because of their pleasant olfactory properties: natural products such as oil of wintergreen, aniseed, sassafras, oil of cinnamon and vanilla beans were included at this time in the "aromatic class" of compounds. Here, then, is the first difficulty about the problem we are trying to solve: a scientist who discovers as Kekule did a fundamental new concept must surely introduce a new an well-chosen word to define it. If he does not do so, the risk is that complete confusion very soon occurs both in books and in the minds of men. We are of the opinion that one cause of the confusion which reigns today concerning the question: "What is aromaticity?" lies in Kekule's original choice of word, but it is also clear that the chemists who have worked

Critique of the notion of aromaticity

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.

Jean François Labarre

Papers

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

Analyse conformationnelle théorique du diméthylsulfoxyde (CH3)2SO

Critique of the notion of aromaticity

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