A MNDO study of molecular geometries, enthalpies, formation, atomization, bond separation, and hydrogenation of a series of unsaturated boron-nitrogen compounds, linear with two and three members and cyclic with three, four, five, and six members are presented. For all these molecules, MNDO calculations are in excellent agreement with available ab initio calculations or experimental data. The high rotational barrier in aminoborane HlBNH2, 30.6 kcal/mol, and the length of bond in iminoborane HBNH, 1.1 83 A, imply strong double and triple BN bond character in these two molecules. In the odd membered heterocycles, examination of the molecular geometries and energies of equilibrium states shows that in all cases, the stability of the compounds grows with the number of boron atoms and decreases with that of the nitrogen atoms. Moreover, compared study of the two BN-fulvenes with their homologous hydrocarbon shows that only BN (BB)-fulvene has a polyenic structure similar to that of fulvene.

ResumeBien que la rCgion M des polyactnes soit beaucoup moins rCactive que leur rigion K, les rCactivitCs de AbstractThis study is a unified approach to quantum theories of polyacen carcinogenesis. Part I discusses the M, L, and B regions theory. Although the M region of polyacens is much less reactive than is the K region, the chemical reactivity of these two regions parallels each other. Consequently, the K-L Pullman's theory may be reformulated by replacing the K region by the M region. The M-L regions theory improves all the quantum structure-carcinogenic activity relationships obtained previously in the K-L framework. Moreover, it would appear that in these correlations, the most important role is played by nonconcerted electrophilic reactions. We suggest that the M-L modified Pullman's theory and the Bay (B) region theory, proposed by Jerina et al., are related to different steps of the metabolic activation mechanism leading to ultimate carcinogen: the first step for the Pullman's theory and the carbocation step for the theory of the Bay region. We have termed the "M, L and B regions theory" as this unified approach to the problem of polyacens carcinogenesis.

ResumeSelon les r6sultat.s obtenus au moyen de MNDO, apds optimisation complkte de la gkomktrie, les anions cyclononatetrwnyle et benzocyclononatetranyle, l'ennkapyrrole et I'enneathiophkne sont des composks plans a tendance aromatique. L'enneafulvkne posskde une conformation pkferentielle plane mais une structure de composk conjugut polybnique alors que I'enneafuranne et le cyclononatetraene sont des molkcules gauches. La methode de Huckel universelle (MHU), qui est une technique emphique simple, tenant compte de tous les klectrons de valence et decrivant les systkmes u au moyen d'oM localis6es, conduit aux m h e s conclusions que la mkthode MNDO. Les calculs effectuks au moyen de MNDO et MHU nous ont permis de d6crire I'ensemble des propri6tks physicochimiques des composks conjuguks enneagonawt pris en considkration.A notre connaissance, nous prksentons ici, la premikre etude tkhrique des composks conjuguks contenant le cycle k neuf membres. AbstractWe present MNLKI calculations of a series of nine-membered conjugated compounds. The geometries have been fully optimized. Cyclononatetraenyl and benzocyclononatetraenyl anions, nonapyrrol, and nonathiophene appear to be planar aromatic compounds. Following MNDO, nonafulvene has a planar prefeted conformation, but a polyenic structure, while nonafuran and cyclononatetraene are gauche molecules. The Universal Huckel Method (UHM), a simple all valence electron empirical technique which describes u systems using localized MO, gives, qualitatively, the same results as the MNDO method. The quantum calculation carried out with MNDO and UHM have been used to describe the physicochemical properties of ninemembered ring conjugated compounds under consideration. To our knowledge, we present here the f i t theoretical study dealing with nine-membered conjugated molecules.

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