Etude MNDO des composes conjugues enneagonaux

Abstract

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.

Azonine, a “Nearly” Forgotten Aromatic Molecule

“…The analogous quantities for Na are 1.0°and 5.3°, and 0.7°and 4.4°. These values clearly demonstrate that the interaction between the free electron pair and H 2 (9) is almost similar to that of the anion for the Na salt, but is smaller for the Li salt. The τ H2(9) values of the Li salt are therefore shifted to a lesser extent.…”

“…Azonine is a planar molecule of C 2V symmetry with a dipole moment of 3.0 D (vs 2.54 D as predicted by MNDO 9 ) that lies on the C 2 axis and points from the center of the ring toward the nitrogen atom. The HOMO-LUMO energy gap of azonine is equal to 4.56 eV, which is 1.78 times smaller than the value determined with MNDO, 9 but corresponds well with the B3LYP/ 6-311G(2d,p) value 27 of 4.57 eV.…”

“…The 1 H NMR spectra, shown in Table 8, contradict, at first sight, the existence of distorted azonide frames, because the aromaticity is apparently enhanced. Evidence for this enhancement is provided by the large downshift of the τ value of H 2 (9) and by the coalescence of the remaining hydrogen peaks. The apparent contradiction is easily lifted for the DMSO results.…”

“…This is also indicated by the respective HOMA values of 0.929 and 0.900. large downshift of the H 2(9) τ values originates from the deshielding influence of the free electron pair. This effect is strong because of the shape of the large azonine ring, which has the H 2 (9) atoms residing close to the free electron pair.…”

“…8 No further experimental research has been performed on this molecule, and its N-substituted derivatives, since the pioneering 1 H NMR, UV-vis, and thermal stability studies by Anastassiou (see ref 8 and references therein). The first computational study of azonine, 9 using the MNDO approach, was carried out in 1986, and the more recent theoretical studies only consider it as a reference system. [10][11][12] The current work presents a thorough theoretical study of azonine and its water complexes to explain the previously mentioned experimental results of Anastassiou.…”