Peter Birner

Quantum chemical investigations of the π-acceptor ability of α-diimine ligands

Abstract The bonding properties of α-diimine complexes of transition metals in low oxidation states are essentially determined by π-back donation. The energies of the lowest unoccupied molecular orbitals of the ligands have been used to estimate their acceptor ability. MO calculations of selected α-diimines are reported in the present paper. N-unsubstituted and apliphatic diimines are found to be better π-acceptors than 2,2′-bipyridine which is in agreement with experimental results. It turns out that the LUMO energies calculated by the NDDO method are a good measure for the π-acceptor behaviour of ligands with similar structure towards transition metal ions.

Quantum chemical calculations for the determination of the molecular structure of conjugated compounds: Part XIV. On the conformational structure of α-diimine ligands

Abstract The conformational structure of some typical α-diimine ligands (2,2′-bipyridyl, glyoxaldiimine, glyoxal-N,N′-dimethyldiimine) is examined using the NDDO method. An analysis of the potential energy curves indicates the influence of specific lone-pair effects on the molecular structure. Based on a continuum model, changes of the molecular arrangement caused by solvent effects are estimated.

Quantum chemical calculations for the determination of the molecular structure of conjugated compounds

Abstract The conformational structure of trans - and cis -stilbene and trans - and cis -azobenzene was examined using the NDDO method. Contrary to the results of other semi-empirical all-valence electron methods, the main features of the molecular geometry are correctly accounted for. According to the results of the NDDO method, the nitrogen inversion mechanism is favoured over the rotational mechanism for the cis - trans isomerization of azobenzene.

CH acidity comparative CNDO/2 and NDDO calculations on the reactivity of azabenzenes

Abstract It is shown that in contradiction to results obtained with the CNDO/2 method the NDDO method is successful in accounting for the effect of nitrogen lone pairs on the stability of π-delocalized and σ-localized carbanions derived from various azabenzenes.

Success of the NDDO method to predict conformations in conjugated systems

Abstract The potential functions for the rotation about the essential single bonds in some conjugated compounds as nitrobenzene, nitrosobenzene, benzoic acid, benzamide, acrolein, furfural and phenol have been calculated using the NDDO formalism. Contrary to the known results of the CNDO/2 method, the preferred conformations of these systems are correctly accounted for.

Quantum chemical investigations of the conformational structure of protonated 2,2'-bipyridine

Abstract The structure and conformational behaviour of mono-and di-protonated 2,2′-bipyridine are investigated by a semi-empirical NDDO method. Whereas monoprotonated 2,2′-bipyridine exists in a planar cisoid arrangement, two stable twisted conformations are obtained for the diprotonated cation.