Langsame Inversion am pyramidal gebundenen Stickstoff: Konfiguration und Konformation beim Strukturtyp der N, N‐Dialkoxy‐alkylamine aus der Sicht eines semiempirischen MO‐Modells

Abstract

AbstractNitrogen inversion and rotation around the N‐O single bond in N, N‐dialkoxyalkylamine systems are discussed in terms of a semi‐empirical MO method which is essentially based on the concepts discussed by Mulliken in connection with the “magic formula”. By taking a simplified structural model and adjusting one empirical parameter, a satisfying agreement with experimental results is obtained. The results allow a chemically transparent interpretation and confirm, to a more quantitative extent, the previously discussed concepts [1].The (spx → p) promotion of the nitrogen lone pair strongly inhibits the inversion process and dominates the simultanous lowering of the σ‐bond energies due to (i) the gain of s‐character in the σ‐involved nitrogen hybrid‐AO's and (ii) the increased σ‐bond overlaps. This dominance is considerably enhanced when electronegative ligands are attached to nitrogen. The total repulsion energy turns out to favour strongly the planar transition state and is essentially determined by the repulsions between the lone pair and the σ‐bonds at nitrogen. Factorization into several repulsive contributions reveals that among these only one inhibits the inversion process, namely the repulsions between the nitrogen lone pair and the bonded and non‐bonded electron pairs on the ligands.For the process of rotation around the N‐O single bond a potential curve is obtained with two energy minima. The repulsion energy analysis shows that the shape of the potential curve is governed by the repulsions between the lone pairs on oxygen and nitrogen as well as the formally more or less “lone pair‐like” σNC‐bond. This situation is compared to the more general one in which essentially two lone pairs or formally more or less “lone pair‐like” σ‐bonds, on each of two adjacent centers, repel each other by conjugative destabilization; a situation which is realized for instance in molecules that show the anomeric effect.