Exploring rare chemical phenomena using fractional nuclear charges: The cis‐effect in N2F2

Abstract

AbstractThe so called cis‐effect, in which the cis form of an ethylenic compound is more stable than its trans‐isomer, is a good example of a rare phenomenon described only for a few isomeric pairs. In this work, a novel approach to the study of this rare effect is attempted by changing the nuclear charges of the fluoride and nitrogen in cis‐ and trans‐N2F2. This allowed the survey of a large number of isomeric pairs composed sharing the same molecular topology, while varying the electronegativity of selected atoms. The results show an increasing stabilization of the cis‐isomers when increasing the electronegativity of the fluoride atoms, in line with the accepted zeitgeist linking this effect to the electronegativity of the peripheral groups. However, a similar behavior was also found when varying the nuclear charge of the nitrogen atoms. This prompted a more in‐depth look at the electronic structure of these compounds, gathering descriptors from Bader's Quantum Theory of Atoms in Molecules. A descriptive linear model was derived from these descriptors using a two‐stage stepwise linear regression. Further inspection of the model linked the destabilization of the trans‐isomers with a deviation from the idealized trigonal planar geometry predicted by the Valence Shell Electron Pair Repulsion model. This tendency was explained by an increase in the ionic character of the NF bonds. Furthermore, the current model suggests that the Coulombic repulsion between the two peripheral atoms in the cis‐isomers prevents such large deviations from the idealized geometry, thus, contributing to their relative stabilization. The qualitative rules governing the cis‐effect in N2F2 were further tested on other compounds with a similar bond topology.