FULL PAPER Exploring rare chemical phenomena using fractional nuclear charges: The cis-effect in N 2 F 2 Filipe Teixeira | Andr  e Melo | M. Nat  alia D. S. Cordeiro Department of Chemistry and Biochemistry, LAQV-REQUIMTE, Faculty of Sciences of the University of Porto, Rua do Campo Alegre, 4169-007, Porto, Portugal Correspondence Filipe Teixeira, and Natalia DS Cordeiro, LAQV-REQUIMTE, Faculty of Sciences of the University of Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal. Email: filipe.teixeira@fc.up.pt or Email: ncordeir@fc.up.pt Funding information Fundaç~ ao para a Ci^ encia e a Tecnologia (FCT/MEC; FEDER, under the Partnership Agreement PT2020), Projects: UID/QUI/ 50006/2013 and POCI/01/0145/FEDER/ 007265 Abstract The 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-N 2 F 2 . This allowed the survey of a large number of isomeric pairs composed sharing the same molecular topology, while varying the elec- tronegativity 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 link- ing 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 NAF 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 gov- erning the cis-effect in N 2 F 2 were further tested on other compounds with a similar bond topology. KEYWORDS cis-effect, fractional nuclear charge, QTAIM 1 | INTRODUCTION The development of novel two-dimensional materials has greatly expanded, from the carbon-centered development of graphene applications into an ever-expanding world where new materials using the most common elements on the periodic table are being developed. [1] Among these new materials, boron nitride is one of the most common, yielding interesting applications in electronics [2] and medicine, [3] and re-kindling the interest for nitrogen chemistry. At the same time, azo compounds such as azobenzene play an important role as a stabilizer in polymers and liquid crystal appli- cations. [4] Difluorodiazene (dinitrogen fluoride, N 2 F 2 ) is a lesser known member of the diazene family, remarkable for the fact that cis-difluorodiazene (1, in Figure 1) is considerably more stable than its trans-isomer (2, in Figure 1). [5] This phenomenon was first reported by Craig et al. [6] in the 1970s and is shared with a number of ethylene derivatives, such as 1,2-difluoroethene (CHFCHF), and 1,2-dichloroethene. These compounds share a com- mon feature that DE5E trans 2E cis >0 (1) were E trans and E cis correspond to the energy of the trans- and cis-isomers, respectively. Nevertheless, this so-called cis-effect is much more promi- nent in the case of N 2 F 2 , scoring an energetic difference between trans- and cis-difluoronitrene, DE, of 112:7561:7 kJ.mol 21 . As a comparative Int J Quantum Chem. 2018;e25662. https://doi.org/10.1002/qua.25662 http://q-chem.org V C 2018 Wiley Periodicals, Inc. | 1 of 15 Received: 7 December 2017 | Revised: 3 April 2018 | Accepted: 19 April 2018 DOI: 10.1002/qua.25662 Int J Quantum Chem. 2018;118:e25662. http://q-chem.org © 2018 Wiley Periodicals, Inc. 1 of 15 https://doi.org/10.1002/qua.25662