Abstract
We used next-generation QTAIM (NG-QTAIM) to explain the cis-effect for two families of molecules: C(2)X(2) (X = H, F, Cl) and N(2)X(2) (X = H, F, Cl). We explained why the cis-effect is the exception rather than the rule. This was undertaken by tracking the motion of the bond critical point (BCP) of the stress tensor trajectories T(σ)(s) used to sample the U(σ)-space cis- and trans-characteristics. The T(σ)(s) were constructed by subjecting the C1-C2 BCP and N1-N2 BCP to torsions ± θ and summing all possible T(σ)(s) from the bonding environment. During this process, care was taken to fully account for multi-reference effects. We associated bond-bending and bond-twisting components of the T(σ)(s) with cis- and trans-characteristics, respectively, based on the relative ease of motion of the electronic charge density ρ(r(b)). Qualitative agreement is found with existing experimental data and predictions are made where experimental data is not available.