The Cis-Effect Explained Using Next-Generation QTAIM.

Area of Science:

• Quantum chemistry
• Molecular modeling
• Chemical bonding theory

Background:

• The cis-effect, a phenomenon influencing molecular geometry and reactivity, is not universally observed.
• Understanding the underlying electronic factors governing the cis-effect is crucial for predicting molecular behavior.

Purpose of the Study:

• To elucidate the origins of the cis-effect in C2X2 and N2X2 molecular families (X = H, F, Cl).
• To determine why the cis-effect is an exception rather than the rule in chemical bonding.

Main Methods:

• Application of next-generation Quantum Theory of Atoms in Molecules (NG-QTAIM) for detailed electronic structure analysis.
• Analysis of stress tensor trajectories (Tσ(s)) to track bond critical point (BCP) motion under torsional stress.
• Consideration of multi-reference effects and electronic charge density (ρ(r)) dynamics.

Main Results:

• The cis- and trans-characteristics were correlated with bond-bending and bond-twisting components of Tσ(s), respectively.
• The relative ease of motion of electronic charge density dictates the prevalence of the cis-effect.
• Qualitative agreement was achieved with existing experimental data.

Conclusions:

• The cis-effect's rarity is explained by the specific dynamics of electronic charge density and BCP motion.
• NG-QTAIM provides a robust framework for understanding subtle bonding phenomena.
• Predictions for uncharacterized systems were made based on the established methodology.