Theoretical Study of the Effect of Cis–Trans Isomerism on 1,2-Dihaloethylene–Formic Acid Complexes

). Four stable complexes were identified, each exhibiting distinct structural characteristics. Complexes involving the trans isomers are stabilized exclusively through cyclic, double hydrogen-bonding (HB-HB) interactions. In contrast, the cis isomers form structures stabilized by a cooperative triad of hydrogen- and halogen-bonding interactions (HB-XB-HB). Interaction energy analyses indicate that the trans complexes are, overall, the most stable among the identified structures. Although energy decomposition analysis (EDA) confirms that electrostatic interactions constitute the dominant contribution to the stabilization of all complexes, complementary analyses reveal that specific orbital-driven charge-transfer interactions particularly lp(X) → σ* (O-H) play a crucial role in stabilizing the cooperative HB-XB-HB networks formed by the cis isomers.