Hybrid-DFT study and NBO interpretations of the conformational behavior of 1,2-dihalodisilanes

Hybrid-density functional theory (B3LYP/Def2-TZVPP) based method and NBO interpretation were used to investigate the conformational behavior of 1,2-dihalodisilanes [halo=F (1), Cl (2), Br (3), I (4)]. The B3LYP/Def2-TZVPP results showed that the anti conformations of compounds 1-4 are more stable than their corresponding gauche conformations. The stability of the anti conformation compared to the gauche conformation increases from compound 1 to compound 4. The NBO analysis of donor-acceptor interactions showed that the generalized anomeric effect (GAE) is in favor of the gauche conformations of compounds 1 and 2. Contrary to compounds 1 and 2, GAE is in favor of the anti conformations of compounds 3 and 4. The GAE values calculated (i.e. GAEanti-GAEgauche) increase from compound 1 to compound 4. On the other hand, the calculated dipole moment values for the gauche conformations increase from compound 1 to compound 3 but decreases from compound 3 to compound 4. Based on the results obtained, there is no conflict between the GAE and the electrostatic model impacts on the conformational preferences in compounds 1-3 but the electrostatic model can not rationalize the increase of the instability of the gauche conformation of compound 4 compared to its anti conformation on going from compound 3 to compound 4. Consequently, in the conflict between the GAE and the electrostatic model, the former succeeded in accounting for the increase of the anti conformation stability from compound 1 to compound 4. There is a direct correlation between the calculated GAE, Δ[rSi-Si(G)-rSi-Si(A)] parameters. The correlations between the GAE, bond orders, ΔGAnti-Gauche, ΔG‡(Gauche→Gauche′, C2v), ΔG‡(Anti→Gauche, C2), dipole-dipole interactions, structural parameters and conformational behaviors of compounds 1-4 have been investigated.