Complete basis set, hybrid-DFT study, and NBO interpretations of the conformational behavior of 1,2-dihaloethanes

The conformational behavior of 1,2-difluoroethane (1), 1,2-dichloroethane (2), 1,2-dibromoethane (3), and 1,2-diiodoethane (4) have been analyzed by means of complete basis set CBS-QB3, hybrid-density functional theory (B3LYP/Def2-TZVPP) based methods and natural bond orbital (NBO) interpretation. Both methods showed the expected greater stability of the gauche conformation of compound 1 compared to its anti conformation. Contrary to compound 1, the anti conformations of compounds 2-4 are more stable than their gauche conformation. The stability of the anti conformation compared to the gauche conformation increases from compound 1 to compound 4. The NBO analysis of donor-acceptor (sigma -> sigma*) interactions showed that the generalized anomeric effect (GAE) is in favor of the gauche conformation of compound 1. Contrary to compound 1, GAE is in favor of the anti conformations of compounds 2-4. The GAE values calculated (i.e., GAE(anti) - GAE(gauche)) increase from compound 1 to compound 4. On the other hand, the calculated dipole moment values for the gauche conformations decrease from compound 1 to compound 4. In the conflict between the GAE and dipole moments, 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, a dagger[r (c-c)(G) - r (c-c)(A)] and a dagger[r (c-x)(A) - r (c-x)(G)] parameters. The correlations between the GAE, bond orders, total steric exchange energies (TSEEs), Delta G (Anti-Gauche), Delta G (aEuro)(Gauche -> Gauche', C (2v)), Delta G (aEuro)(Anti -> Gauche, C (2)), dipole-dipole interactions, structural parameters, and conformational behaviors of compounds 1-4 have been investigated.