Systematic theoretical study of structures and bondings of the charge-transfer complexes of ammonia with HX, XY, and X₂ (X and Y are halogens)

The hybrid Hartree-Fock density functional theory (HF-DFT) method was employed with the combined use of CEP-121G(d,p) for H, N, F, and Cl, and RCEP(d) for Br and I to produce the equilibrium structures for both component molecules and the title charge-transfer (CT) complexes. Results are close to experimental data. The intermolecular stretching force constant k derived from the parabolic fit of its energy curve is parallel to the experimental value. The excellent linear relationship between the calculated k and the binding energy E_B shows the consistency of the present work in describing the intermolecular bonding intensity. The step-by-step charge-transfer model was proposed to directly evaluate the extent of intra- and intermolecular CT. The intermolecular CT amount, δ₁, is not large, in accordance with the weak bonding in the complex. But the CT effect is found to be significant in determining the bonding strength. δ₁ is parallel to the dipole moment of the complex. The change of the total dipole moment in the complex formation can be used in the quantitative assessment of the electrostatic and nonelectrostatic contributions of the intermolecular bonding.