A MP2(full) theoretical investigation on the π-halogen interaction between OCBBCO and X1X2 (X1, X2 = F, Cl, Br)

The π-halogen bond interactions are found between the BB triple bond and X1X2 (X1, X2 = F, Cl, Br) employing MP2(full) method at 6-311+G(2d), aug-cc-pVDZ and aug-cc-pVTZ levels according to the “CP (counterpoise) corrected potential energy surface (PES)” methodology, accompanied by the BB bond contraction. The (2, 3) extrapolated energies using the two-point approximation are also reported. All the π-halogen complexes are of electronic state ¹A₁ with the C_2V symmetry. The dipole moment of dihalogen, the effects of the polarization of the halogen atom X1 and the electron withdrawing of X2 influence the strength of π-halogen bond interaction. The analyses of the natural charges, natural bond orbital (NBO), atoms in molecules (AIM) theory and electron density shifts reveal the nature of the π-halogen bond interactions, and explain the origin of the BB bond contraction. The energy decomposition analysis at B3LYP/TZ2P level shows that the interaction energy in the OCBBCO⋯X1X2 is mainly determined by the orbital energy. The values of ΔE_int, ΔE_elstat, ΔE_pauli and ΔE_orb are all arranged in the order of OCBBCO⋯BrF > OCBBCO⋯ClF ≈ OCBBCO⋯FCl > OCBBCO⋯BrCl > OCBBCO⋯Br₂ > OCBBCO⋯Cl₂ > OCBBCO⋯ClBr > OCBBCO⋯FBr. The binding energy of the complex of OCBBCO with X1X2 is stronger than that of the corresponding HCCH⋯X1X2 complex. OCBBCO⋯F₂ is indicative of covalent interaction. These results confirm that OCBBCO can be as π-electron donor to form the π-halogen bond interaction.