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 C2V 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 ΔEint, ΔEelstat, ΔEpauli and ΔEorb 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.