Ab Initio Molecular Orbital Study of the Benzene–Chlorine Complex

The geometrical structures and stabilization energies of benzene–chlorine complex were studied theoretically using an ab initio molecular orbital method. Full geometry optimization was carried out for four models ((1) [On Center (C6v symmetry)], (2) [On Atom (Cs symmetry)], (3) [On Bond (Cs symmetry)] and (4a,b) [Resting (Cs symmetry)]) with restricted Hartree–Fock (RHF) and second-order Møller–Plesset perturbation (MP2) methods, MP2 (Frozen), and MP2 (Full), using various basis sets. The RHF method is not good enough to describe the molecular structure and interaction energy of the complex. The geometry obtained with the MP2/DZP + d method has good agreement with the X-ray crystal structure. An unusual basis set superposition error (BSSE) was found when the MP2 method with a frozen-core approximation was applied to evaluate the interaction energy. In the MP2 (Full) method, in which the inner-core wavefunctions was correlated, the reasonable stabilization energy is obtained. The BSSE-corrected interaction energies (ΔE(corr.)) of models(1), (2), and (3) are −6.63, −8.44, and −8.14 kJ mol−1, respectively. The most stable structure of the benzene–chlorine complex was calculated to be model (2). Model (3) lies at only 0.3 kJ mol−1 higher energy than model (2) in the MP2(Full)/DZP + d calculation. The position of chlorine would be very flexible on the benzene ring, and the molecular structure of the complex is easily interchangeable between models (2) and (3).