INTERMOLECULAR INTERACTIONS IN P-CHLOROBENZOIC ACID DIMERS

The molecular structure of the parallel orientation of the p-chloroben zoic acid dimer has been studied at the second-order Moller-Plesset le vel using split-valence-quality basis sets augmented by polarization f unctions. In the obtained dimer structure, the Cl atoms are located be hind the carboxyl groups of the neighboring monomer, while in crystall ine copper and nickel p-chlorobenzoate complexes, the Cl atoms and the benzene nuclei form a parallel displaced umbrella structure with the Cl atoms mutually above the center of the respective benzene ring. The interaction energy for the parallel p-chlorobenzoic acid dimer comple x is calculated at the second-order Moller-Plesset level to be 5.1 kca l mol(-1). The intermolecular attraction is mainly due to dispersion e ffects between the aromatic rings. The obtained distance between the b enzene nuclei of 343 pm is in agreement with the experimental values o f 341.6 and 366.6 pm measured for nickel and copper p-chlorobenzoate c omplexes, respectively. At the second-order Moller-Plesset level, the umbrella structure is found to be bound by 2.6 kcal mol(-1), as compar ed to infinite separations, with a chlorine-ring distance of 364 pm. H owever, at the second-order Moller-Plesset level, the umbrella structu re is not a local minimum.