Structure and stability of fluorine-substituted benzene-argon complexes: The decisive role of exchange-repulsion and dispersion interactions

The van der Waals complexes benzene-argon (BAr), fluorobenzene-argon (FAr), p-difluorobenzene-argon (DAr) are investigated at the second-order Moller- Plesset (MP2) level of theory using the 6-31+G(d), cc-pVDZ, aug-cc-pVTZ, an d [7s4p2d1f/4s3p1d/3s1p] basis sets. Geometries, binding energies, harmonic vibrational frequencies, and density distribution are calculated where bas is set superposition errors are corrected with the counterpoise method. Bin ding energies turn out to be almost identical (MP2/[7s4p2d1f/4s3p1d/3s1p]: 408, 409, 408 cm(-1)) for BAr, FAr, and DAr. Vibrationally corrected bindin g energies (357, 351, 364 cm(-1)) agree well with experimental values (340, 344, and 339 cm(-1)). Symmetry adapted perturbation theory (SAPT) is used to decompose binding energies and to examine the influence of attractive an d repulsive components. Fluorine substituents lead to a contraction of the pi density of the benzene ring, thus reducing the destabilizing exchange-re pulsion and exchange-induction effects. At the same time, both the polarizi ng power and the polarizability of the pi -density of the benzene derivativ e decreases thus reducing stabilizing induction and dispersion interactions . Stabilizing and destabilizing interactions largely cancel each other out to give comparable binding energies. The equilibrium geometry of the Ar com plex is also a result of the decisive influence of exchange-repulsion and d ispersive interactions. (C) 2001 American Institute of Physics.