Structure of the chlorobenzene-argon dimer: Microwave spectrum and ab initio analysis

The rotational spectra of the Cl-35 and Cl-37 isotopes of the chlorobenzene -argon van der Waals dimer have been assigned using Fourier transform micro wave spectroscopy techniques. Rotational constants and chlorine nuclear qua drupole coupling constants were determined which confirm that the complex h as C-s symmetry. The argon is over the aromatic ring, shifted from a positi on above the geometrical ring center towards the substituted carbon atom, a nd at a distance of about 3.68 Angstrom from it. This distance is 0.1-0.2 A ngstrom shorter than the similar distance in the benzene-argon and fluorobe nzene-argon complexes. Experimental results are confirmed and explained wit h the help of second-order Moller-Plesset perturbation calculations using a VDZP+diff basis set. The complex binding energy of the chlorobenzene-argon complex is 1.28 kcal/mol (fluorobenzene-argon, 1.17; benzene-argon, 1.12 k cal/mol) reflecting an increase in stability caused by larger dispersion in teractions when replacing one benzene H atom by F or by Cl. The structure a nd stability of Ar .C6H5-X complexes are explained in terms of a balance be tween stabilizing dispersion and destabilizing exchange repulsion interacti ons between the monomers. (C) 2000 American Institute of Physics. [S0021-96 06(00)01644-5].