On the nature of the interaction energy in the Ar–ClF complex

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The analysis of the potential energy surface of the Ar–ClF complex is performed using the perturbation theory of intermolecular forces. The three minima on the potential energy surface correspond to the linear Ar—Cl–F configuration (global minimum De=233 cm−1), the linear Ar—F–Cl configuration (De=133 cm−1), and the T structure in which the Ar atom is nearly perpendicular to the molecular axis of Cl–F (De=146 cm−1). The calculated parameters of the minima are in full accord with the recent ab initio study by Tao and Klemperer [J. Chem. Phys. 97, 440 (1992)]. The absolute minimum results from the attractive dispersion and polarization energies which help overcome a considerable exchange repulsion. The secondary linear minimum Ar—F–Cl, is due, in large measure, to the dispersion energy accompanied by a weaker exchange repulsion. The T configuration is characterized by the weakest repulsion and the dispersion energy roughly equal to that in Ar—F–Cl. The analysis of the angular behavior of the Heitler–London interaction energy leads us to believe that the charge distribution of the Cl–F molecule possesses a concave shape along the molecular axis at the Cl end of the molecule. This indentation in the charge cloud allows subsystems to approach close to one another in the linear Ar—Cl–F arrangement, and also causes an appreciable stiffness of the Ar–Cl–F bending mode.