Calculated rotational spectrum of Ar-CO from an ab initio potential energy surface: A very floppy van der Waals molecule

Using the ab initio potential of Shin ef al. (to be published), we have calculated the bound states and infrared absorption spectrum of the van der Waals complex Ar.+CO. The results show that Ar.*CO cannot be treated as a quasirigid rotor, nor as a molecule with a free internal rotor. In particular, a transition to the first excited van der Waals bending level is predicted to be present in the spectrum, and its frequency varies with s1 (the projection quantum number of the total angular momentum onto the intermolecular axis going from the center of mass of CO to the Ar atom). It is also shown that, although the spectrum cannot be analyzed by the use of a rigid rotor model, rotational “constants” can still be defined for each value of a. This is consistent with the available experimental data and the predicted bending excitation can account for unassigned transitions in the infrared spectrum of this complex. Finally, a sensitivity analysis of the calculated spectrum with respect to the potential anisotropy has been performed.