HINDERED ROTATION IN VAN DER WAALS' MOLCULES

Author Institution: Deparatment of Chemistry, Indiana UniversityThe internal rotation of a diatomic molecule, $A_{2}$, in an $A_{2}$- X Van der Waals ‘ molecule can be strongly perturbed by anisotropic intermolecular interactions. Such perturbations are clearly revealed in the vibraional-rotational spectra of $O_{2} - Ar$ and $N_{2}-Ar$ reported elsewhere in this symposium (paper H4). A variational calculation of the internal rotatory energies will be presented in which the angular dependent intermolecular ineraction is approximated by a sinusoidal potential function. As the perturbation increases the free rotor levels are shifted and split somewhat like a Stark effect. An energy diagram corrlating the free rotor levels with bound librator levels is obtained by systematically varying the rotational barrier height. A comparison of calculated librator and hindered rotor transition energies with observed fine strucure yields he preferred geometry and an estimate of the internal rotational barrier of the $A_{2}$ -X Van der Waals ‘ molecule