Knowledge Bank

Vibration-rotation transitions of the $C2H2-Ar$ complex have been observed in the doubly degenerate monomer $\nu 5$ region via direct IR absorption of tunable diode-laser radiation in a pulsed supersonic free jet. The spectrum is composed of in- and out-of-plane C---H bending bands, which are coupled with each other through a strong b-type Coriolis interaction. Anomalous Coriolis-coupling and A rotational constants indicate the failure in a semirigid-molecule description of this highly nonrigid system in the doubly degenerate bending manifold. A dynamical model calculation, where the large-amplitude vdW bending is treated as an internal rotation of the $C2H2$ subunit hindered by an anisotropic intermolecular potential, has revealed that the characteristic energy--level structure originates from its specific angular momentum coupling scheme involving an additional angular momentum for the intramolecular degenerate bending. The difference potential providing the parity doubling in the C---H bending manifold is estimated to be relatively small, - $-0.1cm-1$.