Free-jet infrared absorption spectroscopy of the C2H2–Ar complex in the doubly degenerate monomer C–H bending region

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Abstract
Vibration–rotation transitions of the C2H2–Ar complex have been observed in the doubly degenerate monomer C–H bending (ν5) region via direct IR absorption of tunable diode laser radiation in a pulsed supersonic free jet. The spectrum is composed of in‐plane and out‐of‐plane C–H bending bands, which are coupled with each other through a strong Coriolis interaction. Anomalous Coriolis coupling and A rotational constants indicates 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 van der Waals bending motion is treated as an internal rotation of the C2H2 monomer 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 only about −0.1 cm−1.