Infrared absorption spectroscopy of the CO2–Ar complex in the 2376 cm−1 combination band region: The intermolecular bend

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Abstract
The rovibrational absorption spectrum of CO2–Ar has been recorded in the 2376 cm−1 region by using a pulsed, slit nozzle expansion and tunable IR diode laser. The spectrum is ascribed to the 410510 combination band, where ν4 corresponds to the asymmetric stretch of the CO2 moiety and ν5 is the intermolecular bend. This combination band is ∼102 times less intense and 27.82 cm−1 higher in energy than the 410 fundamental. Effective upper‐state rotational constants were determined for this nearly prolate rotor, and the ν4+ν5 moments of inertia show a large inertial defect, Δ=9.48 amu Å2. In planar molecules, this indicates large vibrational amplitude, and reflects inadequacy of the rotational Hamiltonian. From the moments of inertia derived using a Watson Hamiltonian, geometric properties of ν4+ν5 can be calculated. The average O–C–Ar angle is 76.9°, while the center‐of‐mass separation between CO2 and Ar is 3.60 Å. These values differ from those of the ground state (83.1° and 3.51 Å) and ν4=1 (83.0° and 3.50 Å). Results are compared with previous work, which used different models and approximations to obtain intermolecular frequencies. In addition, inertial defects were calculated for the ground and vibrationally excited states using a simple normal mode model developed by Herschbach and Laurie, and Oka and Morino. With harmonic force fields, it accounts for most of the inertial defect in the ground and excited states, and yields reasonable R and θ values. However, we conclude that the good agreement between experimental and calculated inertial defects may be fortuitous, since several positive and negative contributions cancel.