Infrared spectrum, structure, and properties of the O2–Ar van der Waals molecule

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Abstract
The infrared spectrum of a gaseous mixture of oxygen and argon at 93 °K reveals descrete features which are attributed to absorption by the O2–Ar van der Waals molecule. The spectrum occurs near the infrared inactive vibration of O2. One band, ν1, corresponds to the stretching frequency of O2 within O2–Ar. An analysis of the P and R envelope of this band provides an approximate intermolecular distance for O2–Ar of Re = 3.5 Å. Other absorption features extend as much as 50 cm−1 to the high and low frequency of the ν1 fundamental. These features are assigned as libration or hindered rotation of O2 within the van der Waals molecule in combination with the ν1 fundamental. A model is developed to account for these rotatory energy levels in which the anisotropic intermolecular interaction in O2–Ar is approximated by a simple sinosoidal potential. The internal rotatory energy levels are then obtained from a quantum mechanical calculation. Using this model the infrared data are shown to be consistent with an angular interaction which favors a ``T'' orientation with a barrier to internal rotation of 30 cm−1 (85 cal/mole). Thus a picture of O2–Ar emerges in which molecules with rotatory energy less than the barrier height undergo librational motion and will appear semirigid or ``locked'' into a ``T'' configuration. van der Waals molecules with rotatory energies greater than the barrier height will undergo hindered internal rotation. This study, in addition to other recent work, indicates the power of spectroscopy to provide a mapping of intermolecular interactions of small molecules.