Infrared spectrum of HCl–O2 dimer and tentative assignment of HCl–(O2)3 tetramer

Spectra of the weakly bound dimer HCl–O2 were studied in the mid-infrared region of the HCl fundamental band (≈2890 cm−1) using a tunable optical parametric oscillator source to probe a pulsed supersonic slit jet expansion. This is the first reported gas-phase spectrum of HCl–O2 in any region. Over 180 H35Cl–O2 and H37Cl–O2 rotational transitions were assigned and interpreted in terms of four sub-states: a Σ ground state, a Π state at about 2.3 cm−1, another Σ state at about 3.3 cm−1, and another Π state at an unknown energy (possibly 1–2 cm−1). For each isotopologue, rotational energies for 75 levels in the ground and excited vibrational states [v(H–Cl) = 0 and 1] were derived directly from the spectra. Efforts to interpret the spectra using an existing theory of weakly bound dimers containing O2 were not very successful, probably due to the presence of large amplitude intermolecular motions. Spectra of two larger clusters, likely each containing one HCl and multiple O2 molecules, were also assigned. One of these spectra, is well simulated by a symmetric top molecule with B = 0.028 cm−1 and is likely due to the tetramer HCl–(O2)3. Although the other spectrum is not understood at all, its dependence on O2 concentration and apparent B-value (≈0.045 cm−1) suggest that it could be due to the trimer HCl–(O2)2.