Hindered internal rotation in jet cooled H2HF complexes

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Abstract
The vibration–rotation spectrum of the HF stretch mode in ortho‐H2HF complexes has been obtained via infrared laser direct absorption detection in a slit supersonic jet expansion. The spectrum resembles a Ka =1←1 parallel band of a prolate near‐symmetric top and can be reasonably well fit with a Watson A‐type Hamiltonian; however, no rigid molecular structure can reproduce the observed Ka splittings without invoking unphysically large changes in the constituent bond lengths upon complexation. The splittings are more correctly analyzed in terms of a j=1 hindered H2 rotor in an anisotropic potential, with a minimum energy T‐shaped geometry. Matrix calculations determine barriers to H2 rotation between 120 and 170 cm−1 that depend systematically both on vibrational and rotational state in the complex. These data are consistent with a strong increase in potential anisotropy with decreasing intermolecular separation, with both upper and lower vibrational states close to the dissociation limit. No evidence for a corresponding Σ←Σ para‐H2HF spectrum is observed, despite adequate experimental sensitivity. The matrix calculations indicate that the ground Σ state of para‐H2HF is less stabilized by the potential anisotropy than the ground Π state in ortho‐H2HF, and may therefore be much less efficiently formed in the jet expansion. The preferential observation of a ground Π vs Σ state in ortho‐H2HF clearly indicates a minimum in the potential surface for a T‐shaped vs collinear geometry. The observed rotational constants strongly suggest a H2⋅⋅⋅H–F ordering. The results provide direct evidence for vibrationally averaged structure, internal rigidity, and intermolecular bond strength that are significantly quantum state dependent, but can be qualitatively understood in terms of simple steric interactions between the H2 and HF subunits.