Search

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. [ABSTRACT FROM AUTHOR]

High sensitivity, tunable laser direct absorption methods are exploited to obtain high resolution IR spectra (Δν≾0.001 cm-1) of weakly bound CO2HF complexes in a pulsed supersonic slit jet expansion. Transitions from the ground vibrational state corresponding to a single quantum excitation of the ν1 HF stretch are observed and analyzed with a semirigid linear molecule Hamiltonian. The observed increase in both B (+1.75%) and D (+55%) upon ν1 excitation is inconsistent with the commonly used diatomic approximation, and is not possible to rationalize for a nearly linear upper state geometry with small amplitude zero point motion of the intermolecular CO2 bend coordinate. We consider an alternative centrifugal straightening mechanism which predicts large centrifugal distortion effects due to end over end rotation of a complex with a nonlinear vibrationally averaged geometry in a weak bending potential. In support of this interpretation, hot band spectra are observed arising from bend excited complexes significantly populated in the 16 K expansion; intensity based estimates of the internal excitation indicate a surprisingly low bend frequency of 10±5 cm-1. A preliminary analysis of the spectra as l doubling in a Π←Π vibrational hot band for a linear equilibrium geometry is presented. An alternative interpretation of the spectrum as asymmetry doubling of a K=1←1 rotational hot band for a bent geometry is also considered. This latter interpretation is more consistent with the data and predicts a CO2 bend angle in the complex for K=1 between 25° and 30°. Linewidths for the upper vibrational states in CO2HF exhibit homogeneous broadening 3–4 times in excess of the apparatus resolution. Voigt analysis of the absorption line shapes indicates linewidths (FWHM) of 136±16 MHz and independent of J state; this corresponds to a relaxation lifetime of 1.1±0.2 ns for HF in the complex. [ABSTRACT FROM AUTHOR]