High resolution, jet-cooled infrared spectroscopy of (HCl)2: Analysis of ν1 and ν2 HCl stretching fundamentals, interconversion tunneling, and mode-specific predissociation lifetimes

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Abstract
An extensive series of near-infrared absorption spectra are recorded for jet-cooled (6–14 K) hydrogen chloride dimer (HCl)2. Both ΔKa=0 and ΔKa=±1 bands are observed for both the free (ν1) and bonded (ν2) HCl stretches; all three chlorine isotopomers (H 35Cl–H 35Cl, H 35Cl–H 37Cl, and H 37Cl–H 37Cl) are observed and analyzed for K″a ≤ 2. The slit jet spectrum extends significantly the previous cooled cell infrared study of this complex and provides a measure of tunneling splittings for Ka=0 and 1 for each of the HCl ground (v=0) and excited (v=1) states. Mode specific vibrational predissociation is observed via analysis of the absorption line shapes, with Lorentzian contributions to the line profiles of Δν1≲1.6 MHz and Δν2=5.1±1.2 (2σ) MHz full width at half-maximum for ν1 and ν2 excitation, respectively. Stronger coupling in (HCl)2 of the bonded (ν2) vs free (ν1) HCl vibration to the dissociation coordinate is consistent with the comparable trends observed in other hydrogen bonded dimers. Quantum mechanical variational calculations on an electrostatic angular potential energy surface are used to model the internal HCl rotor dynamics using a coupled rotor formalism; analysis of the internal rotor eigenfunctions provides direct evidence for large amplitude ‘‘geared’’ internal rotation of the HCl subunits.