Probing hydrogen bond potential surfaces for out-of-plane geometries: Near-infrared combination band torsional (ν6) spectroscopy in (HCl)2

High-resolution near-infrared (IR) spectra of jet cooled (HCl)2 are obtained in the 3000–3070 cm−1 combination band region corresponding to simultaneous excitation of intermolecular out-of-plane torsion (ν6) with intramolecular HCl stretching (ν1,ν2) degrees of freedom. Two (H35Cl)2 bands (plus the isotopically mixed H35Cl–H37Cl bands) due to ν6 torsional excitation are observed. High-resolution spectral analysis of ground-state combination differences and nuclear spin intensity alternation effects permits unambiguous identification of J labeling and tunneling level symmetry, yielding precise rotational constants and vibrational band origins. Comparison with previous far-IR studies suggests assignment of the 3036.52(5) and 3014.86 cm−1 bands to “bound HCl stretch+torsion” (ν2+ν6) combination excitation out of the lower (Γvt=A−←A+) and upper (Γvt=B−←B+) tunneling levels, respectively. A third much weaker band (3035.37 cm−1) is also observed in close proximity with the ν2+ν6 (Γvt=A−←A+) origin, which most likely acquires oscillator strength by state-mixing between the “dark” perturbing state and the ν2+ν6 upper level. These data provide the first opportunity for comparison with exact quantum calculations by Qui and Bačić and co-workers on high level ab initio/semiempirical (HCl)2 potential surfaces, which should facilitate further refinement of intermolecular potentials for out-of-plane twisting of the hydrogen bond.