Probing hydrogen bond potential surfaces for out-of-plane geometries: Near-infrared combination band torsional (v(6)) spectroscopy in (HCl)(2)

Abstract

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 (nu(6)) with intramolecular HCl stretching (nu(1), nu(2)) degrees of freedom. Two ((HCl)-Cl-35)(2) bands (plus the isotopically mixed (HCl)-Cl-35-(HCl)-Cl-37 bands) due to nu(6) 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'' (nu(2) + nu(6)) combination excitation out of the lower (Gamma(vt) = A(-) <-- A(+)) and upper (&UGamma;(vt) = B- <-- B+) tunneling levels, respectively. A third much weaker band (3035.37 cm(-1)) is also observed in close proximity with the nu(2) + nu(6) (Gamma(vt) = A(-) <-- A(+)) origin, which most likely acquires oscillator strength by state-mixing between the "dark'' perturbing state and the &nu;(2) + &nu;(6) upper level. These data provide the first opportunity for comparison with exact quantum calculations by Qui and Bacic 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. (C) 2003 American Institute of Physics.