Slit-jet near-infrared diode laser spectroscopy of (DCl)2: ν1, ν2 DCl stretching fundamentals, tunneling dynamics, and the influence of large amplitude ‘‘geared’’ intermolecular rotation

Abstract

The first high resolution spectra of (DCl)2 are reported using direct IR laser absorption spectroscopy in a slit supersonic expansion. The spectral data are analyzed to obtain vibrational frequencies, rotational constants, and tunneling (interconversion) level splittings for isotopically symmetric (D35Cl)2 and (D37Cl)2, and mixed D35Cl–D37Cl dimers. Six dimer absorption bands are observed and analyzed for both (D35Cl)2 and D35Cl–D37Cl. These include two perpendicular Ka=1←0, v1=1←0 (i.e., ‘‘free’’ DCl stretch) bands, one each originating from the lower (+) and the upper (−) tunneling sublevels in the ground vibrational state. Four parallel v2=1←0 (i.e., ‘‘bound’’ DCl stretch) bands are also observed, one for each of the Ka=0←0 and Ka=1←1 subbands originating from both the lower (+) and upper (−) tunneling components. In addition, two bands are observed only for the isotopically mixed dimer (i.e., complexes from D35Cl and D37Cl), which acquire oscillator strength by virtue of the breaking of inversion symmetry. This complete set of bands provides the necessary data to determine interconversion splittings for the mixed dimer in the ground [5.9595(6) cm−1] and the two DCl vibrationally excited states [3.2286(6) cm−1 for v1=1 and 2.9935(6) cm−1 for v2=1], as well as to make accurate predictions for the symmetric (D35Cl)2 dimer. These experimental splittings for the ground state are compared to results from (i) a 1D quantum calculation for adiabatic motion over a minimum energy tunneling path; and (ii) a 3D variational calculation in a basis set of free DCl rotors which treats all three internal bend and torsion angles (Θ1, Θ2, and φ1–φ2). These calculations, performed on an approximate dipole and quadrupole model of the electrostatic potential surface, reproduce the ground state tunneling splittings to within 15%. The corresponding eigenfunctions provide direct evidence for highly correlated, ‘‘geared’’ internal rotation of the two DCl subunits over a low barrier. The fivefold decrease in tunneling splitting for the symmetric (DCl)2 upon v1=1 or v2=1 excitation is qualitatively consistent with previous models of vibrationally diminished tunneling rates due to intramolecular V→V energy transfer at the C2h transition state. However, this decrease is nearly identical to the 4.8-fold decrease observed in (HCl)2, which is quantitatively inconsistent with a simple dipole–dipole vibrational energy transfer mechanism. Measured linewidths in these dimer spectra are all at the resolution limit of the diode laser apparatus, which translates into vibrational predissociation lifetimes in excess of 3 ns.