High resolution near infrared spectroscopy of HCl–DCl and DCl–HCl: Relative binding energies, isomer interconversion rates, and mode specific vibrational predissociation

Abstract

Both D- and H-bonded isomers of the mixed dimers formed between HCl and DCl are investigated via high resolution infrared difference frequency and diode laser spectroscopy in the 2885 and 2064 cm−1 regions. From an analysis of the relative integrated absorption intensities, the D-bonded complex (i.e., HCl–DCl) is determined to be more stable by 16±4 cm−1 than the H-bonded (i.e., DCl–HCl) species. All four chlorine isotopic combinations of the lower energy (HCl–DCl) complex are probed via excitation of both HCl (vHClacc=1←0) and DCl (vDCldon=1←0) stretches. Additionally, two chlorine isotopomers of the higher energy (DCl–HCl) complex are investigated through HCl excitation. Compared to the facile tunneling observed in both (HCl)2 or (DCl)2 complexes, these mixed dimers exhibit more rigid behavior characteristic of two distinct isomeric species. However, the relatively small energy difference (16±4 cm−1) between the two isomers still allows the wave functions for both species to sample both the HCl–DCl and DCl–HCl local minima on the potential surface. This intermediate level of angular localization of the wave function is modeled via 3D quantum mechanical calculations including all three internal rotor angular degrees of freedom. Additionally, a 1D treatment along the minimum energy tunneling path is investigated, which quantifies the asymmetry in the tunneling coordinate due to isotopic dependence of the H- and D-bonded zero point bending and torsion energies. Vibrational predissociation lifetimes in excess of the slit jet instrument line shape are determined from homogeneous broadening of the spectral line widths. The HCl stretch excited lifetime of H-bonded DCl–HCl [ΔνHCldon=44(6) MHz, τHCldon=3.6(5) ns] is threefold shorter than the corresponding lifetime of D-bonded HCl–DCl [ΔνHClacc=16(3) MHz, τHClacc=9.6(16) ns]. This ratio is quite comparable to the results obtained in investigations of (HCl)2 and consistent with a stronger, mode specific coupling to the dissociation coordinate for excitation of the bonded-HX vs free-HX moiety. However, the absolute lifetimes of both vHClacc=1 HCl–DCl and vHCldon=1 DCl–HCl complexes are tenfold shorter than the corresponding excited vibrational state lifetimes in (HCl)2. This suggests a near resonant channel for predissociation into HCl(v=0)+DCl(v=1) which minimizes the energy deposited into rotation and relative translation of the diatomic fragments.