HIGH-RESOLUTION NEAR-INFRARED SPECTROSCOPY OF HCL-DCL AND DCL-HCL - RELATIVE BINDING-ENERGIES, ISOMER INTERCONVERSION RATES, AND MODE-SPECIFIC VIBRATIONAL PREDISSOCIATION

Both D- and H-bonded isomers of the mixed dimers formed between HCl an d DCl are investigated via high resolution infrared difference frequen cy and diode laser spectroscopy in the 2885 and 2064 cm(-1) regions. F rom an analysis of the relative integrated absorption intensities, the D-bonded complex (i.e., HCl-DCl) is determined to be more stable by 1 6+/-4 cm(-1) than the H-bonded (i.e., DCl-HCl) species. All four chlor ine isotopic combinations of the lower energy (HCl-DCl) complex are pr obed via excitation of both HCl(v(acc)(HCl) = 1 <-- 0) and DCl (v(don) (DCl) = 1 <-- 0) stretches. Additionally, two chlorine isotopomers of the higher energy (DCl-HCl) complex are investigated through HCl excit ation. Compared to the facile tunneling observed in both (HCl)(2) or ( DCl)(2) complexes, these mixed dimers exhibit more rigid behaviour cha racteristic 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 l evel of angular localization of the wave function is modeled via 3D qu antum mechanical calculations including all three internal rotor angul ar 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 predissoc iation lifetimes in excess of the slit jet instrument line shape are d etermined from homogeneous broadening of the spectral line widths. The HCl stretch excited lifetime of H-bonded DCl-HCl [Delta nu(don)(HCl) = 44(6) MHz, tau(don)(HCl) = 3.6(5) ns] is threefold shorter than the corresponding lifetime of D-bonded HCl-DCl [Delta nu(acc)(HCl) = 16(3) MHz, tau(acc)(HCl) = 9.6(16) ns]. This ratio is quite comparable to t he results obtained in investigations of (HCl)2 and consistent with a stronger, mode specific coupling to the dissociation coordinate for ex citation of the bonded-HX vs free-HX moiety. However, the absolute lif etimes of both v(acc)(HCl) = 1 HCl-DCl and vdonHCl = 1 DCl-HCl complex es are tenfold shorter than the corresponding excited vibrational stat e lifetimes in (HCl)(2). This suggest a near resonant channel for pred issociation into HCl(v = 0) + DCl(v = 1) which minimizes the energy de posited into rotation and relative translation of the diatomic fragmen ts.