Acidic protons before take-off:: A comparative jet Fourier transform infrared study of small HCl- and HBr-solvent complexes

A comprehensive study of the proton vibrational dynamics in isolated (HX)(m).(M)(n) (X = Cl, Br, and M = H2O, D2O, CH3OH) clusters is presented. Mid-IR (1900-3900 cm(-1)) spectra are measured in pulsed, seeded slit nozzle expansions. A close similarity among the HCl and HBr systems is revealed and exploited for cluster assignment. Distinct absorptions by small HX and mixed HX-M clusters are superimposed on broadbands, partly due to electrolytically dissociated HX in larger mixed clusters. Assignments of bands corresponding to mixed dimers and trimers based on their X-H and O-H stretching fundamentals are provided. The broad Cl-H stretching band profile of the 1:2 complex is indicative of sub-picosecond intracluster vibrational energy redistribution. The assignments are compared to matrix isolation experiments, when available. Systematic quantum chemistry calculations are performed for HCl-water complexes with up to three molecules and their isotopomers. Hydrogen bond induced frequency shifts of Cl-H stretching vibrations are best described at the coupled-cluster [CCSD(T)] level of theory. Lower level correlated quantum-chemical calculations clearly overestimate Cl-H frequency shifts. The low abundance of mixed clusters in the size range close to electrolytic dissociation (n = 3-5) is discussed. (C) 2003 American Institute of Physics.