Zero-pressure thermal-radiation-induced dissociation of gas-phase cluster ions: Comparison of theory and experiment for (H2O)(2)Cl(-) and (H2O)(3)Cl(-)

The kinetics of dissociation of (H2OCl- and (H2OCl- at essentially zero pressure by absorption of infrared photons from the background radiation field (ZTRID) are compared with theoretical expectations and kinetic modeling. New experimental dissociation results as a function of temperature are reported for (H2OCl-. Density functional calculations of the complex structures and the normal mode frequencies and integrated infrared absorption intensities are described. A master equation approach was used to model the dissociation rates predicted from these molecular properties. Using literature dissociation enthalpies in the modeling, order-of-magnitude agreement between experimental and predicted dissociation rates with no adjustable parameters was found. Alternatively, taking the ZTRID measurements as independent determinations of the dissociation enthalpies, values of Delta H-diss(298) of 11.6 kcal ((H2OCl-) and 9.5 kcal ((H2OCl-) were derived, in modest disagreement with literature. Redetermination of the (H2OCl- dissociation enthalpy by high-pressure mass spectrometry gave 11.9 kcal mol(-1), in excellent accord with the ZTRID value. The ZTRlD Delta H-diss(298) value for (H2OCl- was confirmed by analysis of the temperature dependence, and we consider it to be reliable. The success of this modeling is taken as a strong validation of our picture of the ZTRID process, and this is suggested as a promising new approach to estimation of ion dissociation thermochemistry.