Library

Notes: Rate constants have been measured for the reactions of O− with CH4, CH2D2, and CD4 as a function of ion-neutral average center-of-mass kinetic energy, 〈KEcm〉, at several temperatures over the range 93 K–565 K using a selected ion flow drift tube apparatus. For the CH4 reaction we also report measurements made using a high-temperature flowing afterglow (HTFA) instrument over the temperature range 300 K–1313 K. The rate constants are found to have a very large isotope effect, with the CH4 rate constant a factor of 15 higher than the CD4 rate constant at 93 K. The rate constants generally have a minimum with respect to temperature and 〈KEcm〉, except for the higher-temperature data for CD4 where the rate constants show only an increase with increasing kinetic energy. The data indicate that increasing rotational temperature decreases the rate constants and that rotational energy behaves similarly to translational energy. Single excitations of bending and twisting vibrations have a negligible effect on the rate constant. Either the stretching vibrations or overtones of the bending vibrations increase the rate constants. If the stretches are responsible for the increase in the rate constants, the derived rate constant for a single quantum of stretch excitation (v=1) is 5×10−10 cm3 s−1, a factor of 6 larger than the rate constant for v=0. The CH2D2 rate constants are approximately equal to the averages of the rate constants for the pure isotopes. The product branching ratio (OH−/OD−) shows no dependence on CH2D2 rotational temperature or low-frequency CH2D2 vibrations. A theoretical study of the minimum energy reaction path was performed to help elucidate the reaction dynamics. The minimum energy reaction surface was characteristic of the standard double minimum pathway for ion molecule reactions. The height of the central barrier was found to be close to the energy of the reactants and varied with isotopic substitution. Conformationally different transition states are found for these isotopic reactions. Theoretical studies at the QCISD(T) level of theory find distinct transition states corresponding to O−+CH4, O−+H-CHD2, O−+D-CH2D, O−+CD4. The transition state barriers increase in the order O−+CH4, O−+H-CHD2, O−+D-CH2D, and O−+CD4, in agreement with experimental reaction rates. The main features of the reactivity are explained by the characteristics of the reaction surface. © 1997 American Institute of Physics.