Excitation functions of slow proton transfer reactions involving negative ions

Excitation functions were determined for a series of slow proton and deuteron transfer reactions involving negative ions using a tandem mass spectrometer. Some of the reactions observed, for example, ND/sub 2//sup -/ + D/sub 2/ ..-->.. D/sup -/ + ND/sub 3/, exhibit translational energy thresholds, even though they are exothermic. Other reactions, such as CH/sub 3/COCH/sub 2//sup -/ + CD/sub 3/COCD/sub 3/ ..-->.. CH/sub 3/COCH/sub 2/D + CD/sub 3/COCD/sub 2//sup -/, exhibit a complex functional dependence of the cross section upon relative translational energy, which in turn is quite sensitive to the internal energy of the neutral reactant. Rate coefficients determined for these negative ion reactions in the present investigation are compared with analogous data previously reported for thermal energy reactants. The translational energy thresholds observed for several of the reactions are consistent with the existence of a potential energy barrier in the reaction coordinate between the reactants and products. It is demonstrated that the deconvoluted excitation function for the ND/sub 2//sup -//D/sub 2/ reaction is of the form sigma varies as (E/sub rel/ -- E/sub 0/)/sup 1/2//E/sub rel/, as required by theory, and that the translational energy threshold corresponds to the Arrhenius activation energy for this process.