Flow–drift tube measurements of kinetic energy dependences of some exothermic proton transfer rate constants

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Abstract
The rate constants for several exothermic proton transfer reactions have been measured as a function of relative ion kinetic energy in a flow–drift tube. The reactions include the exothermic proton transfers of O2H+ with N2, CO2, and NH3; H3+, CH5+, COH+, and NH3+ with NH3. The energy range covered is from thermal to about an electron volt. The energy dependences of the proton transfers are of interest in evaluating the reaction efficiency, with both polar and nonpolar neutrals. It is found that the thermal energy rate constants are given reasonably well by the Langevin (nonpolar) or average dipole orientation (polar) collision rate constants in most cases. However the energy dependence of the reaction rate constants is not that expected for the collision rate constant in most cases. It is thus concluded that models assuming a constant reactant efficiency with energy are not always valid.