Cross Sections and Intramolecular Isotope Effects in AB—HD Ion—Molecule Reactions

Abstract

Isotopic ion—molecule reactions producing ABH⁺ and ABD⁺ ions (AB=N₂, CO, O₂, and CO₂) when AB—HD mixtures are subjected to electron bombardment in the mass‐spectrometer ion source have been studied as a function of reactant ion velocity. Good agreement between the experimental and theoretical cross sections for secondary ion formation is obtained when both AB⁺—HD and HD⁺—AB interactions are taken as possible reaction paths. Ionization‐efficiency curves of the secondary ion products furnish supporting evidence for the assumed reaction mechanisms. Identification of reactant ions by ionization‐efficiency curves is not sufficient to establish the thermochemistry of the ion—molecule reaction; distributions of vibronic states produced by electron impact are also required. The vibrational population of excited reactant ions formed in the electron impact process has been estimated from the squares of the overlap integrals of respective ground and excited ionized states. Data taken from published photoionization experiments were used to estimate the distribution of electronic excitation in the N₂⁺, CO⁺, and O₂⁺ reactant ions. This data served to better define the reactant ion species in the above ion—molecule interactions and aided in the analysis of energy transfer processes and intramolecular isotope effects in these reactions. Intramolecular isotope effects showed an increase in the ratio of ABH⁺/ABD⁺ with increasing reactant ion velocity. The isotope effects are described in terms of a unimolecular decomposition of the collision complex and ion or atom transfer mechanisms. The isotope effects indicate that atom transfer mechanisms are favored both in reactions of ions with high kinetic energy and in strongly exothermic reactions of relatively slow ions.