Charge Transfer and Proton Transfer in Polyatomic Ion-Molecule Systems

Abstract

Ion impact mass spectra of some hydrocarbon molecules have been investigated using mass analyzedbeams of H+, D+, H2 +, D2 +,H3 +, D3 +, ArH+, ArD+, Ar+ and Ne+ ions. Charge‐transfer and proton‐ or deuteron‐transfer mechanisms suffice to account qualitatively for the observed spectra. The energetics of the respective processes in the systems studied provide a basis for assigning various ion fragments and parent molecule ions to either charge‐transfer or ion‐molecule reaction formation mechanisms. The role of the neutral molecule as an energy sink in molecular charge transfer has been examined in detail for H2 +charge‐transferreactions. The distribution of recombination energies experimentally observed in H2 + collisions with a variety of molecular species is rationalized with the aid of a model that considers the neutralization reaction as a Franck‐Condon process. Estimates are made of the distribution of excited neutral product H2 molecules from the squares of the overlaps of H2 +−H2 vibrational wavefunctions. This model also accounts for relatively low cross sections for H3 + ion‐impact reactions because of the production of vibrationally excited H2 which renders some processes endothermic. The necessity of formation of vibrationally excited neutral molecular products in ion‐molecule reactions that proceed via stripping mechanisms accounts for energy barriers not predicted from usual thermochemical considerations. Cyclopropane and propylene spectra were the subject of extensive study. With ArH+ projectile ions the velocity dependence of the component of ion‐impact spectra attributed to ion‐molecule reactions was found to be in good agreement with the 1 / υ dependence of Langevin collision cross sections, and the magnitude of experimental cross sections were found in good agreement with those calculated from simple classical theory. Significant differences were observed in the ion‐impact spectra of cyclopropane and propylene. The ion‐molecule reaction component was attributed primarily to proton‐ or deuteron‐transfer reactions which initially produce excited C3H7 + ions. Processes which superficially appeared to be hydride ion‐transfer processes were plausibly explained as the loss of H2 from excited C3H7 +, and this argument was supported by the isotopic exchange observed with deuterium‐substituted projectile ions. Studies on hydrocarbons up to and including neopentane showed further consistency with predictions made for ion‐molecule reaction cross sections based on the Langevin model.