Evidence for a difference in the dissociation mechanisms of acetylene (HCCH) and vinylidene (H2CC:) from charge inversion mass spectrometry

Abstract

Vinylidene and acetylene are the simplest hydrocarbon isomers, and vinylidene is the simplest unsaturated carbene. The charge inversion mass spectra of C₂H₂⁺ cations derived from acetylene using Na, K, Rb and Cs targets were found to be clearly different from those derived from vinylidenechloride (1,1-dichloroethylene). The process of formation of the negative ions in charge inversion mass spectrometry is via near-resonant neutralization followed by spontaneous dissociation, and then endothermic negative ion formation. The intensity of the C₂⁻ peak relative to the C₂H⁻ peak in these spectra increased with decreasing ionization potential of the targets for both of the isomeric C₂H₂⁺ cations. The formation of the C₂⁻ anion is proposed to result from the dissociation of excited C₂H₂ neutrals into C₂ and H₂. The dependence on target species of the intensities of the C₂⁻ peak relative to the C₂H⁻ peak for HCCH and H₂CC: cannot be rationalized by the internal energy of the excited C₂H₂ neutrals. The differences indicate that the isomeric C₂H₂ neutrals dissociate into C₂H and H prior to 1,2-hydrogen atom migration.