Hydrogen migrations in mass spectrometry. III–Energetics of formation of [R′CO2H2]+ in the mass spectra of R′CO2R

The appearance potentials for the [R'CO₂H₂]⁺ ion produced in the fragmentation process documentclass{article}pagestyle{empty}begin{document}$ left[{{rm R}^{rm '} {rm CO}_{rm 2} {rm R}} right]_{}^{_.^ + } $end{document} → [R'CO₂H₂]⁺+[R? 2H] have been measured using mono-energetic electron impact techniques for ethyl, n-propyl, and i-propyl formates and acetates. The results indicate that at the threshold the product ion has the protonated acid structure with the hydrogen on the carbonyl and not the hydroxyl group, and that the neutral product for the propyl esters is the allyl radical and not the cyclopropyl radical. For the propyl formates and acetates the appearance potential of the [R'CO₂H₂]⁺ ion is identical with the adiabatic ionization potential of the parent ester (measured by photoelectron spectroscopy) indicating that fragmentation occurs for ground state molecular ions. A two-step mechanism is proposed to rationalize the results.