Lactone enol cation-radicals: gas-phase generation, structure, energetics, and reactivity of the ionized enol of butane-4-lactone.

The cation-radical of 2-hydroxyoxol-2-ene (1(+)) represents the first lactone enol ion whose structure and gas-phase ion chemistry have been studied by experiment and theory. Ion 1(+) was generated by the McLafferty rearrangement in ionized 2-acetylbutane-4-lactone and characterized by accurate mass measurements, isotope labeling, metastable ion and collisionally activated dissociation (CAD) spectra. Metastable 1(+) undergoes competitive losses of H-4 and CO that show interesting deuterium and (13)C isotope effects. The elimination of CO from metastable 1(+) shows a bimodal distribution of kinetic energy release and produces ()CH(2)CH(2)CHdbond;OH(+) (14(+)) and CH(3)CHdbond;CHOH(+) (15(+)) in ratios which are subject to deuterium isotope effects. Ab initio calculations at the G2(MP2) level of theory show that 1(+) is 105 kJ mol(-1) more stable than its oxo form, butane-4-lactone(2(+)). The elimination of CO from 1(+) involves multiple isomerizations by hydrogen migrations and proceeds through ion-molecule complexes of CO with 14(+) and 15(+). In addition, CO is calculated to catalyze an exothermic isomerization 14(+) --> 15(+) in the ion-molecule complexes. Multiple consecutive hydrogen migrations in metastable 1(+), as modeled by RRKM calculations on the G2(MP2) potential energy surface, explain the unusual deuterium kinetic isotope effects on the CO elimination.