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 CHOH + ( 14 +· ) and CH 3 CHCHOH +· ( 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. Copyright © 2002 John Wiley & Sons, Ltd.