An experimental and computational study of the ions formed by the reaction of cyclopentanone with O−

The structures and reactivities of the ions formed by the reaction of cyclopentanone with O− have been studied using flowing afterglow-selected ion flow tube (FA-SIFT) experiments in conjunction with density functional theory (DFT) calculations. Three C5H6O− isomers were found to be generated - cyclopentanone-2,5-diyl radical anion (4−), 2-carbenacyclopentanone radical anion (5−), and cyclopentanone-2,4-diyl radical anion (6−). The large amount of signal loss observed in this reaction is attributed to formation of 2-cyclopentenone radical anion (10−), in which the electron is predicted to be unbound. DFT calculations predict 4− to be the most stable of the bound C5H6O− ions, and FA-SIFT experiments confirm 4− is the major ion formed in this reaction. Bracketing experiments found the proton affinity (PA) of 4− to be 362 ± 5 kcal/mol and the electron binding energy (EBE) to be ca. 0.5 eV. Although the PA of this species predicted by DFT calculations (363.2 kcal/mol) is consistent with the experimental value, both DFT and ab initio calculations predict an EBE of ca. 1.6 eV for this radical anion. The apparent conflict between the calculated and experimental EBE is resolved by proposing that, in the gas phase bracketing experiments, the electron transfer process leads adiabatically, not to cyclopentanone-2,5-diyl (4), but, by a retro-Nazarov reaction, to the more stable 1,4-pentadien-3-one (18). DFT calculations show that the difference between the computed and measured EBEs of 4− can be accounted for by the calculated difference between the energies of 18 and 4.