Long-lived parent negative ions formed via nuclear-excited Feshbach resonances. Part 1.—Benzene derivatives

Abstract

Long-lived parent negative ions formed via nuclear-excited Feshbach resonances at thermal electron energies have been observed for six NO₂-containing benzene derivatives—namely, o-, m- and p-nitrophenol, and o-, m- and p-nitroaniline. The lifetimes, τ, of these ions at thermal electron energies, as determined by the slope method using a time-of-flight mass spectrometer, are 460 ± 34, 31 ± 1.3, 13.9 ± 0.4, 46 ± 2, 21 ± 1, and 15 ± 0.8 µs for o-, m- and p-nitrophenol, and o-, m- and p-nitroaniline, respectively. The measured lifetimes, as well as those reported previously for all known long-lived parent negative ions of benzene derivatives, are discussed and related to the respective molecular structures. The measured lifetimes depend on the electron donor-acceptor properties of the substituent groups and the interamolecular interaction between them, which is a function of their relative position around the benzene periphery. The results of this study suggest that, quite generally, NO₂- containing benzene derivatives capture thermal electron very efficiently and form long-lived (τ > 10^–6 s) parent negative ions in the absence of competing dissociative electron attachment processes which are usually very fast.