Long-lived parent negative ions formed via nuclear-excited Feshbach resonances. Part 4.—Systematic study of NO2-containing benzene derivatives

Abstract

Forty substituted nitrobenzenes (substituents NO₂ and X) have been studied in a time-of-flight mass spectrometer and found to capture thermal and near-thermal energy electrons forming long-lived (lifetimes τ > 10^–6 s) nuclear-excited Feshbach resonant states. The parent negative ion autodetachment lifetimes increase with increasing value of the polarographic half-wave potential of these molecules as might be expected theoretically. They also depend on the electron donor-acceptor properties of the substituent X and the intra-molecular interaction between NO₂ and X. When the substituent X is an electron acceptor, the ions have lifetimes about ten times longer than when X is an electron donor. In this connection, CNDO-2 molecular orbital calculations on the neutral molecules have shown that when X is an electron acceptor, the magnitude of τ correlates with the amount of π-electron charge which is removed from the benzene ring while τ is little affected by the amount of π-electron charge which is donated to the benzene ring by X when X is an electron donor. Similar calculations on the negative ions of twelve para-substituted compounds indicated an increase in the ring π-electron charge by 0.2 to 0.3 electron units when X is an electron donor and by 0.4 to 0.7 electron units when X is an electron acceptor. At thermal energies the measured lifetimes for the ions of o-nitrophenol, o-nitrobenzaldehyde and o-nitroaniline were found to be much longer than those of the respective para-isomers, and this is attributed to the intramolecular interaction between the substituents NO₂ and X at the ortho-position. The results of this investigation lead to the conclusion that all NO₂-containing benzene derivatives form long-lived parent negative ions at thermal and epithermal energies, unless a fast dissociative attachment process is energetically possible. They also provide an insight into the details of electron attachement to polyatomic molecules and suggest that the degree of intramolecular charge transfer between a substituent and the benzene ring, as well as the extent of intramolecular interaction between the substituents around the benzene periphery can be inferred from the measured values of τ.