Attachment of Electrons to Substituted Benzenes

Attachment of slow (0–5 eV) electrons to a number of aromatic species has been studied in a time-of-flight (TOF) mass spectrometer using an automatic retarding potential difference (RPD) electron beam with resolution of ∼ 0.1 eV. Autodetachment lifetimes were measured for parent ions formed by two-body attachment of thermal (∼ 0.03 eV) electrons to the following molecules (lifetimes in microseconds are given in parentheses): C6F5Br (21), C6F5Cl (17.6), C6F5CN (17), C6F5CHO (36), C6H5CN (≈ 5), C6H5NO2 (17.5), C6D5NO2 (22), m-C6H4ClNO2 (47), and perfluoronaphthalene (123). The C10F8− ion and ions of the form C6F5X− had lifetimes exhibiting a systematic increase with the number of vibrational degrees of freedom consistent with the trend observed in a previous study of cyclic fluorocarbons. Cl−, Br− and I− were formed from C6F5X (X==Cl, Br, I) along with C6F5− at incident electron energies below 0.1 eV. Current ratios for X− to C6F5− were found to be approxmaltely 100/1, 3/1, and 1/30, respectively. No nondissociative attachment was observed for C6F5I. Monochlorination of nitrobenzene essentially doubled the autodetachment lifetime and added a new mode of dissociation. Cl− and NO2− ion currents from m-chloronitrobenzene showed almost identical energy dependences with peaks at ∼ 1 eV and ∼ 3.5 eV, corresponding closely to the energy dependence of NO2− dissociation from nitrobenzene. This was taken as evidence that the Cl− and NO2− ion currents result from unimolecular dissociation of the same intermediate negative ion state with the incident electron in a π-electronic orbital. Iodine substitution in nitrobenzene had the effect of elimination of long-lived parent ion formation with dissociative attachment producing I− dominating at thermal electron energy. An SF6 scavenger spectrum of benzaldehyde indicated the presence of a temporary negative ion resonance at ∼ 0.65 eV. Negative ion breakdown curves were also determined for C6H5OH, C6F5OCH3, C6F5NH2, C6H5CN, o-C6H4ClF, m-C6H4ClF, m-C6F4ClBr, and m-C6H4ICl.