ChemInform Abstract: Gas‐Phase Electron Photodetachment Spectroscopy of the Molecular Anions of Nitroaromatic Hydrocarbons at Atmospheric Pressure

Abstract

Low-resolution, electron photodetachment (PD) spectra of the molecular radical anions of nitrobenzene and 30 other nitroaromatic hydrocarbons bearing methyl, fluoro, chloro, bromo, and cyano substituents are reported for the first time. A unique feature of these measurements is that they have been obtained at atmospheric pressure by use of a specialized electron capture detector (ECD) in which the molecular anions are formed by resonance electron capture at 200{degree}C. Absolute PD cross sections, {sigma}{sub d}, for these anions over the spectral range 300 to 1200 nm have been obtained by use of I{sup {minus}} as a calibration reference. It is shown that the nitroaromatic anions undergo photodetachment by two mechanisms, direct PD and resonance PD. Through measurements of direct PD in the spectral region of photodetachment onset, the minimum photon energy, E{sub th}, necessary to cause PD has been determined for each nitroaromatic anion. These measurements have been compared with the adiabatic electron affinities (EA) of the corresponding molecules which had been determined previously from studies of gas-phase electron transfer equilibria. This comparison indicates a high level of agreement between these two data sets and is consistent with a simple model of the energetics and structural changes which are thought tomore » accompany the negative ionization of the nitroaromatic hydrocarbons. The occurrence of resonance PD for these anions is supported by a comparison of several of the PD spectra with UV-vis absorption spectra of the corresponding anions previously measured in {tau}-irradiated frozen glassy solutions. By the resonance PD mechanism, peak maxima of high absolute cross section are observed for many of the nitroaromatic anions. 33 refs., 10 figs., 1 tab.« less