Empty level structure and dissociative electron attachment in gas-phase nitro derivatives

Abstract

The gas-phase empty level structures of nitromethane, nitrobenzene, 2-nitroanisole and 2-nitrophenol are studied by means of electron transmission spectroscopy (ETS), dissociative electron attachment spectroscopy (DAS) and ab initio calculations, in the 0–6 eV energy range. Both the ETS and theoretical results indicate that the vertical electron affinity is positive in nitrobenzene, but negative in nitromethane. Consistently, the DA spectra of the nitrobenzenes display an intense molecular anion current at zero energy, while in nitromethane only the NO 2 − fragment (peaking at 0.6 eV) is observed. The most abundant anion is NO 2 −, also in the DA spectra of the nitrobenzenes, although in 2-nitrophenol the loss of the hydroxy radical from the molecular anion gives rise to a signal of the same intensity. Interestingly, the ET spectra of nitrobenzene and 2-nitrophenol display one and two narrow features, respectively, below 4 eV not associated with simple electron capture into π∗ orbitals. These signals are ascribed to core-excited resonances lying at exceptionally low energy. Comparison between the anion current measured with a quadrupole mass filter and that measured at the collision chamber suggests that the occurrence of intramolecular hydrogen bonding in 2-nitrophenol increases the lifetime of the molecular anion formed at zero energy.