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

The gas-phase empty level structures of nitromethane, nitrobenzene, 2-nitro anisole and 2-nitrophenol are studied by means of electron transmission spe ctroscopy (ETS), dissociative electron attachment spectroscopy (DAS) and ab initio calculations, in the 0-6 eV energy range. Both the ETS and theoreti cal results indicate that the vertical electron affinity is positive in nit robenzene, but negative in nitromethane. Consistently, the DA spectra of th e nitrobenzenes display an intense molecular anion current at zero energy, while in nitromethane only the NO2- fragment (peaking at 0.6 eV) is observe d. The most abundant anion is NO2-, also in the DA spectra of the nitrobenz enes, although in 2-nitrophenol the loss of the hydroxy radical from the mo lecular 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 cap ture into pi* orbitals. These signals are ascribed to core-excited resonanc es lying at exceptionally low energy. Comparison between the anion cut-rent 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. (C) 2001 Elsevier Science B.V.