Spectroscopically resolved competition between dissociation and detachment from nitrobenzene radical anion

Abstract

We report the vibrational spectrum of the gas-phase isolated nitrobenzene radical anion. The spectrum has been acquired by infrared multiple-photon absorption induced dissociation and electron detachment using the FT mass spectrometer coupled to the infrared free-electron laser FELIX. Upon wavelength-dependent multiple-photon absorption of intense IR irradiation, the vibrational spectrum acquired by on-resonance dissociation to NO 2 − was shown to correlate with the more sensitive electron detachment channel which is indirectly observed by using SF 6 as electron scavenger. The spectrum is compared to previous spectroscopic studies and novel DFT calculations. The frequency and intensity changes of the vibrational bands for the radical anion with respect to the neutral are interpreted with the aid of molecular orbital calculations and mode projection analysis. The vibrations of the neutral and the anion are interpreted in terms of the component benzene modes. The anion shows a reversal of the familiar strongly deactivating meta-directing electrophilic aromatic substitution effect of the neutral due to a resonance effect placing electron density at the ortho- and para-positions, resulting in a structure of distonic character. The greater abundance of the electron detachment channel over the NO 2 − loss dissociation channel is interpreted in terms of statistical models of the energy-dependent unimolecular rates. The anion and neutral vibrational frequencies employed in a quasi-equilibrium theory (QET) model of electron detachment compare favorably to previous experimental results of metastable anion autodetachment lifetimes. The ratio of dissociation to detachment is investigated as a function of FEL power and the competition between these channels is in agreement with a statistical model.