On the binding of electrons to nitromethane: Dipole and valence bound anions

Conventional (valence) and dipole-bound anions of the nitromethane molecule are studied using negative ion photoelectron spectroscopy, Rydberg charge exchange and field detachment techniques. Reaction rates for charge exchange between Cs(ns,nd) and Xe(nf ) Rydberg atoms with CH3NO2 exhibit a pronounced maximum at an effective quantum number of n*≊13±1 which is characteristic of the formation of dipole-bound anions [μ(CH3NO2)=3.46 D]. However, the breadth (Δn≊5, FWHM) of the n-dependence of the reaction rate is also interpreted to be indicative of direct attachment into a valence anion state via a ‘‘doorway’’ dipole anion state. Studies of the electric field detachment of CH3NO−2 formed through the Xe(nf ) reactions at various n values provide further evidence for the formation of both a dipole-bound anion as well as a contribution from the valence bound anion. Analysis of the field ionization data yields a dipole electron affinity of 12±3 meV. Photodetachment of CH3NO−2 and CD3NO−2 formed via a supersonic expansion nozzle ion source produces a photoelectron spectrum with a long vibrational progression indicative of a conventional (valence bound) anion with a substantial difference in the equilibrium structure of the anion and its corresponding neutral. Assignment of the origin (v′=0, v″=0) transitions in the photoelectron spectra of CH3NO−2 and CD3NO−2 yields adiabatic electron affinities of 0.26±0.08 and 0.24±0.08 eV, respectively.