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*approximate to 13+/-1 which is characteristic of the formation of dipole-bound anions [mu(CH3NO2)=3.46 D]. However, the breadth (Delta n approximate to 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 CH3NO2- 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 CH3NO2- and CD3NO2- 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 CH3NO2- and CD3NO2- yields adiabatic electron affinities of 0.26+/-0.08 and 0.24+/-0.08 eV, respectively. (C) 1996 American Institute of Physics.