A dipole-supported state in cyanomethyl anion, the conjugate base of acetonitrile. Rotational band assignments in the electron photodetachment spectrum of −CH2CN

We report the 1 cm−1 resolution electron photodetachment spectra of cyanomethyl anion and its deuterated analog, trapped and detected in an ion cyclotron resonance spectrometer. Many sharp resonances were observed in the threshold region corresponding to rovibrational transitions from the ground electronic state to a dipole-supported state of the anion which subsequently undergo electron autodetachment. An assignment of the rotational transitions has been carried out, yielding rotational constants. Using spectral band intensities, we have estimated the electron binding energy of the dipole-supported state as 0.020 (±0.006) eV. The adiabatic electron affinities of the neutral radicals ⋅CH2CN and ⋅CD2CN are found to be 1.560±0.006 and 1.549±0.006 eV, respectively. Features of the dipole-supported state in this system have been compared to those in acetaldehyde enolate.