Carbanion Spectroscopy: CH₂CN^-

We have measured the photoelectron spectra of CH2CN- and CD2CN- and report the following electron affinities: EA(CH2CN) = 1.543 ± 0.014 eV and EA(CD2CN) = 1.538 ± 0.012 eV. From an analysis of the peak splittings and intensities, we extract potential-energy curves for the umbrella mode of the CH2CN- negative ion and the CH2CN radical. The radical is found to be a planar, C2ν, species. The structure of the cyanomethide ion is almost described as CH2=C=N-, but the hydrogens are bent slightly out of the molecular plane. A Franck-Condon analysis leads to a value of the out-of-plane deformation angle as 30 ± 5° with a very small inversion barrier of 100 ± 50 cm-1. Our structural conclusions are reinforced by a series of ab initio Hartree—Fock and M⊘ller-Plesset perturbation calculations on both the cyanomethyl radical and cyanomethide ion. Using the gas-phase acidity of CH3CN, we obtain the following bond-dissociation energy for acetonitrile: DH°298(H-CH2CN) = 94.2 ± 2.0 kcal/mol. © 1987, American Chemical Society. All rights reserved.