Photoelectron Spectroscopy of the Methide Anion: Electron Affinities of (•)CH3 and (•)CD3 and Inversion Splittings of CH3(-) and CD3(-).

We report high-resolution photoelectron spectra of the simplest carbanions, CH3⁻ and CD3⁻. The vibrationally resolved spectra are dominated by a long progression in the umbrella mode (ν2) of ˙CH3 and ˙CD3, indicating a transition from a pyramidal C(3v) anion to the planar D(3h) methyl radical. Analysis of the spectra provides electron affinities of ˙CH3 (0.093(3) eV) and ˙CD3 (0.082(4) eV). These results enable improved determination of the corresponding gas-phase acidities: Δ(acid)H(0K)°(CH4) = 414.79(6) kcal/mol and Δ(acid)H(0K)°(CD4) = 417.58(8) kcal/mol. On the basis of the photoelectron anisotropy distribution, the electron is photodetached from an orbital with predominant p-character, consistent with the sp³-hybridized orbital picture of the pyramidal anion. The double-well potential energy surface along the umbrella inversion coordinate leads to a splitting of the vibrational energy levels of the umbrella mode. The inversion splittings of CH3⁻ and CD3⁻ are 21(5) and 6(4) cm⁻¹, respectively, and the corresponding anion umbrella vibrational frequencies are 444(13) and 373(12) cm⁻¹, respectively. Quantum mechanical calculations reported herein show good agreement with the experimental data and provide insight regarding the electronic potential energy surface of CH3⁻.