Photoelectron Spectroscopy of the Methide Anion: Electron Affinities of • CH 3 and • CD 3 and Inversion Splittings of CH 3 – and CD 3 –

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 C3v anion to the planar D3h 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: ΔacidH0K°(CH4) = 414.79(6) kcal/mol and ΔacidH0K°(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–.