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 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–.