Photoelectron Spectroscopy of the Methide Anion: Electron Affinities of ·CH_3 and ·CD_3 and Inversion Splittings of CH_3~- and CD_3~-

摘要

We report high-resolution photoelectron spectra of the simplest carbanions, CH_3~- and CD_3~. The vibrationally resolved spectra are dominated by a long progression in the umbrella mode (v_2) of ·CH_3 and ·CD_3 indicating a transition from a pyramidal C3v anion to the planar D_(3h) methyl radical. Analysis of the spectra provides electron affinities of ·CH_3 (0.093(3) eV) and ·CD_3 (0.082(4) eV). These results enable improved determination of the corresponding gas-phase acidities: Δ_(acid)H_(OK)°(CH_4) = 414.79(6) kcal/mol and Δ_(acid)H_(OK)°(CD_4) = 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 sp~3-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 CH_3~- and CD_3~- 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 CH_3~-.