title: Photoelectron Spectroscopy of the Methide Anion: Electron Affinities of (•)CH3 and (•)CD3 and Inversion Splittings of CH3(-) and CD3(-).
abstract: 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⁻.
authors.forename: Allan M
authors.initials: AM
authors.lastname: Oliveira
authors.forename: Yu-Ju
authors.initials: YJ
authors.lastname: Lu
authors.forename: Julia H
authors.initials: JH
authors.lastname: Lehman
authors.forename: P Bryan
authors.initials: PB
authors.lastname: Changala
authors.affiliation: Department of Chemistry, University of Texas , Austin, Texas 78712, United States.
authors.forename: Joshua H
authors.initials: JH
authors.lastname: Baraban
authors.affiliation: Department of Chemistry, University of Texas , Austin, Texas 78712, United States.
authors.forename: John F
authors.initials: JF
authors.lastname: Stanton
authors.forename: W Carl
authors.initials: WC
authors.lastname: Lineberger