Inversion-vibration energies of CH 3− and adiabatic electron affinity of CH 3

Abstract

The inversion potential of the X 1 A 1 electronic ground state of the methyl anion CH 3 − is investigated at the SCF/single-reference configuration interaction ( SCF SR-CI ) and at the completeactive-space SCF/multireference CI ( CASSCF MR-CI ) levels of theory employing large Gaussian basis sets augmented by diffuse functions to represent the weakly bound carbon lone-pair electrons. These calculations, together with the corresponding out-of-plane bending potential of the CH 3 radical, demonstrate that the Born-Oppenheimer separation of the electronic and nuclear motions becomes invalid when moving along the inversion potential of CH 3 − ion by conventional methods, a quasi-diabatic representation of the full-dimensional potential energy hypersurface is evaluated applying standard SCF SR-CI calculations. From the resulting anharmonic potential function the vibration-rotation energies of the CH 3 − and CD 3 − isotopic variants are calculated using an improved version of the nonrigid inverter Hamiltonian approximation. Combining the present results with the previously determined vibrational potential function for the methyl radical CH 3, the different stabilities of the CH 3 − and CD 3 − isotopes against autodetachment are discussed in terms of effective potentials for the inversion motion and the adiabatic electron affinity of CH 3 is determined as EA = 0.09 eV close to the experimental value of 0.08 ± 0.03 eV. The photoelectron spectrum of CH 3 − is calculated within the Franck-Condon approximation in good agreement with the experimental spectrum. Present predictions of the lower inversion-vibration energy levels and the corresponding rotation constants of the CH 3 − ion are expected to be reliable.