A SUCCESSFUL ABINITIO STUDY OF THE ADIABATIC ELECTRON-AFFINITY OF THE METHYL RADICAL

A stable methyl anion, i.e. a positive adiabatic electron affinity for the methyl radical, has been calculated directly by ab initio methods for the first time. The basis set was uncontracted in the valence region and augmented by four d and two f functions on carbon and three p and one d sets on hydrogen. At the CCD+ST(CCD) level the electron affinity is 0.23 kcal/mol or 0.010 eV. The CCD+ST(CCD) and QCISD(T) results are nearly identical for all basis sets considered. Zero-point energy corrections are estimated to contribute 0.034 eV. Thus, the calculated EA is nearly within the experimental error bound of 0.08 +/- 0.03 eV. The computed equilibrium geometry of CH3- is pyramidal also at a MP2/large basis set level where the anion is bound. The wavefunction is well represented by a single determinant (the coefficient of the dominant configuration in the CCD+ST(CCD) expansion being 0.976, the next largest coefficient 0.039). The MP4 and even the MP2 values are surprisingly close to the CCD+ST(CCD) and QCISD(T) results.