Molecular structure of the methyl anion CH−3. An investigation of the effects of electron correlation using the theory of self-consistent electron pairs (SCEP)

Since Hartree–Fock theory often predicts stable negative ions to lie above the analogous neutral +e−, there have been legitimate concerns that structural predictions at this level of theory may be of dubious value. To investigate this question, ab initio electronic structure theory has been applied to the methyl anion using the large and flexible basis of Duke. Electron correlation was taken into account explicitly using the recently developed theory of self-consistent electron pairs (SCEP). The geometrical structure of CH3− is found to be essentially unchanged by electron correlation. The near Hartree–Fock inversion barrier for CH3− is 1.72 kcal/mole, and this is reduced to 1.50 kcal/mole by correlation effects.