Affinities of Ammonia and Methylamine for H⁺, Li⁺, K⁺ and CH₃⁺ An Ab Initio SCF Study with Large Basis Sets

Computations of Li + ‐affinities of NH 3 and NH 2 CH 3 performed with the SCF ab initio method, using the 4‐31G, 6‐31G*, 6‐31G** basis sets as well as Dunning's (9s, 5p/4s) basis contracted to double‐zeta without and with polarization functions, show that only the last basis is able to reproduce the experimental order of the gas‐phase affinities. A study is made of the evolution of the various components of the binding energy upon methylation, and of the modifications observed upon improvement of the basis sets: reversal of the order of the binding energies to the correct order occurs when the decrease of the Coulomb term becomes smaller than the increase of the delocalization term upon methylation. The best of the previous sets is shown to account in a satisfactory way for the H + and CH 3 + affinities of the same compounds. For K + ‐affinities, the same basis still yields an incorrect ordering upon methylation. A further improvement of the basis is seen to work in the right direction. The K + ‐affinities of H 2 O and NH 3 computed with the most extended basis are in numerical agreement with the gas‐phase values.