Effect of the Number of Methyl Groups on the Cation Affinity of Oxygen, Nitrogen, and Phosphorus Sites of Lewis Bases.

Abstract

The effect of number of CH3 groups (n) on the cation (H+, Li+, Na+, Al+, CH3+) affinity, polarizability, and dipole moment of 14 simple molecules was investigated. Linear correlations were observed between the polarizabilities and the number of methyl groups. The variations of the cation affinities and dipole moments with the number of methyl groups (n) were not linear, and a quadratic function was proposed for obtaining a good fit of the experimental data. Also, because the proton affinities (PA), lithium cation affinities (LCA), sodium cation affinities (SCA), aluminum cation affinity (AlCA), and methyl cation affinity (MCA) varied quadratically with polarizabilities (α), a formula of the form [cation affinities] = a + bα + cα2 was proposed. After correction of the PAs, LCAs, SCAs, AlCA, and MCA for the dipole/charge interaction (Eμ), linear relationships were observed between the corrected cation affinities and n or α. The contribution of Eμ to PA and MCA was small (less than 20%), and its contribution to LCA and SCA was large (>50%). The electrostatic contribution to AlCA was considerable (20-50%); however, it was smaller than the electrostatic contribution to LCA and SCA.