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 CH groups (n) on the cation (H, Li, Na, Al, CH) 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 + cα was proposed. After correction of the PAs, LCAs, SCAs, AlCA, and MCA for the dipole/charge interaction (E), linear relationships were observed between the corrected cation affinities and n or α. The contribution of E 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.

On the Significance of Lone Pair/Lone Pair and Lone Pair/Bond Pair Repulsions in the Cation Affinity and Lewis Acid/Lewis Base Interactions

“…22,23 It is known that larger alkali metal cations (Na + , K + ) attach to Lewis bases mainly via electrostatic interactions, whereas H + , CH 3 + , and to some extent Cu + interact covalently. 19,24 Li + and Al + exhibit intermediate behaviors with comparable electrostatic and covalent contributions. Effects of polarizability, dipole moment, and EWG and EDG on CA have been extensively studied.…”

Directionality of Cation/Molecule Bonding in Lewis Bases Containing the Carbonyl Group

“…For all cations (X) the O–X bond lengths in CH 2 O adduct ions are longer than those in CH 3 CHO adduct ions, in line with the stronger cation/ligand interaction in the latter case (Table S1). This may be attributed to the larger dipole moment of CH 3 CHO, which strengthens the electrostatic interaction, and an extra methyl group, which reinforces the covalent interaction . It should be mentioned that the methyl group increases both the electrostatic and covalent contributions.…”

“…This may be attributed to the larger dipole moment of CH 3 CHO, which strengthens the electrostatic interaction, and an extra methyl group, which reinforces the covalent interaction. 20 It should be mentioned that the methyl group increases both the electrostatic and covalent contributions. Addition of a methyl group increases the polarizability, which enhances the electrostatic ion/induced dipole interaction.…”

“…13,20 On the other hand, the interactions of Li + , Na + , and K + with the heteroatoms are mainly electrostatic ion/dipole interactions. 20 For covalent interactions, the direction of the cation/molecule bond is determined by the axis of putative sp, sp 2 , and sp 3 orbitals of lone pair electrons of heteroatoms. 5 For a pure electrostatic interaction, the cation is aligned with the direction of the dipole moment of the molecule.…”

“…This interaction may be covalent via the electron lone pair(s) on the heteroatoms or electrostatic via cation/dipole interaction. On the one hand, it has been reported that the covalent character of the interactions of H + , CH 3 + , Cu + , and to some extent Al + , with Lewis bases containing these heteroatoms is considerable. , On the other hand, the interactions of Li + , Na + , and K + with the heteroatoms are mainly electrostatic ion/dipole interactions . For covalent interactions, the direction of the cation/molecule bond is determined by the axis of putative sp, sp 2 , and sp 3 orbitals of lone pair electrons of heteroatoms .…”