The phenoxyl group-modulated interplay of cation–π and σ-type interactions in the alkali metal series

Abstract

The interaction potential energy surfaces (IPESs) of four alkaline metal cations (Na⁺, K⁺, Rb⁺ and Cs⁺) complexed with phenol and catechol were explored by accurate ab initio calculations to investigate the interplay of different noncovalent interactions and their behavior along the alkali metal series and upon –OH substitution. Selected one-dimensional interaction energy curves revealed two different minimum energy configurations for all phenol– and catechol–metal complexes, characterized either by cation–π or σ-type interactions. For each investigated complex several two-dimensional IPES maps were also computed, exploiting the computational advantages of the MP2^mod approach. The size of the alkali cation was found to play a similar role in modulating both kinds of complexes, as the interaction strength always decreases along the metal series, from Na⁺ to Cs⁺. Conversely, the number of hydroxyl substituents markedly affected cation–π complexes vs. σ-type ones. As a most relevant finding, in catechol–metal complexes the strength of cation–π interactions is around half that of the σ-type ones. It is argued that the combined effect of cation dimensions and hydroxyl substitution in catechol–Na⁺ complexes makes σ-type configurations remarkably more stable and easily accessible than cation–π ones. Besides shedding new light on the origin of biological phenomena connected with underwater adhesion, the quantum mechanical interaction energy database provided herein may offer a useful reference for tuning accurate force fields, suitable for molecular dynamics simulations, where environmental effects might be also taken into account.