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

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-pi or sigma-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-pi complexes vs. sigma-type ones. As a most relevant finding, in catechol-metal complexes the strength of cation-pi interactions is around half that of the sigma-type ones. It is argued that the combined effect of cation dimensions and hydroxyl substitution in catechol-Na+ complexes makes sigma-type configurations remarkably more stable and easily accessible than cation-pi 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.