Effect of microhydration on the guanidinium···benzene interaction.

The effect of microhydration on the interaction of guanidinium cation with benzene has been studied by employing ab initio calculations. Four different structural arrangements were considered for the guanidinium···benzene interaction to which up to six water molecules were added. T-shaped structures are usually the most stable, but as water molecules are included the energy differences with the parallel structures decrease, reaching a point where parallel complexes are even more stable than T-shaped ones. Therefore, the inclusion of water molecules promotes a change in the structure of the cation···π contact. The analysis reveals that these stability changes are more related with the structure of the hydrating water molecules than to a modulation of the cation···π interaction. Already with three water molecules, one water molecule in the T-shaped complex has to be located in the second solvation shell, whereas in parallel structures this occurs with four water molecules. As a consequence energy differences among structures decrease. The calculations show that the nature of the interaction is almost unaffected in T-shaped structures, whereas an important dispersion increment is observed in parallel ones, though its overall effect is small.