The influence of introducing water molecules into a cation-e complex on the interaction between the cation
and the e system was investigated using the MP2/6-311++G** method to explore how a cation-e complex
changes in terms of both its geometry and its binding strength during the hydration. The calculation on the
methylammonium-benzene complex showed that the cation-e interaction is weakened by introducing H2O
molecules into the system. For example, the optimized interaction distance between the cation and the benzene
becomes longer and longer, the transferred charge between them becomes less and less, and the cation-e
binding strength becomes weaker and weaker as the water molecule is introduced one by one. Furthermore,
the introduction of the third water molecule leads to a dramatic change in both the complex geometry and the
binding energy, resulting in the destruction of the cation-e interaction. The decomposition on the binding
energy shows that the influence is mostly brought out through the electrostatic and induction interactions.
This study also demonstrated that the basis set superposition error, thermal energy, and zero-point vibrational
energy are significant and needed to be corrected for accurately predicting the binding strength in a hydrated
cation-e complex at the MP2/6-311++G** level. Therefore, the results are helpful to better understand the
role of water molecules in some biological processes involving cation-e interactions.