Enhancing the function, non-additivity, and substitution position effect of the Li atom in the cation-π interaction and its mechanism: An ab initio study of Li +Li-substituted benzene complexes

Quantum chemical calculations have been performed to study the cation-pi interaction in the Li+center dot center dot center dot C6H6-nLin (n = 0-6) complex. The results show that the cation-pi interaction is enhanced by lithium substitution in the aromatic ring. With increasing number of lithium substituents in the aromatic ring, the total interaction energy becomes more negative. However, the average contribution from one Li substituent to the interaction energy becomes smaller. The lithium substituent in the aromatic ring displays negative non-additivity in enhancing the cation-pi interaction. The substitution position of the lithium substituent in the aromatic ring has a prominent effect on the strength of the cation-pi interaction. The largest interaction energy (-97.7 kcal mol(-1)) is found for the Li+center dot center dot center dot 1,3,5-C6H3Li3 complex. The enhancing effect of the Li atom can be understood in terms of the natural population analysis (NPA) charge on the carbon atom in the aromatic ring, the most negative electrostatic potential at the center of the aromatic ring, and energy decomposition.