Substituent effects in ion-pi interactions: fine-tuning via the ethynyl group.

In this work, we report a high level theoretical study (RI-MP2(full)/aug-cc-pVDZ) that deals with the effect of electron-withdrawing substituents on cation-pi and anion-pi interactions in the absence/presence of triple bonds between the substituent and the aromatic ring. The ethynyl group is able to finely tune the interaction energy of the complexes. Interestingly, for the cation-pi complexes, it reduces the effect of the electron withdrawing groups (EWG), improving the interaction. For anion-pi complexes, it boosts the effect of the EWG, improving the interaction as well. This dual behavior has been studied by examining the geometric and energetic features of the complexes, "atoms-in-molecules" analysis and charge transfer effects.