Quadrupole moment versus Molecular Electrostatic Potential: Strange behavior of ethynyl-substituted benzenes

In this Letter we demonstrate that the quadrupole moment (Qzz) of aromatic compounds shows a strong correlation with the MEP value computed along the main symmetry axis perpendicular to the ring plane. The Qzz value correlates well with both the LUMO energy level and the dipoles created by the substituents. Latter result allows conciliating two different points of view regarding the source of the electrostatic contribution to the total interaction energy ion–π complexes of substituted benzenes. The ethynyl group presents a strange behavior that is analyzed in this Letter, using several tools including MEP and SAPT analysis among others.