Direct and indirect effects of dispersion interactions on the electric properties of weakly bound complexes

Direct (electronic) and indirect (geometrical) modifications of the molecular properties of weakly interacting complexes between the push-pull p-aminobenzoic acid (pABA) molecule and the nonpolar benzene (Bz) have been studied with a large panel of wave function (WF) and density functional theory (DFT) based methods using carefully selected atomic basis sets. For pABA, both the canonical (pABA-c) and zwitterionic (pABA-z) forms have been investigated. Owing to strongly distinct charge distributions, the two forms of pABA enable us to mimic different interaction modes with Bz. In this work, we assessed the performances of dispersion-corrected DFT methods, as well as of long-range corrected exchange-correlation functionals. It follows from the SAPT analysis that both the structure and the interaction energy of the first complex (pABA-c···Bz) is mainly controlled by dispersion interactions whereas, in the second complex (pABA-z···Bz), electrostatic and induction forces play also an important role. Our results suggest that the (non)linear electric properties of push-pull and zwitterionic molecules can be significantly reduced by the presence of a nonpolar compound. We also show that even for a complex with stability strongly determined by dispersion forces, the direct dispersion contributions to its electric properties can be small. Nevertheless, the intersystem distance is influenced by dispersion forces, which, in turn indirectly tune the induced properties. The zwitterionic derivative appears to be more challenging in the context of molecular properties.