Substituent effects in parallel-displaced π–π interactions

High-quality quantum-mechanical methods are used to examine how substituents tune π–π interactions between monosubstituted benzene dimers in parallel-displaced geometries. The present study focuses on the effect of the substituent across entire potential energy curves. Substituent effects are examined in terms of the fundamental components of the interaction (electrostatics, exchange-repulsion, dispersion and induction) through the use of symmetry-adapted perturbation theory. Both second-order Møller–Plesset perturbation theory (MP2) with a truncated aug-cc-pVDZ′ basis and spin-component-scaled MP2 (SCS-MP2) with the aug-cc-pVTZ basis are found to mimic closely estimates of coupled-cluster with perturbative triples [CCSD(T)] in an aug-cc-pVTZ basis. Substituents can have a significant effect on the electronic structure of the π cloud of an aromatic ring, leading to marked changes in the π–π interaction. Moreover, there can also be significant direct interactions between a substituent on one ring and the π-cloud of the other ring.