Intuitive and counterintuitive noncovalent interactions of aromatic π regions with the hydrogen and the nitrogen of HCN

•We have investigated intuitive and counterintuitive complex formation between eight aromatic molecules and HCN.•The intuitive interactions all led to bound complexes, and five of the counterintuitive did as well.•The Hellmann–Feynman theorem was invoked to help explain the formation of the five counterintuitive complexes in terms of polarization/dispersion.•Very good correlations were obtained, for the intuitive and the counterintuitive complexes separately, between the computed interaction energies and values predicted solely on the basis of the most positive and the most negative electrostatic potentials in the π regions and on the HCN.

We have investigated intuitive and counterintuitive complex formation between eight aromatic molecules and HCN. In four of the former, the π regions had negative electrostatic potentials; in the other four, the π regions had positive potentials. Each aromatic molecule was allowed to interact through its π region with both the hydrogen (positive potential) of HCN and also the nitrogen (negative potential). In eight cases, therefore, interaction was intuitively favorable (positive/negative) while in the other eight, attractive interaction would be counterintuitive on the basis of the ground state electrostatic potentials (positive/positive or negative/negative). The intuitive interactions all led to bound complexes, and five of the counterintuitive did as well. The Hellmann–Feynman theorem was invoked to help explain the formation of the five counterintuitive complexes in terms of polarization/dispersion. Very good correlations were obtained, for the intuitive and also the counterintuitive complexes, between the computed interaction energies and values predicted solely on the basis of the most positive and the most negative electrostatic potentials in the π regions and on the HCN.