An Interacting Quantum Atoms and Multipolar Electrostatics Study of XH…π Interactions

The interaction energies of nine XH…π (X = C, N, O) benzene-containing van der Waals complexes were analysed, at atomic and fragment level, using QTAIM multipolar electrostatics and the energy partitioning method Interacting Quantum Atoms/Fragment (IQA/IQF). These descriptors were paired with the Relative Energy Gradient (REG) method, which solidifies the connection between quantum mechanical properties and chemical interpretation. This combination provides a precise understanding, both qualitative and quantitative, of the nature of these interactions, which are ubiquitous in biochemical systems. The formation of the OH…π and NH…π systems is electrostatically driven, with the Qzz component of the quadrupole moment of the benzene carbons interacting with the charges of X and H in XH. There is the unexpectedly intra-monomeric role of X-H (X=O,N) where its electrostatic energy helps the formation of the complex and its covalent energy thwarts it. However, the CH…π interaction is governed by exchange-correlation energies, thereby establishing a covalent character, as opposed to the literature’s designation as a non-covalent interaction. Moreover, dispersion energy is relevant, statically and in absolute terms, but less relevant compared to other energy components in terms of the formation of the complex. Multipolar electrostatics are similar across all systems.