Structures of protonated benzene dimer and intermolecular interaction decomposition via symmetry-adapted perturbation theory

Intermolecular interactions of the protonated benzene dimer are examined through computed interaction energies of three distinct dimer configurations and the decomposition of the interaction into electrostatic, induction, exchange-repulsion, and dispersion components. Protonated benzene dimer is an effective prototype for the study of positively charged, non-covalent interactions commonly encountered throughout aromatic regions of complex biological systems. Three distinct configurations of the dimer are identified: parallel-displaced, T-shaped, and canted. Equilibrium interaction energies computed with CCSD(T) range from −11.4 to −10.0 kcal mol−1 where the lowest energy structures involve direct interaction of the protonation site with the top face of benzene. All three conformers contain unorthodox, cationic, CH–π hydrogen bonds. Comparison of the parallel-displaced structures of C 6 H 6 – C 6 H 7 + and ( C 6 H 6 ) 2 + reveals that the interaction between benzene and protonated benzene is weaker than the interaction between benzene and benzene radical cation. Symmetry adapted perturbation theory provides evidence that the benzene–benzenium attractive interaction consists primarily of equal contributions from dispersion and electrostatics.